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-{"id": "0620_ch04", "subject": "Chemistry", "code": "0620", "chapter": "Chemical formulae and equations", "text": "Status: model-generated, unreviewed by a subject teacher. Generated 2 Oct 2026.\n\n# 0620 Chemistry — Chapter 4: Chemical formulae and equations (syllabus 3.1 Formulae)\n\nScope: syllabus section 3.1, Core 1–4 and Supplement 5–8 (students sit Extended). Arithmetic with formulae (Mr, moles, empirical formula from data, molecular formula from Mr) belongs to chapter 5 (`ch05_chemical_calculations.md`); ionic half-equations belong to electrolysis (section 4). Paragraphs [P1]… are the anchors the knowledge atoms point to. Stage-1 pipeline text; see ATOMS_PLAN.md.\n\n## A. One rule underneath everything\n\n[P1] Chemistry needs a written language because the things it describes are invisible and come in vast numbers. The language rests on two conservation laws. **Atoms are conserved**: a reaction regroups atoms but never creates, destroys or converts them into atoms of another element. **Charge is conserved**: electrons can move from one particle to another, but the total charge before and after a change is the same. A formula is a record of *which atoms, and how many, are grouped together*; an equation is a record of *how the groups change while the atoms and the charge stay the same*. Every rule in this chapter is a way of keeping those two counts honest on paper; a forgotten rule can be rebuilt by asking \"is every atom accounted for, and does the charge add up?\"\n\n## B. Symbols and the formulae of elements\n\n[P2] Each element has a one- or two-letter symbol; the first letter is a capital and the second is lower case (Co is cobalt, CO is carbon monoxide — case carries meaning). The formula of an element shows the particle it is made of. Most elements are written as single atoms, because in the solid or liquid they form a giant structure (metals, carbon, silicon) or exist as separate atoms (the noble gases, He, Ne, Ar). Seven elements exist as **diatomic molecules** and must always be written with a 2 when they appear as the element: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂. Writing O, H or Cl for the element in an equation is the commonest formula error in the examiner reports. Inside a compound the 2 disappears, because the atoms are bonded to something else: oxygen is O₂, but water is H₂O.\n\n## C. Reading a formula\n\n[P3] A formula is a count. A subscript counts the atom (or bracketed group) immediately before it: H₂SO₄ contains 2 H, 1 S and 4 O. A subscript after a bracket multiplies everything inside the bracket: Ca(NO₃)₂ contains 1 Ca, 2 N and 6 O. Brackets are needed only when a group of atoms that travels together (a compound ion such as NO₃⁻ or OH⁻) appears more than once; NaNO₃ needs no bracket, and writing Na(NO₃) is redundant. A large number written *in front of* a formula in an equation is a **coefficient**; it multiplies the whole formula: 3H₂SO₄ means 6 H, 3 S and 12 O. The raised dot in a hydrated salt such as CuSO₄·5H₂O means \"together with\": one formula unit of copper(II) sulfate is combined with five water molecules, so the unit contains 1 Cu, 1 S, 9 O and 10 H. The dot is not a multiplication sign. The formula of a compound never carries the charges of its ions: magnesium chloride is MgCl₂, not Mg²⁺Cl₂⁻.\n\n## D. Molecular formula: what is in one molecule\n\n[P4] A substance made of separate molecules has a definite particle, so it can be described exactly. The **molecular formula** gives the number and type of each atom in one molecule: ethane is C₂H₆, glucose is C₆H₁₂O₆, ammonia is NH₃. To deduce a molecular formula from a model, a ball-and-stick picture, a displayed formula or a structural formula, count each kind of atom, including every atom hidden inside a group (CH₃, OH, COOH). For a displayed formula it helps to tick each atom as it is counted. Organic molecular formulae are written C first, then H, then the other elements (C₂H₆O for ethanol). A structural formula such as CH₃CH₂OH carries the same atoms as C₂H₆O; the molecular formula adds them up and loses the arrangement, which is why isomers can share one molecular formula.\n\n## E. Empirical formula: the simplest ratio\n\n[P5] Many substances have no molecule to count. An ionic lattice or a giant covalent structure is the same pattern repeated an indefinite number of times, so the only thing that can be stated is the *ratio* of the particles. The **empirical formula** is the simplest whole-number ratio of the different atoms or ions in a compound. For ionic compounds (NaCl, MgO, Al₂O₃) and giant covalent substances (SiO₂) the formula written is always the empirical formula. For molecular substances the empirical formula is found from the molecular formula by dividing every subscript by their highest common factor: C₂H₆ → CH₃; C₆H₁₂O₆ → CH₂O; H₂O₂ → HO; C₄H₈O₂ → C₂H₄O. If the highest common factor is 1, the two formulae are identical (H₂O, CH₄, C₃H₈, CO₂). The same reasoning explains why a polymer has no fixed molecular formula: its chains are of indefinite and varying length, so only its repeat unit (and an empirical formula) can be written. Finding an empirical formula from masses or percentages, and a molecular formula from an empirical formula plus Mr, are calculations; they are treated in chapter 5.\n\n## F. Where ionic charges come from\n\n[P6] An ion is an atom or group of atoms that has gained or lost electrons. For main-group elements the charge follows from the electronic configuration: atoms lose or gain the fewest electrons that leave a full outer shell. So Group I metals form 1+ ions (Na⁺, K⁺, Li⁺), Group II form 2+ (Mg²⁺, Ca²⁺, Ba²⁺), aluminium in Group III forms Al³⁺; non-metals in Group V, VI and VII gain 3, 2 or 1 electrons (N³⁻, O²⁻, S²⁻, F⁻, Cl⁻, Br⁻, I⁻). Hydrogen forms H⁺. Group IV and Group VIII elements do not normally form simple ions. Transition elements can form more than one ion, so their names carry a **Roman numeral** that gives the oxidation number, which for a simple metal ion equals its charge: iron(II) is Fe²⁺, iron(III) is Fe³⁺, copper(II) is Cu²⁺. The numeral is a positive number only; it says nothing about the other ion. Zinc (Zn²⁺) and silver (Ag⁺) have one common ion and usually carry no numeral.\n\n[P7] Some ions are groups of atoms covalently bonded together that carry an overall charge: the **compound ions**. They must be learned because they cannot be deduced from a group number: ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, carbonate CO₃²⁻, sulfate SO₄²⁻, hydrogencarbonate HCO₃⁻, phosphate PO₄³⁻, and the organic anion ethanoate CH₃COO⁻ (in general, a carboxylic acid RCOOH gives the anion RCOO⁻). Names give clues: an ending *-ide* means a single element (or hydroxide), *-ate* means the ion contains oxygen. Sulfide S²⁻ and sulfate SO₄²⁻ are different ions; confusing them is a recorded examiner complaint. The charge of a compound ion belongs to the whole group, which is why the group goes in a bracket when there is more than one of it.\n\n## G. Formulae of ionic compounds from charges\n\n[P8] An ionic compound is electrically neutral, so its formula is the smallest whole-number ratio of ions in which the positive and negative charges cancel. The method: write both ions with their charges; find the lowest common multiple of the two charge sizes; take enough of each ion to reach it. Sodium oxide: Na⁺ and O²⁻; LCM 2; two Na⁺ per O²⁻ → Na₂O. Aluminium sulfate: Al³⁺ and SO₄²⁻; LCM 6; two Al³⁺ and three SO₄²⁻ → Al₂(SO₄)₃. Ammonium carbonate: NH₄⁺ and CO₃²⁻ → (NH₄)₂CO₃. Iron(III) hydroxide: Fe³⁺ and OH⁻ → Fe(OH)₃. The \"swap the charge numbers\" shortcut gives the same answer because it builds the LCM automatically, but it must be followed by simplifying (Mg²⁺ and O²⁻ swap to Mg₂O₂, which simplifies to MgO). The method works for unfamiliar ions too: given a chromate ion CrO₄²⁻, potassium chromate must be K₂CrO₄ and silver chromate Ag₂CrO₄; given phosphate PO₄³⁻, calcium phosphate is Ca₃(PO₄)₂. If a compound contains more than two kinds of ion, every positive charge is added and every negative charge is added, and the two totals must be equal.\n\n[P9] The same neutrality argument runs backwards. From a formula, the charge on an unknown ion follows from the known one: in Cr₂(SO₄)₃ three sulfate ions carry 6−, so two chromium ions carry 6+ and each is Cr³⁺. From a salt formula, the anion can be read off: in Na₂SeO₃ two Na⁺ balance one SeO₃²⁻, and the parent acid (which supplies H⁺ in place of Na⁺) is H₂SeO₃. A formula can also be deduced from a **model or diagram** of a lattice or molecule by counting the relative numbers of each particle: a lattice diagram with equal numbers of + and − ions of single charge represents a 1 : 1 compound such as KBr, while one with twice as many anions as cations represents a compound like MgCl₂. If the charges are shown, neutrality is a check on the count. A harder version names a mixed-ion solid: if it contains only Co²⁺, Co³⁺ and O²⁻, the formula must be neutral *and* need both cobalt ions; Co₃O₄ (2+ plus 3+ plus 3+ = 8+, four O²⁻ = 8−) fits, Co₂O₃ does not.\n\n## H. Formulae to know by heart\n\n[P10] Statement 3.1.1 asks for the formulae of the substances named anywhere in the syllabus. Most follow from [P6]–[P8], but a working set must be instant: H₂O, CO₂, CO, NH₃, CH₄, SO₂, SO₃, NO₂, NO; the acids HCl, H₂SO₄, HNO₃, CH₃COOH; NaOH, Ca(OH)₂, CaCO₃, CaO, Fe₂O₃, Al₂O₃, SiO₂, CuO, ZnCl₂, Cu(NO₃)₂, (NH₄)₂SO₄; and the two hydrated salts named in the syllabus, CuSO₄·5H₂O (blue) and CoCl₂·6H₂O (pink). When a formula is asked for, a name earns nothing; when a formula is printed in the question, copy it exactly — examiner reports repeatedly note candidates changing a formula they were given.\n\n## I. Equations: from words to balanced symbols\n\n[P11] A **word equation** names reactants and products: magnesium + hydrochloric acid → magnesium chloride + hydrogen. It records what happens but not in what proportions. A **symbol equation** replaces names with formulae and must then obey conservation of atoms: the same number of each kind of atom on each side. Writing the correct formulae comes first and is a separate mark in every Paper 4 equation item (the mark scheme's M1 is usually \"correct species\" or \"correct product\"); balancing is the second mark. A word equation is never accepted where a \"chemical equation\" or \"symbol equation\" is asked for.\n\n[P12] **Balancing changes coefficients, never subscripts.** Changing H₂O to H₂O₂ to \"fix\" the oxygen count produces a different substance, hydrogen peroxide, and the equation then describes a different reaction. A coefficient only changes how many particles of the same substance take part. A dependable procedure: (1) write correct formulae for everything; (2) balance an element that appears in only one substance on each side, starting with the formula containing the most atoms; (3) balance elements that appear as the free element (O₂, H₂, Cl₂, metals) last, because their coefficient changes only that one element; (4) treat a compound ion that survives unchanged (SO₄, NO₃) as a single unit; (5) recount every element, and reduce coefficients to the smallest whole numbers. Example: Al + O₂ → Al₂O₃. Oxygen needs a common multiple of 2 and 3, so 3O₂ and 2Al₂O₃; then 4Al: 4Al + 3O₂ → 2Al₂O₃. Never balance by writing Al₂ or 2Al₂ for the metal, or by inserting extra species such as H or O atoms that the reaction does not contain.\n\n[P13] **Combustion and \"odd oxygen\".** For a hydrocarbon or other C–H–O compound burning completely, balance C (into CO₂), then H (into H₂O), then count the oxygen atoms needed on the right and supply them as O₂. If that number is odd, double every coefficient. Butane: C₄H₁₀ + O₂ → 4CO₂ + 5H₂O needs 13 O atoms, i.e. 6½ O₂; doubling gives 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. If the fuel contains oxygen, subtract its oxygen atoms before counting O₂. The same atom-counting identifies an unknown product: if C₁₀H₂₂ cracks into C₂H₄ and one other molecule, the other must be C₈H₁₈, because all ten carbons and twenty-two hydrogens must be accounted for. Reversible reactions are written with ⇌ instead of →; the balancing rules do not change.\n\n## J. State symbols\n\n[P14] State symbols show the physical state of each substance under the conditions of the reaction: **(s)** solid, **(l)** liquid, **(g)** gas, **(aq)** aqueous, i.e. dissolved in water. Aqueous is not a synonym for liquid: dilute hydrochloric acid is HCl(aq), because hydrogen chloride is dissolved in water, while water itself is H₂O(l) and is never (aq). Rules that cover nearly every examined case: a metal, a metal oxide, a carbonate added as a powder and any precipitate are (s); acids, alkalis and soluble salts in solution are (aq); water formed in a reaction at room temperature is (l); gases at room temperature and pressure (H₂, O₂, CO₂, Cl₂, NH₃, SO₂) are (g); a halogen produced in solution, such as bromine in a displacement, is (aq). The state depends on the conditions given: water from a combustion at high temperature is H₂O(g), and a liquid product like silicon(IV) chloride at r.t.p. is (l). State symbols go in brackets, lower case, after each formula; when a question says \"include state symbols\", every species needs one.\n\n## K. Deducing an equation from information\n\n[P15] Supplement statement 3.1.8 asks for the equation of a reaction the student may never have seen, from information supplied. The move is always the same: (1) list the reactants and products the question names, and write their formulae (using any formula printed in the question exactly); (2) if a product is not named, deduce it from a reaction pattern — acid + metal → salt + hydrogen; acid + base (oxide or hydroxide) → salt + water; acid + carbonate → salt + water + carbon dioxide; metal + water → hydroxide + hydrogen (or oxide + hydrogen with steam); element + oxygen → oxide; a more reactive metal or halogen displaces a less reactive one from its compound; thermal decomposition of a carbonate → oxide + carbon dioxide; complete combustion → carbon dioxide + water — or from the atom count, as in [P13]; (3) balance; (4) add state symbols if asked. In a typical unfamiliar case a metal oxide reacts with sodium hydroxide to give water and a salt of a stated anion; the salt's formula comes from charges ([P8]) and the rest is balancing. Examiner reports show the commonest failure is a wrong product formula (K₂O for potassium hydroxide, ZnCl for zinc chloride), not the balancing.\n\n## L. Ionic equations\n\n[P16] An ionic compound dissolved in water is not a set of molecules but a collection of separate, mobile ions; aqueous sodium chloride is Na⁺(aq) and Cl⁻(aq). When two such solutions are mixed and a reaction occurs, only some of the ions change. The ones that are present unchanged before and after are **spectator ions**, and an **ionic equation** leaves them out, showing only the particles that actually react. The procedure: (1) write the full balanced symbol equation with state symbols; (2) split every aqueous ionic substance (soluble salts, strong acids, alkalis) into its ions; (3) do *not* split solids, precipitates, water, gases, covalent molecules such as halogens, or metals; (4) cross out ions that appear identically on both sides; (5) check that the result balances in atoms **and** in total charge. Mixing silver nitrate and sodium chloride solutions: Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq), leaving Ag⁺(aq) + Cl⁻(aq) → AgCl(s).\n\n[P17] Three families cover almost every ionic equation examined. **Precipitation**: two aqueous ions on the left, one solid on the right, state symbols always (aq) + (aq) → (s); a shortcut is to take the precipitate's formula and split it into its ions, so PbI₂ comes from Pb²⁺(aq) + 2I⁻(aq) and Cu(OH)₂ from Cu²⁺(aq) + 2OH⁻(aq). **Neutralisation** of any acid by any alkali reduces to H⁺(aq) + OH⁻(aq) → H₂O(l), which is why every such reaction has the same essential change. **Displacement**: a metal and a metal ion, Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s), or a halogen molecule and a halide ion, Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq); here the sodium or potassium ions are the spectators, and the halogens stay as diatomic molecules because they are covalent. In every case the charge check is decisive: 2Al(s) + 3Cu²⁺(aq) → 2Al³⁺(aq) + 3Cu(s) has 6+ on each side, while Al + Cu²⁺ → Al³⁺ + Cu balances atoms but not charge. An ionic half-equation (Cu²⁺ + 2e⁻ → Cu) is a different object: it shows one electrode process with electrons written explicitly, and belongs to electrolysis and redox.\n\n## M. Conventions and bridges\n\n[P18] Marking conventions for this chapter are stable across the archive. Formula items are one mark, formula only; a name, a formula with charges left in, a wrong case (CL, Co for CO) or a subscript written full size is not credited. Equation items are worth 1–3 marks: M1 for the correct species (often just the key product), M2 for the fully balanced equation, M3 for state symbols when asked; multiples of a correct equation and fractions such as ½O₂ are usually accepted, but an equation with extra species, charges in compound formulae or the reactants on the wrong side is not. Ionic precipitation equations earn M1 for the precipitate as the only product, M2 for exactly the right two ions as the only reactants, M3 for (aq) + (aq) → (s). Bridges: chapter 5 reads balanced equations as mole ratios and calculates formulae from data; electrolysis (section 4) builds half-equations on [P16]–[P17]; redox (section 6) explains the Roman numeral; acids and salts (section 7) supply the reaction patterns and solubility rules used in [P15]–[P17]; organic chemistry (section 11) supplies the molecules to which [P4], [P5] and [P13] are applied.\n\n## Coverage matrix\n\n| Syllabus statement | Wording (abridged, official) | Tier | Paragraphs |\n|---|---|---|---|\n| 3.1.1 | State the formulae of the elements and compounds named in the subject content | Core | P2, P3, P7, P10 |\n| 3.1.2 | Define the molecular formula of a compound as the number and type of different atoms in one molecule | Core | P4, P5 |\n| 3.1.3 | Deduce the formula of a simple compound from the relative numbers of atoms present in a model or a diagrammatic representation | Core | P4, P9 |\n| 3.1.4 | Construct word equations and symbol equations to show how reactants form products, including state symbols | Core | P11, P12, P13, P14 |\n| 3.1.5 | Define the empirical formula of a compound as the simplest whole number ratio of the different atoms or ions in a compound | Supplement | P5 |\n| 3.1.6 | Deduce the formula of an ionic compound from the relative numbers of the ions present in a model or a diagrammatic representation or from the charges on the ions | Supplement | P6, P7, P8, P9 |\n| 3.1.7 | Construct symbol equations with state symbols, including ionic equations | Supplement | P14, P16, P17 |\n| 3.1.8 | Deduce the symbol equation with state symbols for a chemical reaction, given relevant information | Supplement | P13, P15 |\n| — bridge, no statement | The two conservation laws behind every rule | — | P1 |\n| — bridge, no statement | Marking conventions; links to chapter 5, sections 4, 6, 7, 11 | — | P18 |\n\nGaps: none of 3.1.1–3.1.8 lacks a paragraph. Scope notes: ion charges from group number (P6) formally belong to section 2 (ionic bonding) but are the prerequisite for 3.1.6 and are kept as a bridge; the meaning of the Roman numeral (P6) is formally 6.4.1 but is needed to write formulae from names. Not covered on purpose: empirical formula from composition data and molecular formula from Mr (chapter 5, 3.3.7); ionic half-equations (section 4); general formulae of homologous series and displayed formulae (section 11).\n", "atoms": [{"id": "0620-3.1-A01", "type": "concept", "statement": "Atoms are conserved in a chemical reaction: they are regrouped, never created, destroyed or turned into another element.", "why": "Reactions rearrange bonds and electrons; nuclei are untouched.", "prerequisites": [], "tier": "core", "paragraphs": "P1"}, {"id": "0620-3.1-A02", "type": "concept", "statement": "Charge is conserved: the total charge on each side of an equation is the same.", "why": "Electrons move between particles but are not created or destroyed.", "prerequisites": ["0620-3.1-A01"], "tier": "supplement", "paragraphs": "P1"}, {"id": "0620-3.1-A03", "type": "fact", "statement": "Element symbols start with a capital letter; a second letter is lower case, and case carries meaning (Co is cobalt, CO is carbon monoxide).", "why": "Symbols are a shared international code.", "prerequisites": [], "tier": "core", "paragraphs": "P2"}, {"id": "0620-3.1-A04", "type": "fact", "statement": "Seven elements exist as diatomic molecules and are written H2, N2, O2, F2, Cl2, Br2, I2 whenever they appear as the element.", "why": "Their atoms pair up by a covalent bond to fill outer shells.", "prerequisites": ["0620-3.1-A03"], "tier": "core", "paragraphs": "P2"}, {"id": "0620-3.1-A05", "type": "fact", "statement": "Other elements are written as single atoms in equations: metals, carbon, silicon, sulfur and the noble gases.", "why": "Giant structures and monatomic gases have no small molecule to count.", "prerequisites": ["0620-3.1-A03"], "tier": "core", "paragraphs": "P2"}, {"id": "0620-3.1-A06", "type": "representation", "statement": "A subscript counts the atom (or bracketed group) immediately before it: H2SO4 has 2 H, 1 S, 4 O.", "why": "A formula is a count of atoms.", "prerequisites": ["0620-3.1-A03"], "tier": "core", "paragraphs": "P3"}, {"id": "0620-3.1-A07", "type": "representation", "statement": "A subscript after a bracket multiplies everything inside; brackets are used only when a compound ion appears more than once (Ca(NO3)2, but NaNO3).", "why": "The compound ion travels as one unit.", "prerequisites": ["0620-3.1-A06"], "tier": "supplement", "paragraphs": "P3"}, {"id": "0620-3.1-A08", "type": "representation", "statement": "A coefficient in front of a formula multiplies the whole formula: 3H2SO4 contains 6 H, 3 S, 12 O.", "why": "The coefficient counts whole formula units.", "prerequisites": ["0620-3.1-A06"], "tier": "core", "paragraphs": "P3"}, {"id": "0620-3.1-A09", "type": "representation", "statement": "The raised dot in a hydrated salt (CuSO4.5H2O) means together with: whole water molecules are added, not multiplied.", "why": "Water of crystallisation is part of the crystal formula unit.", "prerequisites": ["0620-3.1-A06"], "tier": "core", "paragraphs": "P3"}, {"id": "0620-3.1-A10", "type": "fact", "statement": "A compound formula never shows the ion charges: MgCl2, not Mg2+Cl2-.", "why": "The charges cancel in the neutral compound; they are shown only on separate ions.", "prerequisites": ["0620-3.1-A06"], "tier": "supplement", "paragraphs": "P3"}, {"id": "0620-3.1-A11", "type": "fact", "statement": "Molecular formula: the number and type of different atoms in one molecule.", "why": "A molecular substance has a definite particle that can be counted.", "prerequisites": ["0620-3.1-A06"], "tier": "core", "paragraphs": "P4"}, {"id": "0620-3.1-A12", "type": "procedure", "statement": "Deduce a molecular formula from a model, displayed or structural formula by counting each kind of atom, including atoms inside groups (CH3, OH, COOH).", "why": "The formula is just the atom count of the drawn particle.", "prerequisites": ["0620-3.1-A11"], "tier": "core", "paragraphs": "P4"}, {"id": "0620-3.1-A13", "type": "concept", "statement": "A molecular formula loses the arrangement of atoms, so different compounds (isomers) can share one molecular formula; organic formulae are written C, then H, then other elements.", "why": "Counting atoms discards how they are joined.", "prerequisites": ["0620-3.1-A11"], "tier": "core", "paragraphs": "P4"}, {"id": "0620-3.1-A14", "type": "fact", "statement": "Empirical formula: the simplest whole-number ratio of the different atoms or ions in a compound.", "why": "A ratio is all that can be stated when there is no fixed particle size.", "prerequisites": ["0620-3.1-A11"], "tier": "supplement", "paragraphs": "P5"}, {"id": "0620-3.1-A15", "type": "concept", "statement": "Ionic lattices and giant covalent substances (NaCl, SiO2) have only an empirical formula, because there is no separate molecule.", "why": "A giant structure repeats indefinitely; only the ratio is fixed.", "prerequisites": ["0620-3.1-A14"], "tier": "supplement", "paragraphs": "P5"}, {"id": "0620-3.1-A16", "type": "procedure", "statement": "Find an empirical formula from a molecular formula by dividing every subscript by their highest common factor; if it is 1 the two formulae are the same.", "why": "Dividing keeps the ratio and makes it simplest.", "prerequisites": ["0620-3.1-A14", "0620-3.1-A12"], "tier": "supplement", "paragraphs": "P5"}, {"id": "0620-3.1-A17", "type": "concept", "statement": "A polymer has no single molecular formula because its chains are of indefinite and varying length; only the repeat unit can be written.", "why": "There is no one molecule size to count.", "prerequisites": ["0620-3.1-A11", "0620-3.1-A14"], "tier": "supplement", "paragraphs": "P5"}, {"id": "0620-3.1-A18", "type": "relationship", "statement": "Main-group ion charge follows the group: I 1+, II 2+, Al 3+, V 3-, VI 2-, VII 1-.", "why": "Atoms gain or lose the fewest electrons that leave a full outer shell.", "prerequisites": [], "tier": "supplement", "paragraphs": "P6"}, {"id": "0620-3.1-A19", "type": "fact", "statement": "Group IV and Group VIII elements do not normally form simple ions; hydrogen forms H+.", "why": "Gaining or losing four electrons is too costly; noble gases already have full shells.", "prerequisites": ["0620-3.1-A18"], "tier": "supplement", "paragraphs": "P6"}, {"id": "0620-3.1-A20", "type": "fact", "statement": "A Roman numeral in a name gives the oxidation number of the metal, equal to its ion charge: iron(II) Fe2+, iron(III) Fe3+, copper(II) Cu2+; it is always positive and refers to the metal only.", "why": "Transition metals form more than one ion, so the name must say which.", "prerequisites": ["0620-3.1-A18"], "tier": "supplement", "paragraphs": "P6"}, {"id": "0620-3.1-A21", "type": "fact", "statement": "Zinc forms only Zn2+ and silver only Ag+, so their names usually carry no numeral.", "why": "One common ion needs no label.", "prerequisites": ["0620-3.1-A20"], "tier": "supplement", "paragraphs": "P6"}, {"id": "0620-3.1-A22", "type": "fact", "statement": "Compound ions to know: NH4+, OH-, NO3-, CO3 2-, SO4 2-, HCO3-, PO4 3-.", "why": "Their charges cannot be deduced from a group number, so they are learned.", "prerequisites": ["0620-3.1-A18"], "tier": "supplement", "paragraphs": "P7"}, {"id": "0620-3.1-A23", "type": "fact", "statement": "A carboxylic acid RCOOH gives the anion RCOO- (ethanoate CH3COO-), so its salts are written like other salts: (CH3COO)2Ca.", "why": "The acid loses H+ from the COOH group.", "prerequisites": ["0620-3.1-A22"], "tier": "supplement", "paragraphs": "P7"}, {"id": "0620-3.1-A24", "type": "relationship", "statement": "Name endings: -ide means a single element (or hydroxide), -ate means the ion contains oxygen; sulfide S2- and sulfate SO4 2- are different ions.", "why": "The ending encodes the composition.", "prerequisites": ["0620-3.1-A22"], "tier": "supplement", "paragraphs": "P7"}, {"id": "0620-3.1-A25", "type": "concept", "statement": "An ionic compound is neutral: total positive charge equals total negative charge.", "why": "Opposite charges must cancel in a stable compound.", "prerequisites": ["0620-3.1-A02", "0620-3.1-A18"], "tier": "supplement", "paragraphs": "P8"}, {"id": "0620-3.1-A26", "type": "procedure", "statement": "Formula from charges: write both ions, find the lowest common multiple of the charge sizes, take enough of each ion to reach it, bracket a compound ion used more than once (Al2(SO4)3).", "why": "This is the smallest neutral ratio.", "prerequisites": ["0620-3.1-A25", "0620-3.1-A07", "0620-3.1-A10"], "tier": "supplement", "paragraphs": "P8"}, {"id": "0620-3.1-A27", "type": "procedure", "statement": "The swap-the-charges shortcut must be followed by simplifying (Mg2O2 becomes MgO).", "why": "Swapping builds the LCM but not always the simplest ratio.", "prerequisites": ["0620-3.1-A26"], "tier": "supplement", "paragraphs": "P8"}, {"id": "0620-3.1-A28", "type": "procedure", "statement": "Apply the charge method to supplied or unfamiliar ions, and to compounds with three kinds of ion, by totalling all positive and all negative charges.", "why": "Neutrality does not depend on knowing the ion beforehand.", "prerequisites": ["0620-3.1-A26"], "tier": "supplement", "paragraphs": "P8"}, {"id": "0620-3.1-A29", "type": "procedure", "statement": "Inverse: find an unknown ion charge from a formula and the known ion (Cr2(SO4)3 gives Cr3+).", "why": "Neutrality fixes the missing charge.", "prerequisites": ["0620-3.1-A25"], "tier": "supplement", "paragraphs": "P9"}, {"id": "0620-3.1-A30", "type": "procedure", "statement": "Read the anion from a salt formula, and the parent acid by replacing the metal ions with H+ (Na2SO3 gives SO3 2- and H2SO3).", "why": "A salt is an acid with its H+ replaced by metal ions.", "prerequisites": ["0620-3.1-A29", "0620-3.1-A22"], "tier": "supplement", "paragraphs": "P9"}, {"id": "0620-3.1-A31", "type": "procedure", "statement": "Deduce a formula from a model or lattice diagram by counting the relative numbers of each particle, checking neutrality when charges are shown.", "why": "The ratio in the picture is the ratio in the formula.", "prerequisites": ["0620-3.1-A25", "0620-3.1-A12"], "tier": "supplement", "paragraphs": "P9"}, {"id": "0620-3.1-A32", "type": "procedure", "statement": "For a compound containing two ions of one element (mixed valence), choose the formula that is both neutral and contains every stated ion.", "why": "Neutrality alone admits formulas that lack one of the ions.", "prerequisites": ["0620-3.1-A25"], "tier": "supplement", "paragraphs": "P9"}, {"id": "0620-3.1-A33", "type": "fact", "statement": "Formulae of named molecular compounds: H2O, CO2, CO, NH3, CH4, SO2, SO3, NO2, NO.", "why": "Listed across the syllabus content.", "prerequisites": ["0620-3.1-A03", "0620-3.1-A04"], "tier": "core", "paragraphs": "P10"}, {"id": "0620-3.1-A34", "type": "fact", "statement": "Formulae of the common acids: HCl, H2SO4, HNO3, CH3COOH.", "why": "Each is H+ combined with a known anion.", "prerequisites": ["0620-3.1-A22", "0620-3.1-A33"], "tier": "core", "paragraphs": "P10"}, {"id": "0620-3.1-A35", "type": "fact", "statement": "Hydrated copper(II) sulfate is CuSO4.5H2O (blue) and hydrated cobalt(II) chloride is CoCl2.6H2O (pink).", "why": "Named in the syllabus as tests for water.", "prerequisites": ["0620-3.1-A09"], "tier": "core", "paragraphs": "P10"}, {"id": "0620-3.1-A36", "type": "technique", "statement": "When a formula is asked for, give a formula, not a name; when a formula is printed in the question, copy it exactly.", "why": "A name scores nothing for a formula mark, and the mark scheme assumes the given formula.", "prerequisites": ["0620-3.1-A33"], "tier": "core", "paragraphs": "P10; P18"}, {"id": "0620-3.1-A37", "type": "representation", "statement": "A word equation names reactants and products (reactants -> products); it is not accepted where a chemical or symbol equation is asked for.", "why": "Names carry identity but not proportions.", "prerequisites": [], "tier": "core", "paragraphs": "P11"}, {"id": "0620-3.1-A38", "type": "procedure", "statement": "First step of any symbol equation: write the correct formula of every species; this is usually the first mark (M1).", "why": "Balancing a wrong formula balances the wrong reaction.", "prerequisites": ["0620-3.1-A33", "0620-3.1-A26", "0620-3.1-A04"], "tier": "core", "paragraphs": "P11"}, {"id": "0620-3.1-A39", "type": "concept", "statement": "Balance by changing coefficients only, never subscripts or formulae.", "why": "A changed subscript is a different substance (H2O2 is not H2O).", "prerequisites": ["0620-3.1-A01", "0620-3.1-A06"], "tier": "core", "paragraphs": "P12"}, {"id": "0620-3.1-A40", "type": "procedure", "statement": "Balancing order: an element found in one substance each side first (largest formula first), compound ions as units, free elements last; recount everything and use the smallest whole numbers.", "why": "Free elements change only their own count, so they mop up at the end.", "prerequisites": ["0620-3.1-A39", "0620-3.1-A08"], "tier": "core", "paragraphs": "P12"}, {"id": "0620-3.1-A41", "type": "procedure", "statement": "Combustion: balance C into CO2, then H into H2O, count the O needed, supply it as O2 (subtract any O in the fuel); if the O2 count is odd or half, double all coefficients.", "why": "Oxygen is the only free element and is fixed last.", "prerequisites": ["0620-3.1-A40", "0620-3.1-A49"], "tier": "core", "paragraphs": "P13"}, {"id": "0620-3.1-A42", "type": "procedure", "statement": "Deduce an unknown formula in an equation from the atom count (missing product; cracking products from a given ratio).", "why": "Every atom on the left must appear on the right.", "prerequisites": ["0620-3.1-A01", "0620-3.1-A06"], "tier": "core", "paragraphs": "P13"}, {"id": "0620-3.1-A43", "type": "representation", "statement": "A reversible reaction is written with a double arrow; balancing rules are unchanged.", "why": "The same atoms are conserved in both directions.", "prerequisites": ["0620-3.1-A40"], "tier": "core", "paragraphs": "P13"}, {"id": "0620-3.1-A44", "type": "fact", "statement": "State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water; aqueous is not the same as liquid.", "why": "An equation should show the physical form of each substance.", "prerequisites": [], "tier": "core", "paragraphs": "P14"}, {"id": "0620-3.1-A45", "type": "relationship", "statement": "Assigning states: metals, oxides, powdered carbonates and precipitates (s); acids, alkalis and soluble salts in solution (aq); water (l); gases at r.t.p. (g); a halogen formed in solution (aq).", "why": "Each follows from the substance and whether it is dissolved.", "prerequisites": ["0620-3.1-A44"], "tier": "core", "paragraphs": "P14"}, {"id": "0620-3.1-A46", "type": "relationship", "statement": "The state depends on the conditions stated: water from a hot combustion is (g); a liquid product at r.t.p. is (l).", "why": "State symbols describe the substance under the reaction conditions.", "prerequisites": ["0620-3.1-A45"], "tier": "supplement", "paragraphs": "P14"}, {"id": "0620-3.1-A47", "type": "procedure", "statement": "Deduce an equation from information: list the named species, write formulae (copy given ones), deduce unnamed products from a reaction pattern or the atom count, balance, add state symbols if asked.", "why": "The equation is built from conservation plus a known pattern.", "prerequisites": ["0620-3.1-A38", "0620-3.1-A40", "0620-3.1-A45", "0620-3.1-A48"], "tier": "supplement", "paragraphs": "P15"}, {"id": "0620-3.1-A48", "type": "relationship", "statement": "Acid patterns: acid + metal -> salt + hydrogen; acid + base -> salt + water; acid + carbonate -> salt + water + carbon dioxide; reactive metal + water -> hydroxide + hydrogen.", "why": "Bridge from section 7 (acids) and 9 (metals).", "prerequisites": [], "tier": "core", "paragraphs": "P15"}, {"id": "0620-3.1-A49", "type": "relationship", "statement": "Other patterns: element + oxygen -> oxide; a more reactive metal or halogen displaces a less reactive one; carbonate on heating -> oxide + carbon dioxide; complete combustion -> carbon dioxide + water.", "why": "Bridge from sections 9, 10 and 11.", "prerequisites": [], "tier": "core", "paragraphs": "P15"}, {"id": "0620-3.1-A50", "type": "concept", "statement": "A dissolved ionic compound exists as separate ions; ions present unchanged before and after a reaction are spectator ions.", "why": "Only the particles that change belong in a description of the change.", "prerequisites": ["0620-3.1-A02", "0620-3.1-A44"], "tier": "supplement", "paragraphs": "P16"}, {"id": "0620-3.1-A51", "type": "procedure", "statement": "Ionic equation recipe: full equation; split aqueous ionic substances into ions; do not split solids, water, gases, covalent molecules or metals; cancel spectators; check atoms and charge.", "why": "Leaves exactly the particles that react.", "prerequisites": ["0620-3.1-A50", "0620-3.1-A40"], "tier": "supplement", "paragraphs": "P16"}, {"id": "0620-3.1-A52", "type": "representation", "statement": "Precipitation ionic equation: two aqueous ions -> one solid, (aq) + (aq) -> (s); take the precipitate formula and split it into its ions to get the left-hand side (Pb2+ + 2I- -> PbI2).", "why": "The precipitate is the only new substance; its ions came from the two solutions.", "prerequisites": ["0620-3.1-A51", "0620-3.1-A26"], "tier": "supplement", "paragraphs": "P17"}, {"id": "0620-3.1-A53", "type": "fact", "statement": "Every acid-alkali neutralisation has the ionic equation H+(aq) + OH-(aq) -> H2O(l).", "why": "The salt ions are spectators in every case.", "prerequisites": ["0620-3.1-A51"], "tier": "supplement", "paragraphs": "P17"}, {"id": "0620-3.1-A54", "type": "representation", "statement": "Displacement ionic equations: metal + metal ion (Fe + Cu2+ -> Fe2+ + Cu) or halogen molecule + halide ion (Cl2 + 2I- -> 2Cl- + I2); the halogens stay diatomic and the Group I cations are spectators.", "why": "Covalent halogen molecules are not split; only the halide ions change.", "prerequisites": ["0620-3.1-A51", "0620-3.1-A04"], "tier": "supplement", "paragraphs": "P17"}, {"id": "0620-3.1-A55", "type": "procedure", "statement": "Balance an ionic equation for charge as well as atoms (2Al + 3Cu2+ -> 2Al3+ + 3Cu has 6+ on each side).", "why": "Charge is conserved.", "prerequisites": ["0620-3.1-A02", "0620-3.1-A51"], "tier": "supplement", "paragraphs": "P17"}, {"id": "0620-3.1-A56", "type": "concept", "statement": "An ionic half-equation is a different object: one electrode process with electrons written explicitly; it belongs to electrolysis and redox.", "why": "It shows only half of a transfer.", "prerequisites": ["0620-3.1-A51"], "tier": "supplement", "paragraphs": "P17"}, {"id": "0620-3.1-A57", "type": "technique", "statement": "Marking pattern: formula items one mark, exact; equations M1 species, M2 balanced, M3 state symbols; precipitation ionic M1 precipitate only, M2 its two ions only, M3 (aq)+(aq)->(s); extra species lose M1.", "why": "Fixed conventions across 2023-25 mark schemes.", "prerequisites": ["0620-3.1-A38"], "tier": "core", "paragraphs": "P18"}], "misconceptions": [{"id": "0620-3.1-M01", "wrong": "the free element can be written as a single atom (O, H, N, Cl, Br) in an equation.", "correct": "write the seven diatomic elements as H2, N2, O2, F2, Cl2, Br2, I2.", "atoms": ["0620-3.1-A04"]}, {"id": "0620-3.1-M02", "wrong": "an equation can be balanced by changing a formula or writing Al2, Fe2, Zn2, Hg2.", "correct": "change only coefficients; a changed formula is a different substance.", "atoms": ["0620-3.1-A39"]}, {"id": "0620-3.1-M03", "wrong": "once the formulae are right the equation is finished.", "correct": "count every element on both sides and balance (a separate mark).", "atoms": ["0620-3.1-A40"]}, {"id": "0620-3.1-M04", "wrong": "a coefficient multiplies only the first element of the formula after it.", "correct": "a coefficient multiplies every atom in the formula (4FeS2 is 4 Fe and 8 S).", "atoms": ["0620-3.1-A08"]}, {"id": "0620-3.1-M05", "wrong": "a salt formula can be written without balancing the charges (ZnCl, Pb2SO4, Pb(SO4)2, AlO).", "correct": "use ion charges and the lowest common multiple so the compound is neutral.", "atoms": ["0620-3.1-A26", "0620-3.1-A25"]}, {"id": "0620-3.1-M06", "wrong": "compound ions can be guessed: nitrate with a 2- charge, ammonium as NH3, sulfide and sulfate interchangeable, nitric acid as H2NO4.", "correct": "learn NO3-, NH4+, SO4 2- (sulfate) vs S2- (sulfide), HNO3.", "atoms": ["0620-3.1-A22", "0620-3.1-A24", "0620-3.1-A34"]}, {"id": "0620-3.1-M07", "wrong": "the ion charges belong inside the compound formula (Mg2+F-2), or fluorine is Fl.", "correct": "compound formulae carry no charges; symbols must be exact.", "atoms": ["0620-3.1-A10", "0620-3.1-A03"]}, {"id": "0620-3.1-M08", "wrong": "the Roman numeral is the charge of the compound, of the non-metal, a valency, or just the number 2.", "correct": "the numeral is the positive oxidation number (ion charge) of the metal only.", "atoms": ["0620-3.1-A20"]}, {"id": "0620-3.1-M09", "wrong": "the group numbers of two elements are the subscripts of their compound (Group V + Group VII gives P5Q7).", "correct": "convert group numbers to ion charges first, then make the compound neutral.", "atoms": ["0620-3.1-A18", "0620-3.1-A26"]}, {"id": "0620-3.1-M10", "wrong": "any neutral formula fits a description of the ions present.", "correct": "the formula must be neutral and contain every stated ion.", "atoms": ["0620-3.1-A32"]}, {"id": "0620-3.1-M11", "wrong": "aqueous means liquid, so water is H2O(aq) and dilute acid is HCl(l).", "correct": "(aq) means dissolved in water; water itself is (l); a dilute acid is (aq).", "atoms": ["0620-3.1-A44", "0620-3.1-A45"]}, {"id": "0620-3.1-M12", "wrong": "an ionic equation is the full symbol equation, or spectator ions may be left in (K+, Na+, NO3-).", "correct": "split aqueous ionic substances and cancel the ions that appear unchanged on both sides.", "atoms": ["0620-3.1-A50", "0620-3.1-A51"]}, {"id": "0620-3.1-M13", "wrong": "an ionic equation only needs the right ions, not the right numbers of them (Pb2+ + Cl- -> PbCl2).", "correct": "balance atoms and total charge: Pb2+ + 2Cl- -> PbCl2.", "atoms": ["0620-3.1-A55", "0620-3.1-A52"]}, {"id": "0620-3.1-M14", "wrong": "in a halogen displacement the halogen molecules carry charges, the ions swap sides, or the reaction is written as a precipitation or half-equation.", "correct": "halogens stay neutral diatomic molecules; the halide ion of the less reactive halogen is the reactant.", "atoms": ["0620-3.1-A54"]}, {"id": "0620-3.1-M15", "wrong": "the molecular formula is the empirical formula.", "correct": "divide the subscripts by their highest common factor.", "atoms": ["0620-3.1-A16"]}, {"id": "0620-3.1-M16", "wrong": "complete combustion gives carbon dioxide and hydrogen, or needs no oxygen on the left.", "correct": "complete combustion: fuel + O2 -> CO2 + H2O.", "atoms": ["0620-3.1-A49", "0620-3.1-A41"]}, {"id": "0620-3.1-M17", "wrong": "a formula printed in the question can be rewritten from memory.", "correct": "copy the given formula exactly; it is the formula the mark scheme uses.", "atoms": ["0620-3.1-A36"]}, {"id": "0620-3.1-M18", "wrong": "a name or word equation answers a request for a formula or chemical equation.", "correct": "a formula needs symbols; a chemical equation means a balanced symbol equation.", "atoms": ["0620-3.1-A36", "0620-3.1-A37"]}]}
+{"id": "0620_ch04", "subject": "Chemistry", "code": "0620", "chapter": "Chemical formulae and equations", "text": "Status: model-generated, unreviewed by a subject teacher. Generated 2 Oct 2026.\n\n# 0620 Chemistry — Chapter 4: Chemical formulae and equations (syllabus 3.1 Formulae)\n\nScope: syllabus section 3.1, Core 1–4 and Supplement 5–8 (students sit Extended). Arithmetic with formulae (Mr, moles, empirical formula from data, molecular formula from Mr) belongs to chapter 5 (`ch05_chemical_calculations.md`); ionic half-equations belong to electrolysis (section 4). Paragraphs [P1]… are the anchors the knowledge atoms point to. Stage-1 pipeline text; see ATOMS_PLAN.md.\n\n## A. One rule underneath everything\n\n[P1] Chemistry needs a written language because the things it describes are invisible and come in vast numbers. The language rests on two conservation laws. **Atoms are conserved**: a reaction regroups atoms but never creates, destroys or converts them into atoms of another element. **Charge is conserved**: electrons can move from one particle to another, but the total charge before and after a change is the same. A formula is a record of *which atoms, and how many, are grouped together*; an equation is a record of *how the groups change while the atoms and the charge stay the same*. Every rule in this chapter is a way of keeping those two counts honest on paper; a forgotten rule can be rebuilt by asking \"is every atom accounted for, and does the charge add up?\"\n\n## B. Symbols and the formulae of elements\n\n[P2] Each element has a one- or two-letter symbol; the first letter is a capital and the second is lower case (Co is cobalt, CO is carbon monoxide — case carries meaning). The formula of an element shows the particle it is made of. Most elements are written as single atoms, because in the solid or liquid they form a giant structure (metals, carbon, silicon) or exist as separate atoms (the noble gases, He, Ne, Ar). Seven elements exist as **diatomic molecules** and must always be written with a 2 when they appear as the element: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂. Writing O, H or Cl for the element in an equation is the commonest formula error in the examiner reports. Inside a compound the 2 disappears, because the atoms are bonded to something else: oxygen is O₂, but water is H₂O.\n\n## C. Reading a formula\n\n[P3] A formula is a count. A subscript counts the atom (or bracketed group) immediately before it: H₂SO₄ contains 2 H, 1 S and 4 O. A subscript after a bracket multiplies everything inside the bracket: Ca(NO₃)₂ contains 1 Ca, 2 N and 6 O. Brackets are needed only when a group of atoms that travels together (a compound ion such as NO₃⁻ or OH⁻) appears more than once; NaNO₃ needs no bracket, and writing Na(NO₃) is redundant. A large number written *in front of* a formula in an equation is a **coefficient**; it multiplies the whole formula: 3H₂SO₄ means 6 H, 3 S and 12 O. The raised dot in a hydrated salt such as CuSO₄·5H₂O means \"together with\": one formula unit of copper(II) sulfate is combined with five water molecules, so the unit contains 1 Cu, 1 S, 9 O and 10 H. The dot is not a multiplication sign. The formula of a compound never carries the charges of its ions: magnesium chloride is MgCl₂, not Mg²⁺Cl₂⁻.\n\n## D. Molecular formula: what is in one molecule\n\n[P4] A substance made of separate molecules has a definite particle, so it can be described exactly. The **molecular formula** gives the number and type of each atom in one molecule: ethane is C₂H₆, glucose is C₆H₁₂O₆, ammonia is NH₃. To deduce a molecular formula from a model, a ball-and-stick picture, a displayed formula or a structural formula, count each kind of atom, including every atom hidden inside a group (CH₃, OH, COOH). For a displayed formula it helps to tick each atom as it is counted. Organic molecular formulae are written C first, then H, then the other elements (C₂H₆O for ethanol). A structural formula such as CH₃CH₂OH carries the same atoms as C₂H₆O; the molecular formula adds them up and loses the arrangement, which is why isomers can share one molecular formula.\n\n## E. Empirical formula: the simplest ratio\n\n[P5] Many substances have no molecule to count. An ionic lattice or a giant covalent structure is the same pattern repeated an indefinite number of times, so the only thing that can be stated is the *ratio* of the particles. The **empirical formula** is the simplest whole-number ratio of the different atoms or ions in a compound. For ionic compounds (NaCl, MgO, Al₂O₃) and giant covalent substances (SiO₂) the formula written is always the empirical formula. For molecular substances the empirical formula is found from the molecular formula by dividing every subscript by their highest common factor: C₂H₆ → CH₃; C₆H₁₂O₆ → CH₂O; H₂O₂ → HO; C₄H₈O₂ → C₂H₄O. If the highest common factor is 1, the two formulae are identical (H₂O, CH₄, C₃H₈, CO₂). The same reasoning explains why a polymer has no fixed molecular formula: its chains are of indefinite and varying length, so only its repeat unit (and an empirical formula) can be written. Finding an empirical formula from masses or percentages, and a molecular formula from an empirical formula plus Mr, are calculations; they are treated in chapter 5.\n\n## F. Where ionic charges come from\n\n[P6] An ion is an atom or group of atoms that has gained or lost electrons. For main-group elements the charge follows from the electronic configuration: atoms lose or gain the fewest electrons that leave a full outer shell. So Group I metals form 1+ ions (Na⁺, K⁺, Li⁺), Group II form 2+ (Mg²⁺, Ca²⁺, Ba²⁺), aluminium in Group III forms Al³⁺; non-metals in Group V, VI and VII gain 3, 2 or 1 electrons (N³⁻, O²⁻, S²⁻, F⁻, Cl⁻, Br⁻, I⁻). Hydrogen forms H⁺. Group IV and Group VIII elements do not normally form simple ions. Transition elements can form more than one ion, so their names carry a **Roman numeral** that gives the oxidation number, which for a simple metal ion equals its charge: iron(II) is Fe²⁺, iron(III) is Fe³⁺, copper(II) is Cu²⁺. The numeral is a positive number only; it says nothing about the other ion. Zinc (Zn²⁺) and silver (Ag⁺) have one common ion and usually carry no numeral.\n\n[P7] Some ions are groups of atoms covalently bonded together that carry an overall charge: the **compound ions**. They must be learned because they cannot be deduced from a group number: ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, carbonate CO₃²⁻, sulfate SO₄²⁻, hydrogencarbonate HCO₃⁻, phosphate PO₄³⁻, and the organic anion ethanoate CH₃COO⁻ (in general, a carboxylic acid RCOOH gives the anion RCOO⁻). Names give clues: an ending *-ide* means a single element (or hydroxide), *-ate* means the ion contains oxygen. Sulfide S²⁻ and sulfate SO₄²⁻ are different ions; confusing them is a recorded examiner complaint. The charge of a compound ion belongs to the whole group, which is why the group goes in a bracket when there is more than one of it.\n\n## G. Formulae of ionic compounds from charges\n\n[P8] An ionic compound is electrically neutral, so its formula is the smallest whole-number ratio of ions in which the positive and negative charges cancel. The method: write both ions with their charges; find the lowest common multiple of the two charge sizes; take enough of each ion to reach it. Sodium oxide: Na⁺ and O²⁻; LCM 2; two Na⁺ per O²⁻ → Na₂O. Aluminium sulfate: Al³⁺ and SO₄²⁻; LCM 6; two Al³⁺ and three SO₄²⁻ → Al₂(SO₄)₃. Ammonium carbonate: NH₄⁺ and CO₃²⁻ → (NH₄)₂CO₃. Iron(III) hydroxide: Fe³⁺ and OH⁻ → Fe(OH)₃. The \"swap the charge numbers\" shortcut gives the same answer because it builds the LCM automatically, but it must be followed by simplifying (Mg²⁺ and O²⁻ swap to Mg₂O₂, which simplifies to MgO). The method works for unfamiliar ions too: given a chromate ion CrO₄²⁻, potassium chromate must be K₂CrO₄ and silver chromate Ag₂CrO₄; given phosphate PO₄³⁻, calcium phosphate is Ca₃(PO₄)₂. If a compound contains more than two kinds of ion, every positive charge is added and every negative charge is added, and the two totals must be equal.\n\n[P9] The same neutrality argument runs backwards. From a formula, the charge on an unknown ion follows from the known one: in Cr₂(SO₄)₃ three sulfate ions carry 6−, so two chromium ions carry 6+ and each is Cr³⁺. From a salt formula, the anion can be read off: in Na₂SeO₃ two Na⁺ balance one SeO₃²⁻, and the parent acid (which supplies H⁺ in place of Na⁺) is H₂SeO₃. A formula can also be deduced from a **model or diagram** of a lattice or molecule by counting the relative numbers of each particle: a lattice diagram with equal numbers of + and − ions of single charge represents a 1 : 1 compound such as KBr, while one with twice as many anions as cations represents a compound like MgCl₂. If the charges are shown, neutrality is a check on the count. A harder version names a mixed-ion solid: if it contains only Co²⁺, Co³⁺ and O²⁻, the formula must be neutral *and* need both cobalt ions; Co₃O₄ (2+ plus 3+ plus 3+ = 8+, four O²⁻ = 8−) fits, Co₂O₃ does not.\n\n## H. Formulae to know by heart\n\n[P10] Statement 3.1.1 asks for the formulae of the substances named anywhere in the syllabus. Most follow from [P6]–[P8], but a working set must be instant: H₂O, CO₂, CO, NH₃, CH₄, SO₂, SO₃, NO₂, NO; the acids HCl, H₂SO₄, HNO₃, CH₃COOH; NaOH, Ca(OH)₂, CaCO₃, CaO, Fe₂O₃, Al₂O₃, SiO₂, CuO, ZnCl₂, Cu(NO₃)₂, (NH₄)₂SO₄; and the two hydrated salts named in the syllabus, CuSO₄·5H₂O (blue) and CoCl₂·6H₂O (pink). When a formula is asked for, a name earns nothing; when a formula is printed in the question, copy it exactly — examiner reports repeatedly note candidates changing a formula they were given.\n\n## I. Equations: from words to balanced symbols\n\n[P11] A **word equation** names reactants and products: magnesium + hydrochloric acid → magnesium chloride + hydrogen. It records what happens but not in what proportions. A **symbol equation** replaces names with formulae and must then obey conservation of atoms: the same number of each kind of atom on each side. Writing the correct formulae comes first and is a separate mark in every Paper 4 equation item (the mark scheme's M1 is usually \"correct species\" or \"correct product\"); balancing is the second mark. A word equation is never accepted where a \"chemical equation\" or \"symbol equation\" is asked for.\n\n[P12] **Balancing changes coefficients, never subscripts.** Changing H₂O to H₂O₂ to \"fix\" the oxygen count produces a different substance, hydrogen peroxide, and the equation then describes a different reaction. A coefficient only changes how many particles of the same substance take part. A dependable procedure: (1) write correct formulae for everything; (2) balance an element that appears in only one substance on each side, starting with the formula containing the most atoms; (3) balance elements that appear as the free element (O₂, H₂, Cl₂, metals) last, because their coefficient changes only that one element; (4) treat a compound ion that survives unchanged (SO₄, NO₃) as a single unit; (5) recount every element, and reduce coefficients to the smallest whole numbers. Example: Al + O₂ → Al₂O₃. Oxygen needs a common multiple of 2 and 3, so 3O₂ and 2Al₂O₃; then 4Al: 4Al + 3O₂ → 2Al₂O₃. Never balance by writing Al₂ or 2Al₂ for the metal, or by inserting extra species such as H or O atoms that the reaction does not contain.\n\n[P13] **Combustion and \"odd oxygen\".** For a hydrocarbon or other C–H–O compound burning completely, balance C (into CO₂), then H (into H₂O), then count the oxygen atoms needed on the right and supply them as O₂. If that number is odd, double every coefficient. Butane: C₄H₁₀ + O₂ → 4CO₂ + 5H₂O needs 13 O atoms, i.e. 6½ O₂; doubling gives 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. If the fuel contains oxygen, subtract its oxygen atoms before counting O₂. The same atom-counting identifies an unknown product: if C₁₀H₂₂ cracks into C₂H₄ and one other molecule, the other must be C₈H₁₈, because all ten carbons and twenty-two hydrogens must be accounted for. Reversible reactions are written with ⇌ instead of →; the balancing rules do not change.\n\n## J. State symbols\n\n[P14] State symbols show the physical state of each substance under the conditions of the reaction: **(s)** solid, **(l)** liquid, **(g)** gas, **(aq)** aqueous, i.e. dissolved in water. Aqueous is not a synonym for liquid: dilute hydrochloric acid is HCl(aq), because hydrogen chloride is dissolved in water, while water itself is H₂O(l) and is never (aq). Rules that cover nearly every examined case: a metal, a metal oxide, a carbonate added as a powder and any precipitate are (s); acids, alkalis and soluble salts in solution are (aq); water formed in a reaction at room temperature is (l); gases at room temperature and pressure (H₂, O₂, CO₂, Cl₂, NH₃, SO₂) are (g); a halogen produced in solution, such as bromine in a displacement, is (aq). The state depends on the conditions given: water from a combustion at high temperature is H₂O(g), and a liquid product like silicon(IV) chloride at r.t.p. is (l). State symbols go in brackets, lower case, after each formula; when a question says \"include state symbols\", every species needs one.\n\n## K. Deducing an equation from information\n\n[P15] Supplement statement 3.1.8 asks for the equation of a reaction the student may never have seen, from information supplied. The move is always the same: (1) list the reactants and products the question names, and write their formulae (using any formula printed in the question exactly); (2) if a product is not named, deduce it from a reaction pattern — acid + metal → salt + hydrogen; acid + base (oxide or hydroxide) → salt + water; acid + carbonate → salt + water + carbon dioxide; metal + water → hydroxide + hydrogen (or oxide + hydrogen with steam); element + oxygen → oxide; a more reactive metal or halogen displaces a less reactive one from its compound; thermal decomposition of a carbonate → oxide + carbon dioxide; complete combustion → carbon dioxide + water — or from the atom count, as in [P13]; (3) balance; (4) add state symbols if asked. In a typical unfamiliar case a metal oxide reacts with sodium hydroxide to give water and a salt of a stated anion; the salt's formula comes from charges ([P8]) and the rest is balancing. Examiner reports show the commonest failure is a wrong product formula (K₂O for potassium hydroxide, ZnCl for zinc chloride), not the balancing.\n\n## L. Ionic equations\n\n[P16] An ionic compound dissolved in water is not a set of molecules but a collection of separate, mobile ions; aqueous sodium chloride is Na⁺(aq) and Cl⁻(aq). When two such solutions are mixed and a reaction occurs, only some of the ions change. The ones that are present unchanged before and after are **spectator ions**, and an **ionic equation** leaves them out, showing only the particles that actually react. The procedure: (1) write the full balanced symbol equation with state symbols; (2) split every aqueous ionic substance (soluble salts, strong acids, alkalis) into its ions; (3) do *not* split solids, precipitates, water, gases, covalent molecules such as halogens, or metals; (4) cross out ions that appear identically on both sides; (5) check that the result balances in atoms **and** in total charge. Mixing silver nitrate and sodium chloride solutions: Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq), leaving Ag⁺(aq) + Cl⁻(aq) → AgCl(s).\n\n[P17] Three families cover almost every ionic equation examined. **Precipitation**: two aqueous ions on the left, one solid on the right, state symbols always (aq) + (aq) → (s); a shortcut is to take the precipitate's formula and split it into its ions, so PbI₂ comes from Pb²⁺(aq) + 2I⁻(aq) and Cu(OH)₂ from Cu²⁺(aq) + 2OH⁻(aq). **Neutralisation** of any acid by any alkali reduces to H⁺(aq) + OH⁻(aq) → H₂O(l), which is why every such reaction has the same essential change. **Displacement**: a metal and a metal ion, Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s), or a halogen molecule and a halide ion, Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq); here the sodium or potassium ions are the spectators, and the halogens stay as diatomic molecules because they are covalent. In every case the charge check is decisive: 2Al(s) + 3Cu²⁺(aq) → 2Al³⁺(aq) + 3Cu(s) has 6+ on each side, while Al + Cu²⁺ → Al³⁺ + Cu balances atoms but not charge. An ionic half-equation (Cu²⁺ + 2e⁻ → Cu) is a different object: it shows one electrode process with electrons written explicitly, and belongs to electrolysis and redox.\n\n## M. Conventions and bridges\n\n[P18] Marking conventions for this chapter are stable across the archive. Formula items are one mark, formula only; a name, a formula with charges left in, a wrong case (CL, Co for CO) or a subscript written full size is not credited. Equation items are worth 1–3 marks: M1 for the correct species (often just the key product), M2 for the fully balanced equation, M3 for state symbols when asked; multiples of a correct equation and fractions such as ½O₂ are usually accepted, but an equation with extra species, charges in compound formulae or the reactants on the wrong side is not. Ionic precipitation equations earn M1 for the precipitate as the only product, M2 for exactly the right two ions as the only reactants, M3 for (aq) + (aq) → (s). Bridges: chapter 5 reads balanced equations as mole ratios and calculates formulae from data; electrolysis (section 4) builds half-equations on [P16]–[P17]; redox (section 6) explains the Roman numeral; acids and salts (section 7) supply the reaction patterns and solubility rules used in [P15]–[P17]; organic chemistry (section 11) supplies the molecules to which [P4], [P5] and [P13] are applied.\n\n## Coverage matrix\n\n| Syllabus statement | Wording (abridged, official) | Tier | Paragraphs |\n|---|---|---|---|\n| 3.1.1 | State the formulae of the elements and compounds named in the subject content | Core | P2, P3, P7, P10 |\n| 3.1.2 | Define the molecular formula of a compound as the number and type of different atoms in one molecule | Core | P4, P5 |\n| 3.1.3 | Deduce the formula of a simple compound from the relative numbers of atoms present in a model or a diagrammatic representation | Core | P4, P9 |\n| 3.1.4 | Construct word equations and symbol equations to show how reactants form products, including state symbols | Core | P11, P12, P13, P14 |\n| 3.1.5 | Define the empirical formula of a compound as the simplest whole number ratio of the different atoms or ions in a compound | Supplement | P5 |\n| 3.1.6 | Deduce the formula of an ionic compound from the relative numbers of the ions present in a model or a diagrammatic representation or from the charges on the ions | Supplement | P6, P7, P8, P9 |\n| 3.1.7 | Construct symbol equations with state symbols, including ionic equations | Supplement | P14, P16, P17 |\n| 3.1.8 | Deduce the symbol equation with state symbols for a chemical reaction, given relevant information | Supplement | P13, P15 |\n| — bridge, no statement | The two conservation laws behind every rule | — | P1 |\n| — bridge, no statement | Marking conventions; links to chapter 5, sections 4, 6, 7, 11 | — | P18 |\n\nGaps: none of 3.1.1–3.1.8 lacks a paragraph. Scope notes: ion charges from group number (P6) formally belong to section 2 (ionic bonding) but are the prerequisite for 3.1.6 and are kept as a bridge; the meaning of the Roman numeral (P6) is formally 6.4.1 but is needed to write formulae from names. Not covered on purpose: empirical formula from composition data and molecular formula from Mr (chapter 5, 3.3.7); ionic half-equations (section 4); general formulae of homologous series and displayed formulae (section 11).\n", "atoms": [{"id": "0620-3.1-A01", "type": "concept", "statement": "Atoms are conserved in a chemical reaction: they are regrouped, never created, destroyed or turned into another element.", "why": "Reactions rearrange bonds and electrons; nuclei are untouched.", "prerequisites": [], "tier": "core", "paragraphs": "P1"}, {"id": "0620-3.1-A02", "type": "concept", "statement": "Charge is conserved: the total charge on each side of an equation is the same.", "why": "Electrons move between particles but are not created or destroyed.", "prerequisites": ["0620-3.1-A01"], "tier": "supplement", "paragraphs": "P1"}, {"id": "0620-3.1-A03", "type": "fact", "statement": "Element symbols start with a capital letter; a second letter is lower case, and case carries meaning (Co is cobalt, CO is carbon monoxide).", "why": "Symbols are a shared international code.", "prerequisites": [], "tier": "core", "paragraphs": "P2"}, {"id": "0620-3.1-A04", "type": "fact", "statement": "Seven elements exist as diatomic molecules and are written H2, N2, O2, F2, Cl2, Br2, I2 whenever they appear as the element.", "why": "Their atoms pair up by a covalent bond to fill outer shells.", "prerequisites": ["0620-3.1-A03"], "tier": "core", "paragraphs": "P2"}, {"id": "0620-3.1-A05", "type": "fact", "statement": "Other elements are written as single atoms in equations: metals, carbon, silicon, sulfur and the noble gases.", "why": "Giant structures and monatomic gases have no small molecule to count.", "prerequisites": ["0620-3.1-A03"], "tier": "core", "paragraphs": "P2"}, {"id": "0620-3.1-A06", "type": "representation", "statement": "A subscript counts the atom (or bracketed group) immediately before it: H2SO4 has 2 H, 1 S, 4 O.", "why": "A formula is a count of atoms.", "prerequisites": ["0620-3.1-A03"], "tier": "core", "paragraphs": "P3"}, {"id": "0620-3.1-A07", "type": "representation", "statement": "A subscript after a bracket multiplies everything inside; brackets are used only when a compound ion appears more than once (Ca(NO3)2, but NaNO3).", "why": "The compound ion travels as one unit.", "prerequisites": ["0620-3.1-A06"], "tier": "supplement", "paragraphs": "P3"}, {"id": "0620-3.1-A08", "type": "representation", "statement": "A coefficient in front of a formula multiplies the whole formula: 3H2SO4 contains 6 H, 3 S, 12 O.", "why": "The coefficient counts whole formula units.", "prerequisites": ["0620-3.1-A06"], "tier": "core", "paragraphs": "P3"}, {"id": "0620-3.1-A09", "type": "representation", "statement": "The raised dot in a hydrated salt (CuSO4.5H2O) means together with: whole water molecules are added, not multiplied.", "why": "Water of crystallisation is part of the crystal formula unit.", "prerequisites": ["0620-3.1-A06"], "tier": "core", "paragraphs": "P3"}, {"id": "0620-3.1-A10", "type": "fact", "statement": "A compound formula never shows the ion charges: MgCl2, not Mg2+Cl2-.", "why": "The charges cancel in the neutral compound; they are shown only on separate ions.", "prerequisites": ["0620-3.1-A06"], "tier": "supplement", "paragraphs": "P3"}, {"id": "0620-3.1-A11", "type": "fact", "statement": "Molecular formula: the number and type of different atoms in one molecule.", "why": "A molecular substance has a definite particle that can be counted.", "prerequisites": ["0620-3.1-A06"], "tier": "core", "paragraphs": "P4"}, {"id": "0620-3.1-A12", "type": "procedure", "statement": "Deduce a molecular formula from a model, displayed or structural formula by counting each kind of atom, including atoms inside groups (CH3, OH, COOH).", "why": "The formula is just the atom count of the drawn particle.", "prerequisites": ["0620-3.1-A11"], "tier": "core", "paragraphs": "P4"}, {"id": "0620-3.1-A13", "type": "concept", "statement": "A molecular formula loses the arrangement of atoms, so different compounds (isomers) can share one molecular formula; organic formulae are written C, then H, then other elements.", "why": "Counting atoms discards how they are joined.", "prerequisites": ["0620-3.1-A11"], "tier": "core", "paragraphs": "P4"}, {"id": "0620-3.1-A14", "type": "fact", "statement": "Empirical formula: the simplest whole-number ratio of the different atoms or ions in a compound.", "why": "A ratio is all that can be stated when there is no fixed particle size.", "prerequisites": ["0620-3.1-A11"], "tier": "supplement", "paragraphs": "P5"}, {"id": "0620-3.1-A15", "type": "concept", "statement": "Ionic lattices and giant covalent substances (NaCl, SiO2) have only an empirical formula, because there is no separate molecule.", "why": "A giant structure repeats indefinitely; only the ratio is fixed.", "prerequisites": ["0620-3.1-A14"], "tier": "supplement", "paragraphs": "P5"}, {"id": "0620-3.1-A16", "type": "procedure", "statement": "Find an empirical formula from a molecular formula by dividing every subscript by their highest common factor; if it is 1 the two formulae are the same.", "why": "Dividing keeps the ratio and makes it simplest.", "prerequisites": ["0620-3.1-A14", "0620-3.1-A12"], "tier": "supplement", "paragraphs": "P5"}, {"id": "0620-3.1-A17", "type": "concept", "statement": "A polymer has no single molecular formula because its chains are of indefinite and varying length; only the repeat unit can be written.", "why": "There is no one molecule size to count.", "prerequisites": ["0620-3.1-A11", "0620-3.1-A14"], "tier": "supplement", "paragraphs": "P5"}, {"id": "0620-3.1-A18", "type": "relationship", "statement": "Main-group ion charge follows the group: I 1+, II 2+, Al 3+, V 3-, VI 2-, VII 1-.", "why": "Atoms gain or lose the fewest electrons that leave a full outer shell.", "prerequisites": [], "tier": "supplement", "paragraphs": "P6"}, {"id": "0620-3.1-A19", "type": "fact", "statement": "Group IV and Group VIII elements do not normally form simple ions; hydrogen forms H+.", "why": "Gaining or losing four electrons is too costly; noble gases already have full shells.", "prerequisites": ["0620-3.1-A18"], "tier": "supplement", "paragraphs": "P6"}, {"id": "0620-3.1-A20", "type": "fact", "statement": "A Roman numeral in a name gives the oxidation number of the element it follows; for a metal in a simple ionic compound this equals the charge on the metal ion, e.g. iron(III) = Fe3+, copper(II) = Cu2+ (but silicon(IV) oxide is not ionic).", "why": "Transition metals form more than one ion, so the name must say which.", "prerequisites": ["0620-3.1-A18"], "tier": "supplement", "paragraphs": "P6"}, {"id": "0620-3.1-A21", "type": "fact", "statement": "Zinc forms only Zn2+ and silver only Ag+, so their names usually carry no numeral.", "why": "One common ion needs no label.", "prerequisites": ["0620-3.1-A20"], "tier": "supplement", "paragraphs": "P6"}, {"id": "0620-3.1-A22", "type": "fact", "statement": "Compound ions to know: NH4+, OH-, NO3-, CO3 2-, SO4 2-, HCO3-, PO4 3-.", "why": "Their charges cannot be deduced from a group number, so they are learned.", "prerequisites": ["0620-3.1-A18"], "tier": "supplement", "paragraphs": "P7"}, {"id": "0620-3.1-A23", "type": "fact", "statement": "A carboxylic acid RCOOH gives the anion RCOO- (ethanoate CH3COO-), so its salts are written like other salts: (CH3COO)2Ca.", "why": "The acid loses H+ from the COOH group.", "prerequisites": ["0620-3.1-A22"], "tier": "supplement", "paragraphs": "P7"}, {"id": "0620-3.1-A24", "type": "relationship", "statement": "Name endings: -ide means a single element (or hydroxide), -ate means the ion contains oxygen; sulfide S2- and sulfate SO4 2- are different ions.", "why": "The ending encodes the composition.", "prerequisites": ["0620-3.1-A22"], "tier": "supplement", "paragraphs": "P7"}, {"id": "0620-3.1-A25", "type": "concept", "statement": "An ionic compound is neutral: total positive charge equals total negative charge.", "why": "Opposite charges must cancel in a stable compound.", "prerequisites": ["0620-3.1-A02", "0620-3.1-A18"], "tier": "supplement", "paragraphs": "P8"}, {"id": "0620-3.1-A26", "type": "procedure", "statement": "Formula from charges: write both ions, find the lowest common multiple of the charge sizes, take enough of each ion to reach it, bracket a compound ion used more than once (Al2(SO4)3).", "why": "This is the smallest neutral ratio.", "prerequisites": ["0620-3.1-A25", "0620-3.1-A07", "0620-3.1-A10"], "tier": "supplement", "paragraphs": "P8"}, {"id": "0620-3.1-A27", "type": "procedure", "statement": "The swap-the-charges shortcut must be followed by simplifying (Mg2O2 becomes MgO).", "why": "Swapping builds the LCM but not always the simplest ratio.", "prerequisites": ["0620-3.1-A26"], "tier": "supplement", "paragraphs": "P8"}, {"id": "0620-3.1-A28", "type": "procedure", "statement": "Apply the charge method to supplied or unfamiliar ions, and to compounds with three kinds of ion, by totalling all positive and all negative charges.", "why": "Neutrality does not depend on knowing the ion beforehand.", "prerequisites": ["0620-3.1-A26"], "tier": "supplement", "paragraphs": "P8"}, {"id": "0620-3.1-A29", "type": "procedure", "statement": "Inverse: find an unknown ion charge from a formula and the known ion (Cr2(SO4)3 gives Cr3+).", "why": "Neutrality fixes the missing charge.", "prerequisites": ["0620-3.1-A25"], "tier": "supplement", "paragraphs": "P9"}, {"id": "0620-3.1-A30", "type": "procedure", "statement": "Read the anion from a salt formula, and the parent acid by replacing the metal ions with H+ (Na2SO3 gives SO3 2- and H2SO3).", "why": "A salt is an acid with its H+ replaced by metal ions.", "prerequisites": ["0620-3.1-A29", "0620-3.1-A22"], "tier": "supplement", "paragraphs": "P9"}, {"id": "0620-3.1-A31", "type": "procedure", "statement": "Deduce a formula from a model or lattice diagram by counting the relative numbers of each particle, checking neutrality when charges are shown.", "why": "The ratio in the picture is the ratio in the formula.", "prerequisites": ["0620-3.1-A25", "0620-3.1-A12"], "tier": "supplement", "paragraphs": "P9"}, {"id": "0620-3.1-A32", "type": "procedure", "statement": "For a compound containing two ions of one element (mixed valence), choose the formula that is both neutral and contains every stated ion.", "why": "Neutrality alone admits formulas that lack one of the ions.", "prerequisites": ["0620-3.1-A25"], "tier": "supplement", "paragraphs": "P9"}, {"id": "0620-3.1-A33", "type": "fact", "statement": "Formulae of named molecular compounds: H2O, CO2, CO, NH3, CH4, SO2, SO3, NO2, NO.", "why": "Listed across the syllabus content.", "prerequisites": ["0620-3.1-A03", "0620-3.1-A04"], "tier": "core", "paragraphs": "P10"}, {"id": "0620-3.1-A34", "type": "fact", "statement": "Formulae of the common acids: HCl, H2SO4, HNO3, CH3COOH.", "why": "Each is H+ combined with a known anion.", "prerequisites": ["0620-3.1-A22", "0620-3.1-A33"], "tier": "core", "paragraphs": "P10"}, {"id": "0620-3.1-A35", "type": "fact", "statement": "Hydrated copper(II) sulfate is CuSO4.5H2O (blue) and hydrated cobalt(II) chloride is CoCl2.6H2O (pink).", "why": "Named in the syllabus as tests for water.", "prerequisites": ["0620-3.1-A09"], "tier": "core", "paragraphs": "P10"}, {"id": "0620-3.1-A36", "type": "technique", "statement": "When a formula is asked for, give a formula, not a name; when a formula is printed in the question, copy it exactly.", "why": "A name scores nothing for a formula mark, and the mark scheme assumes the given formula.", "prerequisites": ["0620-3.1-A33"], "tier": "core", "paragraphs": "P10; P18"}, {"id": "0620-3.1-A37", "type": "representation", "statement": "A word equation names reactants and products (reactants -> products); it is not accepted where a chemical or symbol equation is asked for.", "why": "Names carry identity but not proportions.", "prerequisites": [], "tier": "core", "paragraphs": "P11"}, {"id": "0620-3.1-A38", "type": "procedure", "statement": "First step of any symbol equation: write the correct formula of every species; this is usually the first mark (M1).", "why": "Balancing a wrong formula balances the wrong reaction.", "prerequisites": ["0620-3.1-A33", "0620-3.1-A26", "0620-3.1-A04"], "tier": "core", "paragraphs": "P11"}, {"id": "0620-3.1-A39", "type": "concept", "statement": "Balance by changing coefficients only, never subscripts or formulae.", "why": "A changed subscript is a different substance (H2O2 is not H2O).", "prerequisites": ["0620-3.1-A01", "0620-3.1-A06"], "tier": "core", "paragraphs": "P12"}, {"id": "0620-3.1-A40", "type": "procedure", "statement": "Balancing order: an element found in one substance each side first (largest formula first), compound ions as units, free elements last; recount everything and use the smallest whole numbers.", "why": "Free elements change only their own count, so they mop up at the end.", "prerequisites": ["0620-3.1-A39", "0620-3.1-A08"], "tier": "core", "paragraphs": "P12"}, {"id": "0620-3.1-A41", "type": "procedure", "statement": "Combustion: balance C into CO2, then H into H2O, count the O needed, supply it as O2 (subtract any O in the fuel); if the O2 count is odd or half, double all coefficients.", "why": "Oxygen is the only free element and is fixed last.", "prerequisites": ["0620-3.1-A40", "0620-3.1-A49"], "tier": "core", "paragraphs": "P13"}, {"id": "0620-3.1-A42", "type": "procedure", "statement": "Deduce an unknown formula in an equation from the atom count (missing product; cracking products from a given ratio).", "why": "Every atom on the left must appear on the right.", "prerequisites": ["0620-3.1-A01", "0620-3.1-A06"], "tier": "core", "paragraphs": "P13"}, {"id": "0620-3.1-A43", "type": "representation", "statement": "A reversible reaction is written with a double arrow; balancing rules are unchanged.", "why": "The same atoms are conserved in both directions.", "prerequisites": ["0620-3.1-A40"], "tier": "core", "paragraphs": "P13"}, {"id": "0620-3.1-A44", "type": "fact", "statement": "State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water; aqueous is not the same as liquid.", "why": "An equation should show the physical form of each substance.", "prerequisites": [], "tier": "core", "paragraphs": "P14"}, {"id": "0620-3.1-A45", "type": "relationship", "statement": "Assigning states: metals, oxides, powdered carbonates and precipitates (s); acids, alkalis and soluble salts in solution (aq); water (l); gases at r.t.p. (g); a halogen formed in solution (aq).", "why": "Each follows from the substance and whether it is dissolved.", "prerequisites": ["0620-3.1-A44"], "tier": "core", "paragraphs": "P14"}, {"id": "0620-3.1-A46", "type": "relationship", "statement": "The state depends on the conditions stated: water from a hot combustion is (g); a liquid product at r.t.p. is (l).", "why": "State symbols describe the substance under the reaction conditions.", "prerequisites": ["0620-3.1-A45"], "tier": "supplement", "paragraphs": "P14"}, {"id": "0620-3.1-A47", "type": "procedure", "statement": "Deduce an equation from information: list the named species, write formulae (copy given ones), deduce unnamed products from a reaction pattern or the atom count, balance, add state symbols if asked.", "why": "The equation is built from conservation plus a known pattern.", "prerequisites": ["0620-3.1-A38", "0620-3.1-A40", "0620-3.1-A45", "0620-3.1-A48"], "tier": "supplement", "paragraphs": "P15"}, {"id": "0620-3.1-A48", "type": "relationship", "statement": "Acid patterns: acid + metal -> salt + hydrogen; acid + base -> salt + water; acid + carbonate -> salt + water + carbon dioxide; reactive metal + water -> hydroxide + hydrogen.", "why": "Bridge from section 7 (acids) and 9 (metals).", "prerequisites": [], "tier": "core", "paragraphs": "P15"}, {"id": "0620-3.1-A49", "type": "relationship", "statement": "Other patterns: element + oxygen -> oxide; a more reactive metal or halogen displaces a less reactive one; carbonate on heating -> oxide + carbon dioxide; complete combustion -> carbon dioxide + water.", "why": "Bridge from sections 9, 10 and 11.", "prerequisites": [], "tier": "core", "paragraphs": "P15"}, {"id": "0620-3.1-A50", "type": "concept", "statement": "A dissolved ionic compound exists as separate ions; ions present unchanged before and after a reaction are spectator ions.", "why": "Only the particles that change belong in a description of the change.", "prerequisites": ["0620-3.1-A02", "0620-3.1-A44"], "tier": "supplement", "paragraphs": "P16"}, {"id": "0620-3.1-A51", "type": "procedure", "statement": "Ionic equation recipe: full equation; split aqueous ionic substances into ions; do not split solids, water, gases, covalent molecules or metals; cancel spectators; check atoms and charge.", "why": "Leaves exactly the particles that react.", "prerequisites": ["0620-3.1-A50", "0620-3.1-A40"], "tier": "supplement", "paragraphs": "P16"}, {"id": "0620-3.1-A52", "type": "representation", "statement": "Precipitation ionic equation: two aqueous ions -> one solid, (aq) + (aq) -> (s); take the precipitate formula and split it into its ions to get the left-hand side (Pb2+ + 2I- -> PbI2).", "why": "The precipitate is the only new substance; its ions came from the two solutions.", "prerequisites": ["0620-3.1-A51", "0620-3.1-A26"], "tier": "supplement", "paragraphs": "P17"}, {"id": "0620-3.1-A53", "type": "fact", "statement": "Every acid-alkali neutralisation has the ionic equation H+(aq) + OH-(aq) -> H2O(l).", "why": "The salt ions are spectators in every case.", "prerequisites": ["0620-3.1-A51"], "tier": "supplement", "paragraphs": "P17"}, {"id": "0620-3.1-A54", "type": "representation", "statement": "Displacement ionic equations: metal + metal ion (Fe + Cu2+ -> Fe2+ + Cu) or halogen molecule + halide ion (Cl2 + 2I- -> 2Cl- + I2); the halogens stay diatomic and the Group I cations are spectators.", "why": "Covalent halogen molecules are not split; only the halide ions change.", "prerequisites": ["0620-3.1-A51", "0620-3.1-A04"], "tier": "supplement", "paragraphs": "P17"}, {"id": "0620-3.1-A55", "type": "procedure", "statement": "Balance an ionic equation for charge as well as atoms (2Al + 3Cu2+ -> 2Al3+ + 3Cu has 6+ on each side).", "why": "Charge is conserved.", "prerequisites": ["0620-3.1-A02", "0620-3.1-A51"], "tier": "supplement", "paragraphs": "P17"}, {"id": "0620-3.1-A56", "type": "concept", "statement": "An ionic half-equation is a different object: one electrode process with electrons written explicitly; it belongs to electrolysis and redox.", "why": "It shows only half of a transfer.", "prerequisites": ["0620-3.1-A51"], "tier": "supplement", "paragraphs": "P17"}, {"id": "0620-3.1-A57", "type": "technique", "statement": "Marking pattern: formula items one mark, exact; equations M1 species, M2 balanced, M3 state symbols; precipitation ionic M1 precipitate only, M2 its two ions only, M3 (aq)+(aq)->(s); extra species lose M1.", "why": "Fixed conventions across 2023-25 mark schemes.", "prerequisites": ["0620-3.1-A38"], "tier": "core", "paragraphs": "P18"}], "misconceptions": [{"id": "0620-3.1-M01", "wrong": "the free element can be written as a single atom (O, H, N, Cl, Br) in an equation.", "correct": "write the seven diatomic elements as H2, N2, O2, F2, Cl2, Br2, I2.", "atoms": ["0620-3.1-A04"]}, {"id": "0620-3.1-M02", "wrong": "an equation can be balanced by changing a formula or writing Al2, Fe2, Zn2, Hg2.", "correct": "change only coefficients; a changed formula is a different substance.", "atoms": ["0620-3.1-A39"]}, {"id": "0620-3.1-M03", "wrong": "once the formulae are right the equation is finished.", "correct": "count every element on both sides and balance (a separate mark).", "atoms": ["0620-3.1-A40"]}, {"id": "0620-3.1-M04", "wrong": "a coefficient multiplies only the first element of the formula after it.", "correct": "a coefficient multiplies every atom in the formula (4FeS2 is 4 Fe and 8 S).", "atoms": ["0620-3.1-A08"]}, {"id": "0620-3.1-M05", "wrong": "a salt formula can be written without balancing the charges (ZnCl, Pb2SO4, Pb(SO4)2, AlO).", "correct": "use ion charges and the lowest common multiple so the compound is neutral.", "atoms": ["0620-3.1-A26", "0620-3.1-A25"]}, {"id": "0620-3.1-M06", "wrong": "compound ions can be guessed: nitrate with a 2- charge, ammonium as NH3, sulfide and sulfate interchangeable, nitric acid as H2NO4.", "correct": "learn NO3-, NH4+, SO4 2- (sulfate) vs S2- (sulfide), HNO3.", "atoms": ["0620-3.1-A22", "0620-3.1-A24", "0620-3.1-A34"]}, {"id": "0620-3.1-M07", "wrong": "the ion charges belong inside the compound formula (Mg2+F-2), or fluorine is Fl.", "correct": "compound formulae carry no charges; symbols must be exact.", "atoms": ["0620-3.1-A10", "0620-3.1-A03"]}, {"id": "0620-3.1-M08", "wrong": "the Roman numeral is the charge of the compound, of the non-metal, a valency, or just the number 2.", "correct": "the numeral is the positive oxidation number (ion charge) of the element it follows (for a metal in a simple ionic compound, its ion charge).", "atoms": ["0620-3.1-A20"]}, {"id": "0620-3.1-M09", "wrong": "the group numbers of two elements are the subscripts of their compound (Group V + Group VII gives P5Q7).", "correct": "convert group numbers to ion charges first, then make the compound neutral.", "atoms": ["0620-3.1-A18", "0620-3.1-A26"]}, {"id": "0620-3.1-M10", "wrong": "any neutral formula fits a description of the ions present.", "correct": "the formula must be neutral and contain every stated ion.", "atoms": ["0620-3.1-A32"]}, {"id": "0620-3.1-M11", "wrong": "aqueous means liquid, so water is H2O(aq) and dilute acid is HCl(l).", "correct": "(aq) means dissolved in water; water itself is (l); a dilute acid is (aq).", "atoms": ["0620-3.1-A44", "0620-3.1-A45"]}, {"id": "0620-3.1-M12", "wrong": "an ionic equation is the full symbol equation, or spectator ions may be left in (K+, Na+, NO3-).", "correct": "split aqueous ionic substances and cancel the ions that appear unchanged on both sides.", "atoms": ["0620-3.1-A50", "0620-3.1-A51"]}, {"id": "0620-3.1-M13", "wrong": "an ionic equation only needs the right ions, not the right numbers of them (Pb2+ + Cl- -> PbCl2).", "correct": "balance atoms and total charge: Pb2+ + 2Cl- -> PbCl2.", "atoms": ["0620-3.1-A55", "0620-3.1-A52"]}, {"id": "0620-3.1-M14", "wrong": "in a halogen displacement the halogen molecules carry charges, the ions swap sides, or the reaction is written as a precipitation or half-equation.", "correct": "halogens stay neutral diatomic molecules; the halide ion of the less reactive halogen is the reactant.", "atoms": ["0620-3.1-A54"]}, {"id": "0620-3.1-M15", "wrong": "the molecular formula is the empirical formula.", "correct": "divide the subscripts by their highest common factor.", "atoms": ["0620-3.1-A16"]}, {"id": "0620-3.1-M16", "wrong": "complete combustion gives carbon dioxide and hydrogen, or needs no oxygen on the left.", "correct": "complete combustion: fuel + O2 -> CO2 + H2O.", "atoms": ["0620-3.1-A49", "0620-3.1-A41"]}, {"id": "0620-3.1-M17", "wrong": "a formula printed in the question can be rewritten from memory.", "correct": "copy the given formula exactly; it is the formula the mark scheme uses.", "atoms": ["0620-3.1-A36"]}, {"id": "0620-3.1-M18", "wrong": "a name or word equation answers a request for a formula or chemical equation.", "correct": "a formula needs symbols; a chemical equation means a balanced symbol equation.", "atoms": ["0620-3.1-A36", "0620-3.1-A37"]}]}

data/sets/0417_ch04_q03.json

@@ -1 +1 @@
-{"id": "0417_ch04_q03", "chapterId": "0417_ch04", "subject": "ICT", "code": "0417", "chapter": "Networks and the effects of using them", "topicId": "t3", "topicTitle": "Internet, intranet and the cloud", "quizNo": 3, "quizCount": 7, "order": 3, "title": "Internet, intranet and the cloud", "kind": "topic", "intro": "Practise who may use the internet, an intranet and an extranet, and how cloud storage works.", "minutes": 12, "active": true, "questions": [{"id": "q15", "type": "mcq", "prompt": "A supermarket lets its suppliers log in with a username and password over the internet to check its stock levels. Its staff use a separate private site for policies and rotas. What is the suppliers' system?", "options": ["An extranet", "An intranet", "The internet", "A WLAN"], "answer": 0, "marks": 1, "mark_scheme": "A: extranet; an organisation's private network extended to authorised outsiders.", "explanation": "An extranet lets authorised outsiders (customers, suppliers, partners) into part of an organisation's private network. The staff-only site is the intranet. The internet is open to anyone, and a WLAN is a kind of network, not an access environment.", "atom_ids": ["0417-4-A27", "0417-4-A26"], "template_id": "T10", "misconception_id": null, "difficulty": 1}, {"id": "q16", "type": "mcq", "prompt": "Which description of cloud computing would earn the mark in an exam?", "options": ["Storing data and running software on remote servers in data centres run by a third-party provider, reached through the internet.", "Storing data on the internet.", "A way of backing up the photos on a phone.", "Storing data on a pen drive plugged into a computer that is connected to the internet."], "answer": 0, "marks": 1, "mark_scheme": "A: remote servers; data centres / third-party provider; accessed through the internet.", "explanation": "Examiners reject 'stored on the internet' as too vague: the internet is only the route to the provider's servers. Backup is one use of the cloud, not what the cloud is. A pen drive is local storage, wherever it is plugged in.", "atom_ids": ["0417-4-A30", "0417-4-M06"], "template_id": "T11", "misconception_id": "0417-4-M06", "difficulty": 1}, {"id": "q17", "type": "mcq", "prompt": "A car company keeps its new designs on a private cloud. What does \"private\" mean here?", "options": ["The cloud can be used by only one person.", "The cloud service is controlled by the company and access is restricted to its members.", "The designs are kept on the company's own laptops instead of on servers.", "The cloud cannot be reached through the internet at all."], "answer": 1, "marks": 1, "mark_scheme": "B: controlled by the organisation; restricted access.", "explanation": "Private refers to control and who may access, not to the number of users. Examiners report candidates thinking a private cloud is for one person (option A). The data is still on remote servers reached over a network.", "atom_ids": ["0417-4-A32", "0417-4-M05"], "template_id": "T11", "misconception_id": "0417-4-M05", "difficulty": 2}, {"id": "q18", "type": "multi", "prompt": "Select the TWO statements that are true of a company intranet but NOT true of the internet.", "options": ["It is owned by one organisation.", "Only members of the organisation can access it.", "It uses web pages and web browsers.", "Anyone with a connection can publish material on it.", "It can be used to share information and documents."], "answer": [0, 1], "marks": 2, "mark_scheme": "1 mark each for A and B; no marks if more than two options are chosen.", "explanation": "Ownership and restricted access are what make an intranet private. Web pages, browsers and sharing information are similarities with the internet; open publishing is true of the internet only. Reading 'intranet' as 'internet' is the commonest error in these questions.", "atom_ids": ["0417-4-A26", "0417-4-A29", "0417-4-A28"], "template_id": "T10", "misconception_id": null, "difficulty": 2}, {"id": "q19", "type": "multi", "prompt": "Select the THREE statements that are characteristics of cloud storage.", "options": ["It is paid for by subscription or by how much is used.", "Storage space can be increased or reduced when it is needed.", "Files can be reached from any device connected to the internet.", "It works without any internet connection.", "Each file is kept on only one server.", "The user must buy and maintain the servers."], "answer": [0, 1, 2], "marks": 2, "mark_scheme": "2 marks for exactly A, B and C; 1 mark for two of these with no wrong option; 0 otherwise.", "explanation": "Cloud storage is rented from a provider, grows or shrinks on demand and is reached over the internet from anywhere. It needs a connection, the provider keeps copies on several servers, and the provider (not the user) maintains the hardware.", "atom_ids": ["0417-4-A31", "0417-4-A30"], "template_id": "T11", "misconception_id": null, "difficulty": 2}, {"id": "q20", "type": "multi", "prompt": "A project team keeps its files on the cloud. Select the TWO statements that describe how the data is MANAGED (rather than how it is stored).", "options": ["Files are uploaded and saved on remote servers.", "Files are synced so that every device shows the latest version.", "Copies are backed up to other servers.", "The owner or an administrator controls who can open and edit each file.", "The servers are in data centres run by a third-party provider."], "answer": [1, 3], "marks": 2, "mark_scheme": "1 mark each for B and D; no marks if more than two options are chosen.", "explanation": "Mark schemes split cloud answers into 'stored' (uploaded, remote servers in data centres, backed up) and 'managed' (synced, access controlled, changes tracked, billed by use). Candidates often write only the stored half.", "atom_ids": ["0417-4-A34", "0417-4-A33"], "template_id": "T12", "misconception_id": null, "difficulty": 2}, {"id": "q21", "type": "multi", "prompt": "A student wants to move a project file from a college computer to a home computer without using the cloud. Select the THREE methods that would earn marks.", "options": ["Send it to herself as an email attachment", "Copy it onto a pen drive", "USB", "Copy it onto a portable SSD", "Save it on the college computer's hard disk"], "answer": [0, 1, 3], "marks": 2, "mark_scheme": "2 marks for exactly A, B and D; 1 mark for two of these with no wrong option; 0 otherwise.", "explanation": "Each correct method carries the file: a message or a portable storage device. 'USB' on its own is a port, not a method, so examiners do not credit it. Saving to the college hard disk leaves the file at college.", "atom_ids": ["0417-4-A38"], "template_id": "T13", "misconception_id": null, "difficulty": 2}, {"id": "q22", "type": "short", "prompt": "Meera is deciding whether to keep her coursework on cloud storage or on a pen drive. Give two advantages and two disadvantages of using cloud storage rather than the pen drive.", "answer": null, "marks": 4, "mark_scheme": "Advantages, max 2: can be accessed from anywhere / from many devices; easy to share with others; cannot be lost or damaged like a pen drive; automatic backup by the provider; storage can be expanded. Disadvantages, max 2: needs a stable internet connection (not just 'needs internet'); ongoing subscription cost; data held by a third party so loss of control; security risk such as hacking because copies are kept on many servers; data lost if the provider closes or the subscription lapses", "explanation": "Both sides are needed and each point must compare with the pen drive. 'Needs internet' is too vague for credit; 'needs a stable internet connection' scores.", "atom_ids": ["0417-4-A36", "0417-4-A37"], "template_id": "T13", "misconception_id": null, "difficulty": 2}], "atoms": {"0417-4-A27": "An extranet extends an organisation's intranet to authorised outsiders such as customers and suppliers, over the internet with a username and password.", "0417-4-A26": "An intranet is a private network using internet technologies, accessible only to members of one organisation behind logins and a firewall; used for internal news, policies, forms and documents.", "0417-4-A30": "Cloud computing stores data and runs software on remote servers in data centres run by a third-party provider and reached through the internet; uses include storage, backup, sharing, online applications and cloud gaming.", "0417-4-M06": "the cloud just means 'data stored on the internet', or is only for backing up a phone.", "0417-4-A32": "A public cloud is shared by many customers of a provider; a private cloud is used and completely controlled by one organisation, with access restricted to its members.", "0417-4-M05": "a private cloud is a cloud used by one person.", "0417-4-A29": "They differ in who may access (anyone / members / members plus authorised outsiders), ownership (nobody / an organisation), amount of information, and trust and security (an extranet adds outside logins).", "0417-4-A28": "Internet, intranet and extranet all use internet technologies and can hold web pages; the intranet and extranet both restrict access and sit behind security.", "0417-4-A31": "Cloud characteristics: rented by subscription or pay-per-use, capacity expands or shrinks on demand, accessible from any connected device anywhere, copies held on several servers.", "0417-4-A34": "How cloud data is managed: the provider maintains hardware, security and backups; the owner or administrator controls access; files are synced; changes are tracked and versions kept; billing follows usage.", "0417-4-A33": "How cloud data is stored: the user uploads it, it is saved on remote servers, and it is backed up to other servers.", "0417-4-A38": "Non-cloud ways to move files: email attachment, pen drive, portable HDD or SSD, rewritable CD or DVD (USB on its own is a port, not a method).", "0417-4-A36": "Advantages of cloud over local or portable storage: access anywhere on many devices, easy sharing, nothing physical to lose or damage, automatic backup, expandable and pay for what you use.", "0417-4-A37": "Disadvantages and issues of the cloud: needs a stable internet connection, ongoing subscription, loss of control to a third party (possibly under other laws), more copies kept longer increases breach risk, data lost if the provider closes or the subscription lapses."}}
+{"id": "0417_ch04_q03", "chapterId": "0417_ch04", "subject": "ICT", "code": "0417", "chapter": "Networks and the effects of using them", "topicId": "t3", "topicTitle": "Internet, intranet and the cloud", "quizNo": 3, "quizCount": 7, "order": 3, "title": "Internet, intranet and the cloud", "kind": "topic", "intro": "Practise who may use the internet, an intranet and an extranet, and how cloud storage works.", "minutes": 12, "active": true, "questions": [{"id": "q15", "type": "mcq", "prompt": "A supermarket lets its suppliers log in with a username and password over the internet to check its stock levels. Its staff use a separate private site for policies and rotas. What is the suppliers' system?", "options": ["An extranet", "An intranet", "The internet", "A WLAN"], "answer": 0, "marks": 1, "mark_scheme": "A: extranet; an organisation's private network extended to authorised outsiders.", "explanation": "An extranet lets authorised outsiders (customers, suppliers, partners) into part of an organisation's private network. The staff-only site is the intranet. The internet is open to anyone, and a WLAN is a kind of network, not an access environment.", "atom_ids": ["0417-4-A27", "0417-4-A26"], "template_id": "T10", "misconception_id": null, "difficulty": 1}, {"id": "q16", "type": "mcq", "prompt": "Which description of cloud computing would earn the mark in an exam?", "options": ["Storing data and running software on remote servers in data centres run by a third-party provider, reached through the internet.", "Storing data on the internet.", "A way of backing up the photos on a phone.", "Storing data on a pen drive plugged into a computer that is connected to the internet."], "answer": 0, "marks": 1, "mark_scheme": "A: remote servers; data centres / third-party provider; accessed through the internet.", "explanation": "Examiners reject 'stored on the internet' as too vague: the internet is only the route to the provider's servers. Backup is one use of the cloud, not what the cloud is. A pen drive is local storage, wherever it is plugged in.", "atom_ids": ["0417-4-A30", "0417-4-M06"], "template_id": "T11", "misconception_id": "0417-4-M06", "difficulty": 1}, {"id": "q17", "type": "mcq", "prompt": "A car company keeps its new designs on a private cloud. What does \"private\" mean here?", "options": ["The cloud can be used by only one person.", "The cloud service is controlled by the company and access is restricted to its members.", "The designs are kept on the company's own laptops instead of on servers.", "The cloud cannot be reached through the internet at all."], "answer": 1, "marks": 1, "mark_scheme": "B: controlled by the organisation; restricted access.", "explanation": "Private refers to control and who may access, not to the number of users. Examiners report candidates thinking a private cloud is for one person (option A). The data is still on remote servers reached over a network.", "atom_ids": ["0417-4-A32", "0417-4-M05"], "template_id": "T11", "misconception_id": "0417-4-M05", "difficulty": 2}, {"id": "q18", "type": "multi", "prompt": "Select the TWO statements that are true of a company intranet but NOT true of the internet.", "options": ["It is owned by one organisation.", "Only members of the organisation can access it.", "It uses web pages and web browsers.", "Anyone with a connection can publish material on it.", "It can be used to share information and documents."], "answer": [0, 1], "marks": 2, "mark_scheme": "1 mark each for A and B; no marks if more than two options are chosen.", "explanation": "Ownership and restricted access are what make an intranet private. Web pages, browsers and sharing information are similarities with the internet; open publishing is true of the internet only. Reading 'intranet' as 'internet' is the commonest error in these questions.", "atom_ids": ["0417-4-A26", "0417-4-A29", "0417-4-A28"], "template_id": "T10", "misconception_id": null, "difficulty": 2}, {"id": "q19", "type": "multi", "prompt": "Select the THREE statements that are characteristics of cloud storage.", "options": ["It is paid for by subscription or by how much is used.", "Storage space can be increased or reduced when it is needed.", "Files can be reached from any device connected to the internet.", "It works without any internet connection.", "Each file is kept on only one server.", "The user must buy and maintain the servers."], "answer": [0, 1, 2], "marks": 2, "mark_scheme": "2 marks for exactly A, B and C; 1 mark for two of these with no wrong option; 0 otherwise.", "explanation": "Cloud storage is rented from a provider, grows or shrinks on demand and is reached over the internet from anywhere. It needs a connection, the provider keeps copies on several servers, and the provider (not the user) maintains the hardware.", "atom_ids": ["0417-4-A31", "0417-4-A30"], "template_id": "T11", "misconception_id": null, "difficulty": 2}, {"id": "q20", "type": "multi", "prompt": "A project team keeps its files on the cloud. Select the TWO statements that describe how the data is MANAGED (rather than how it is stored).", "options": ["Files are uploaded and saved on remote servers.", "Files are synced so that every device shows the latest version.", "The file is copied from the user's device to the provider's servers.", "The owner or an administrator controls who can open and edit each file.", "The data is written to hard disks or SSDs in the provider's data centre."], "answer": [1, 3], "marks": 2, "mark_scheme": "1 mark each for B and D; no marks if more than two options are chosen.", "explanation": "Stored = uploaded and saved on remote servers (and copied to other servers). Managed = synced, access controlled, changes tracked, hardware maintained and backups run by the provider, billed by use. Candidates often write only the stored half.", "atom_ids": ["0417-4-A34", "0417-4-A33"], "template_id": "T12", "misconception_id": null, "difficulty": 2}, {"id": "q21", "type": "multi", "prompt": "A student wants to move a project file from a college computer to a home computer without using the cloud. Select the THREE methods that would earn marks.", "options": ["Send it to herself as an email attachment", "Copy it onto a pen drive", "USB", "Copy it onto a portable SSD", "Save it on the college computer's hard disk"], "answer": [0, 1, 3], "marks": 2, "mark_scheme": "2 marks for exactly A, B and D; 1 mark for two of these with no wrong option; 0 otherwise.", "explanation": "Each correct method carries the file: a message or a portable storage device. 'USB' on its own is a port, not a method, so examiners do not credit it. Saving to the college hard disk leaves the file at college.", "atom_ids": ["0417-4-A38"], "template_id": "T13", "misconception_id": null, "difficulty": 2}, {"id": "q22", "type": "short", "prompt": "Meera is deciding whether to keep her coursework on cloud storage or on a pen drive. Give two advantages and two disadvantages of using cloud storage rather than the pen drive.", "answer": null, "marks": 4, "mark_scheme": "Advantages, max 2: can be accessed from anywhere / from many devices; easy to share with others; cannot be lost or damaged like a pen drive; automatic backup by the provider; storage can be expanded. Disadvantages, max 2: needs a stable internet connection (not just 'needs internet'); ongoing subscription cost; data held by a third party so loss of control; security risk such as hacking because copies are kept on many servers; data lost if the provider closes or the subscription lapses", "explanation": "Both sides are needed and each point must compare with the pen drive. 'Needs internet' is too vague for credit; 'needs a stable internet connection' scores.", "atom_ids": ["0417-4-A36", "0417-4-A37"], "template_id": "T13", "misconception_id": null, "difficulty": 2}], "atoms": {"0417-4-A27": "An extranet extends an organisation's intranet to authorised outsiders such as customers and suppliers, over the internet with a username and password.", "0417-4-A26": "An intranet is a private network using internet technologies, accessible only to members of one organisation behind logins and a firewall; used for internal news, policies, forms and documents.", "0417-4-A30": "Cloud computing stores data and runs software on remote servers in data centres run by a third-party provider and reached through the internet; uses include storage, backup, sharing, online applications and cloud gaming.", "0417-4-M06": "the cloud just means 'data stored on the internet', or is only for backing up a phone.", "0417-4-A32": "A public cloud is shared by many customers of a provider; a private cloud is used and completely controlled by one organisation, with access restricted to its members.", "0417-4-M05": "a private cloud is a cloud used by one person.", "0417-4-A29": "They differ in who may access (anyone / members / members plus authorised outsiders), ownership (nobody / an organisation), amount of information, and trust and security (an extranet adds outside logins).", "0417-4-A28": "Internet, intranet and extranet all use internet technologies and can hold web pages; the intranet and extranet both restrict access and sit behind security.", "0417-4-A31": "Cloud characteristics: rented by subscription or pay-per-use, capacity expands or shrinks on demand, accessible from any connected device anywhere, copies held on several servers.", "0417-4-A34": "How cloud data is managed: the provider maintains hardware, security and backups; the owner or administrator controls access; files are synced; changes are tracked and versions kept; billing follows usage.", "0417-4-A33": "How cloud data is stored: the user uploads it, it is saved on remote servers, and it is backed up to other servers.", "0417-4-A38": "Non-cloud ways to move files: email attachment, pen drive, portable HDD or SSD, rewritable CD or DVD (USB on its own is a port, not a method).", "0417-4-A36": "Advantages of cloud over local or portable storage: access anywhere on many devices, easy sharing, nothing physical to lose or damage, automatic backup, expandable and pay for what you use.", "0417-4-A37": "Disadvantages and issues of the cloud: needs a stable internet connection, ongoing subscription, loss of control to a third party (possibly under other laws), more copies kept longer increases breach risk, data lost if the provider closes or the subscription lapses."}}

data/sets/0417_ch04_q06.json

@@ -1 +1 @@
-{"id": "0417_ch04_q06", "chapterId": "0417_ch04", "subject": "ICT", "code": "0417", "chapter": "Networks and the effects of using them", "topicId": "t6", "topicTitle": "Electronic conferencing", "quizNo": 6, "quizCount": 7, "order": 6, "title": "Electronic conferencing", "kind": "topic", "intro": "Practise the three kinds of electronic conference, what each needs, and when each is the better choice.", "minutes": 12, "active": true, "questions": [{"id": "q37", "type": "mcq", "prompt": "A teacher runs an online lesson in a web browser. Every student sees the same slides on screen and can answer polls and type questions in a chat box; cameras are optional. What is this type of electronic conferencing called?", "options": ["Web-conferencing", "Audio-conferencing", "Video-conferencing", "An extranet"], "answer": 0, "marks": 1, "mark_scheme": "A: web-conferencing (a webinar).", "explanation": "Web-conferencing runs in a browser or app and its point is the shared screen, with chat and polls. Audio-conferencing is voice only, and video-conferencing centres on moving pictures and sound. Use the generic term: brand names of apps earn no credit.", "atom_ids": ["0417-4-A68"], "template_id": "T21", "misconception_id": null, "difficulty": 1}, {"id": "q38", "type": "multi", "prompt": "A company replaces its monthly in-person meeting of managers from four countries with an electronic conference. Select the TWO benefits.", "options": ["No travel time, and no travel or hotel costs", "Meetings can be arranged at short notice", "Time-zone differences no longer matter", "No internet connection is needed", "Body language is easier to read than in person"], "answer": [0, 1], "marks": 2, "mark_scheme": "1 mark each for A and B; no marks if more than two options are chosen.", "explanation": "The network replaces the journey, so travel time and cost disappear and meetings can be called quickly. Time zones still matter, a stable connection is essential, and body language is harder to read, not easier.", "atom_ids": ["0417-4-A62", "0417-4-A63"], "template_id": "T23", "misconception_id": null, "difficulty": 1}, {"id": "q39", "type": "multi", "prompt": "Select the THREE items of SOFTWARE needed to hold a video-conference.", "options": ["CODEC", "Webcam", "Echo-cancellation software", "Hardware drivers", "Router", "Microphone"], "answer": [0, 2, 3], "marks": 2, "mark_scheme": "2 marks for exactly A, C and D; 1 mark for two of these with no wrong option; 0 otherwise.", "explanation": "The CODEC compresses and decompresses the audio and video, echo-cancellation stops the microphone picking up the speakers, and drivers make the webcam and microphone work. Webcam, router and microphone are hardware.", "atom_ids": ["0417-4-A65", "0417-4-A64"], "template_id": "T22", "misconception_id": null, "difficulty": 2}, {"id": "q40", "type": "mcq", "prompt": "Which statement of a disadvantage of video-conferencing would earn the mark?", "options": ["It needs a fast, stable internet connection; if the connection drops, the picture freezes or people are cut off.", "It needs the internet.", "It uses a computer.", "It is not very good."], "answer": 0, "marks": 1, "mark_scheme": "A: needs a fast / stable / reliable internet connection, with its consequence.", "explanation": "'Needs internet' is too vague to score: mark schemes ask for a stable or reliable connection. Points must be specific and, in a discussion, linked to a consequence.", "atom_ids": ["0417-4-A63", "0417-4-A70"], "template_id": "T23", "misconception_id": null, "difficulty": 2}, {"id": "q41", "type": "multi", "prompt": "Four engineers who know each other well hold a ten-minute check-in every day. Select the TWO advantages of using audio-conferencing rather than video-conferencing for this.", "options": ["It needs less bandwidth, so there is less freezing and lag.", "It needs less equipment: any telephone will do, with no webcam or screen.", "Everyone can see each other's body language.", "Drawings can be shown on screen.", "There are no time-zone problems."], "answer": [0, 1], "marks": 2, "mark_scheme": "1 mark each for A and B; no marks if more than two options are chosen.", "explanation": "Removing the picture removes both its cost and its information: audio is simpler, cheaper and lighter on bandwidth, but loses body language and visual aids (C and D are advantages of video). Time zones affect every kind of conference.", "atom_ids": ["0417-4-A67", "0417-4-A66"], "template_id": "T23", "misconception_id": null, "difficulty": 2}, {"id": "q42", "type": "numeric", "prompt": "A rural clinic wants to hold a six-person video-conference. The software needs 2.5 Mbps of upload bandwidth for each participant. How much upload bandwidth does the call need, in Mbps? (The clinic's connection gives 10 Mbps upload, shared with twelve staff computers.)", "answer": 15, "tolerance": 0.01, "unit": "Mbps", "marks": 1, "mark_scheme": "15 Mbps (6 × 2.5). This is more than the 10 Mbps upload available, so the connection is not adequate.", "explanation": "6 × 2.5 = 15 Mbps, but only 10 Mbps upload is available, even before the staff computers use any. The video would lag, freeze or drop in quality: this is why conferencing needs a fast, stable connection, and why sharing a screen with audio only would suit the clinic better.", "atom_ids": ["0417-4-A63", "0417-4-A64"], "template_id": null, "misconception_id": null, "difficulty": 3}, {"id": "q43", "type": "short", "prompt": "A sales team holds a weekly meeting online. Describe two advantages and one disadvantage of using web-conferencing rather than video-conferencing for this meeting.", "answer": null, "marks": 3, "mark_scheme": "Advantages, max 2: needs less dedicated hardware and software so it is cheaper; works on more kinds of device; fewer lag or lip-sync problems / needs less bandwidth; documents and screens are easy to share; more privacy if no camera is used. Disadvantage, max 1: less personal / cannot see body language; harder to tell whether people are paying attention or present; fewer features than video-conferencing; still needs a stable internet connection", "explanation": "The question asks for a comparison with video-conferencing, so general points about saving travel do not answer it. Use generic names: brand names of apps earn no credit.", "atom_ids": ["0417-4-A69", "0417-4-A68"], "template_id": "T23", "misconception_id": null, "difficulty": 3}], "atoms": {"0417-4-A68": "Web-conferencing (including webinars) runs in a browser or app over the internet: participants see the same screen, presentations and documents, with chat, polls and optional audio and video.", "0417-4-A62": "Electronic conferencing is real-time communication over a network between people in different places; shared benefits: no travel time or costs, meetings at short notice, safer, anyone anywhere can join.", "0417-4-A63": "Shared drawbacks of conferencing: needs a fast stable internet connection, time zones, lag, freezing, lip-sync and echo problems, equipment and training costs, less personal and harder to hold attention.", "0417-4-A65": "Video-conferencing software: conferencing software, hardware drivers (make webcam and microphone work), a CODEC (compresses and decompresses the streams), echo-cancellation (stops the microphone picking up the speakers).", "0417-4-A64": "Video-conferencing hardware: computer, webcam, microphone, speakers or headphones, monitor or large screen, router or internet connection.", "0417-4-A70": "Paper 1 conventions: list points, first answers only marked; 6-mark discussions need both sides (max five a side); step questions earn one mark per distinct step; specific wording ('stable internet connection', 'large geographical area', 'MAC address'); generic names, never brands.", "0417-4-A67": "Audio versus video-conferencing: audio is simpler, cheaper, needs less bandwidth (less freezing), has no lip-sync issues and gives more privacy, but loses body language and visual aids and makes attention and turn-taking hard to judge.", "0417-4-A66": "Audio-conferencing is voice only: participants join one call, often by dialling into a central system with a PIN, using a telephone handset or a computer's microphone and speakers with an audio-conferencing app.", "0417-4-A69": "Web- versus video-conferencing: web needs less dedicated hardware and software, works on more devices and has fewer lag problems, but is less personal; versus meeting in person it saves travel and can be recorded but needs suitable devices and connections for all."}}
+{"id": "0417_ch04_q06", "chapterId": "0417_ch04", "subject": "ICT", "code": "0417", "chapter": "Networks and the effects of using them", "topicId": "t6", "topicTitle": "Electronic conferencing", "quizNo": 6, "quizCount": 7, "order": 6, "title": "Electronic conferencing", "kind": "topic", "intro": "Practise the three kinds of electronic conference, what each needs, and when each is the better choice.", "minutes": 12, "active": true, "questions": [{"id": "q37", "type": "mcq", "prompt": "A teacher runs an online lesson in a web browser. Every student sees the same slides on screen and can answer polls and type questions in a chat box; cameras are optional. What is this type of electronic conferencing called?", "options": ["Web-conferencing", "Audio-conferencing", "Video-conferencing", "An extranet"], "answer": 0, "marks": 1, "mark_scheme": "A: web-conferencing (a webinar).", "explanation": "Web-conferencing runs in a browser or app and its point is the shared screen, with chat and polls. Audio-conferencing is voice only, and video-conferencing centres on moving pictures and sound. Use the generic term: brand names of apps earn no credit.", "atom_ids": ["0417-4-A68"], "template_id": "T21", "misconception_id": null, "difficulty": 1}, {"id": "q38", "type": "multi", "prompt": "A company replaces its monthly in-person meeting of managers from four countries with an electronic conference. Select the TWO benefits.", "options": ["No travel time, and no travel or hotel costs", "Meetings can be arranged at short notice", "Time-zone differences no longer matter", "No internet connection is needed", "Body language is easier to read than in person"], "answer": [0, 1], "marks": 2, "mark_scheme": "1 mark each for A and B; no marks if more than two options are chosen.", "explanation": "The network replaces the journey, so travel time and cost disappear and meetings can be called quickly. Time zones still matter, a stable connection is essential, and body language is harder to read, not easier.", "atom_ids": ["0417-4-A62", "0417-4-A63"], "template_id": "T23", "misconception_id": null, "difficulty": 1}, {"id": "q39", "type": "multi", "prompt": "Select the THREE items of SOFTWARE needed to hold a video-conference.", "options": ["CODEC", "Webcam", "Echo-cancellation software", "Hardware drivers", "Router", "Microphone"], "answer": [0, 2, 3], "marks": 2, "mark_scheme": "2 marks for exactly A, C and D; 1 mark for two of these with no wrong option; 0 otherwise.", "explanation": "The CODEC compresses and decompresses the audio and video, echo-cancellation stops the microphone picking up the speakers, and drivers make the webcam and microphone work. Webcam, router and microphone are hardware.", "atom_ids": ["0417-4-A65", "0417-4-A64"], "template_id": "T22", "misconception_id": null, "difficulty": 2}, {"id": "q40", "type": "mcq", "prompt": "Which statement of a disadvantage of video-conferencing would earn the mark?", "options": ["It needs a fast, stable internet connection; if the connection drops, the picture freezes or people are cut off.", "It needs the internet.", "It uses a computer.", "It is not very good."], "answer": 0, "marks": 1, "mark_scheme": "A: needs a fast / stable / reliable internet connection, with its consequence.", "explanation": "'Needs internet' is too vague to score: mark schemes ask for a stable or reliable connection. Points must be specific and, in a discussion, linked to a consequence.", "atom_ids": ["0417-4-A63", "0417-4-A70"], "template_id": "T23", "misconception_id": null, "difficulty": 2}, {"id": "q41", "type": "multi", "prompt": "Four engineers who know each other well hold a ten-minute check-in every day. Select the TWO advantages of using audio-conferencing rather than video-conferencing for this.", "options": ["It needs less bandwidth, so there is less freezing and lag.", "It needs less equipment: any telephone will do, with no webcam or screen.", "Everyone can see each other's body language.", "Drawings can be shown on screen.", "There are no time-zone problems."], "answer": [0, 1], "marks": 2, "mark_scheme": "1 mark each for A and B; no marks if more than two options are chosen.", "explanation": "Removing the picture removes both its cost and its information: audio is simpler, cheaper and lighter on bandwidth, but loses body language and visual aids (C and D are advantages of video). Time zones affect every kind of conference.", "atom_ids": ["0417-4-A67", "0417-4-A66"], "template_id": "T23", "misconception_id": null, "difficulty": 2}, {"id": "q42", "type": "mcq", "prompt": "A clinic holds audio-conferences on a computer, using an audio-conferencing app over its internet connection with a microphone and speakers. It now wants to hold video-conferences on the same computer. Which extra item of HARDWARE does it need?", "options": ["A webcam", "A router", "A microphone", "A printer"], "answer": 0, "marks": 1, "mark_scheme": "A: a webcam (to capture the video images).", "explanation": "Audio-conferencing on a computer already needs a microphone, speakers and an internet connection through a router, so the clinic has these. Video-conferencing adds the picture, so it also needs a webcam to capture images; the computer's monitor already shows them. A printer is not needed for any type of conference.", "atom_ids": ["0417-4-A64", "0417-4-A66", "0417-4-A67"], "template_id": "T22", "misconception_id": null, "difficulty": 2}, {"id": "q43", "type": "short", "prompt": "A sales team holds a weekly meeting online. Describe two advantages and one disadvantage of using web-conferencing rather than video-conferencing for this meeting.", "answer": null, "marks": 3, "mark_scheme": "Advantages, max 2: needs less dedicated hardware and software so it is cheaper; works on more kinds of device; fewer lag or lip-sync problems / needs less bandwidth; documents and screens are easy to share; more privacy if no camera is used. Disadvantage, max 1: less personal / cannot see body language; harder to tell whether people are paying attention or present; still needs a stable internet connection; audio/video quality may be lower than a dedicated video-conferencing system", "explanation": "The question asks for a comparison with video-conferencing, so general points about saving travel do not answer it. Use generic names: brand names of apps earn no credit.", "atom_ids": ["0417-4-A69", "0417-4-A68"], "template_id": "T23", "misconception_id": null, "difficulty": 3}], "atoms": {"0417-4-A68": "Web-conferencing (including webinars) runs in a browser or app over the internet: participants see the same screen, presentations and documents, with chat, polls and optional audio and video.", "0417-4-A62": "Electronic conferencing is real-time communication over a network between people in different places; shared benefits: no travel time or costs, meetings at short notice, safer, anyone anywhere can join.", "0417-4-A63": "Shared drawbacks of conferencing: needs a fast stable internet connection, time zones, lag, freezing, lip-sync and echo problems, equipment and training costs, less personal and harder to hold attention.", "0417-4-A65": "Video-conferencing software: conferencing software, hardware drivers (make webcam and microphone work), a CODEC (compresses and decompresses the streams), echo-cancellation (stops the microphone picking up the speakers).", "0417-4-A64": "Video-conferencing hardware: computer, webcam, microphone, speakers or headphones, monitor or large screen, router or internet connection.", "0417-4-A70": "Paper 1 conventions: list points, first answers only marked; 6-mark discussions need both sides (max five a side); step questions earn one mark per distinct step; specific wording ('stable internet connection', 'large geographical area', 'MAC address'); generic names, never brands.", "0417-4-A67": "Audio versus video-conferencing: audio is simpler, cheaper, needs less bandwidth (less freezing), has no lip-sync issues and gives more privacy, but loses body language and visual aids and makes attention and turn-taking hard to judge.", "0417-4-A66": "Audio-conferencing is voice only: participants join one call, often by dialling into a central system with a PIN, using a telephone handset or a computer's microphone and speakers with an audio-conferencing app.", "0417-4-A69": "Web- versus video-conferencing: web needs less dedicated hardware and software, works on more devices and has fewer lag problems, but is less personal; versus meeting in person it saves travel and can be recorded but needs suitable devices and connections for all."}}

data/sets/0455_ch2_elasticity_and_market_system_q01.json

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-{"id": "0455_ch2_elasticity_and_market_system_q01", "chapterId": "0455_ch2_elasticity_and_market_system", "subject": "Economics", "code": "0455", "chapter": "PED, PES and the market economic system", "topicId": "t1", "topicTitle": "Measuring PED", "quizNo": 1, "quizCount": 7, "order": 1, "title": "Measuring PED", "kind": "topic", "intro": "Work out PED values, use the formula both ways and say what each value means.", "minutes": 6, "active": true, "questions": [{"id": "q01", "type": "mcq", "prompt": "Which demand curve shows perfectly price-elastic demand?", "options": ["A horizontal line", "A vertical line", "A downward-sloping straight line", "A curve along which price × quantity is always the same"], "answer": 0, "marks": 1, "mark_scheme": "A: any quantity is bought at one price, none above it.", "explanation": "B is perfectly inelastic; D is unitary elasticity along its whole length; C has a PED that changes along the line.", "atom_ids": ["0455-2.6-A09", "0455-2.6-A08"], "template_id": "T9", "misconception_id": null, "difficulty": 1}, {"id": "q02", "type": "numeric", "prompt": "Sign convention for this self-check: write PED with its minus sign. When the price of a carton of mango juice rises from $2.50 to $3.00, weekly sales fall from 1,200 to 1,050 cartons. Calculate the PED (two decimal places).", "answer": -0.625, "tolerance": 0.02, "unit": "", "marks": 1, "mark_scheme": "%ΔQd = -12.5%; %ΔP = +20%; PED = -12.5 ÷ 20 = -0.625 (accept −0.62 or −0.63)", "explanation": "Quantity on top: -12.5 ÷ 20 = −0.625, so demand is price-inelastic. −1.6 would mean the formula was inverted; −300 (that is −150 ÷ 0.50) would mean raw changes were used instead of percentages.", "atom_ids": ["0455-2.6-A02", "0455-2.6-A05", "0455-2.6-A06"], "template_id": "T1", "misconception_id": null, "difficulty": 1}, {"id": "q03", "type": "mcq", "prompt": "Sign convention: PED is written with its minus sign (for example −0.5) and classified by its size. A snack's demand schedule is: at $6, 20 thousand a week; at $5, 22 thousand; at $4, 30 thousand; at $3, 42 thousand; at $2, 60 thousand. Over which price cut is demand price-inelastic?", "options": ["$6 to $5", "$5 to $4", "$4 to $3", "$3 to $2"], "answer": 0, "marks": 1, "mark_scheme": "A: %ΔQd = 10%, %ΔP = -16.7%, PED = -0.6; size below 1.", "explanation": "PEDs for the four cuts are -0.6, -1.82, -1.6, -1.29. Only the first has a size below 1. Revenue confirms it: $120 thousand falls to $110 thousand, and a price cut that lowers revenue is the sign of inelastic demand. -1.29 is close to 1 but still elastic.", "atom_ids": ["0455-2.6-A05", "0455-2.6-A02", "0455-2.6-A07"], "template_id": "T1", "misconception_id": null, "difficulty": 2}, {"id": "q04", "type": "numeric", "prompt": "Sign convention: PED is written with its minus sign (for example −0.5) and classified by its size. A cricket ground sells 20,000 tickets per match. Demand for tickets has a PED of -1.5. The ground cuts its ticket price by 8%. How many tickets per match will it now sell?", "answer": 22400, "tolerance": 0.005, "unit": "tickets", "marks": 1, "mark_scheme": "%ΔQd = −1.5 × −8 = +12%; 20,000 × 1.12 = 22,400", "explanation": "A price cut raises quantity demanded (a movement along the curve). 12% of 20,000 is 2,400 extra tickets. An answer of 17,600 has the direction wrong.", "atom_ids": ["0455-2.6-A11", "0455-2.6-A13"], "template_id": "T2", "misconception_id": null, "difficulty": 2}, {"id": "q05", "type": "numeric", "prompt": "Sign convention: PED is written with its minus sign (for example −0.5) and classified by its size. The PED for sugary soft drinks is -0.6. The government wants to cut the quantity bought by 15%. By what percentage must the price rise?", "answer": 25.0, "tolerance": 0.01, "unit": "%", "marks": 1, "mark_scheme": "%ΔP = %ΔQd ÷ PED = −15 ÷ −0.6 = 25%", "explanation": "Divide, do not multiply: 15 × 0.6 = 9% is the inverted-formula trap. Check: inelastic demand means the price change must be bigger than the quantity change.", "atom_ids": ["0455-2.6-A12", "0455-2.6-A04"], "template_id": "T2", "misconception_id": "0455-2.6-M01", "difficulty": 2}, {"id": "q06", "type": "mcq", "prompt": "Along a single downward-sloping straight-line demand curve, where is demand more price-elastic?", "options": ["At higher prices", "At lower prices", "Nowhere: PED is the same everywhere because the slope is constant", "Only where the line meets the quantity axis"], "answer": 0, "marks": 1, "mark_scheme": "A: the same price change is a small % at high prices but the quantity change is a large % of a small quantity.", "explanation": "For example, with Q = 100 − 10P, PED is -4 at P = 8 but -0.25 at P = 2. C confuses elasticity with slope.", "atom_ids": ["0455-2.6-A10"], "template_id": "T9", "misconception_id": null, "difficulty": 3}], "atoms": {"0455-2.6-A09": "Perfectly inelastic demand is a vertical curve, perfectly elastic a horizontal one; of two curves through the same point, the steeper is less elastic.", "0455-2.6-A08": "Perfectly inelastic demand has PED = 0 (quantity does not change); perfectly elastic demand has PED = infinity (any quantity at one price, none above it).", "0455-2.6-A02": "Percentage change = (new value - original value) / original value x 100.", "0455-2.6-A05": "Calculate PED from two price-quantity pairs, a schedule or given percentages, then classify the result.", "0455-2.6-A06": "PED is normally negative and is a pure number, not a percentage; Cambridge accepts it with or without the minus sign and it is classified by its size.", "0455-2.6-A07": "Size of PED above 1 is elastic, between 0 and 1 inelastic, exactly 1 unitary.", "0455-2.6-A11": "Run PED forwards: % change in quantity demanded = PED x % change in price.", "0455-2.6-A13": "Turn a percentage change into a new quantity: new quantity = original x (1 + % change / 100).", "0455-2.6-A12": "Run PED backwards: % change in price needed = target % change in quantity / PED.", "0455-2.6-A04": "PED = % change in quantity demanded / % change in price.", "0455-2.6-M01": "PED (or PES) = % change in price / % change in quantity.", "0455-2.6-A10": "PED is not slope: along one straight-line demand curve PED falls as price falls."}}
+{"id": "0455_ch2_elasticity_and_market_system_q01", "chapterId": "0455_ch2_elasticity_and_market_system", "subject": "Economics", "code": "0455", "chapter": "PED, PES and the market economic system", "topicId": "t1", "topicTitle": "Measuring PED", "quizNo": 1, "quizCount": 7, "order": 1, "title": "Measuring PED", "kind": "topic", "intro": "Work out PED values, use the formula both ways and say what each value means.", "minutes": 6, "active": true, "questions": [{"id": "q01", "type": "mcq", "prompt": "Which demand curve shows perfectly price-elastic demand?", "options": ["A horizontal line", "A vertical line", "A downward-sloping straight line", "A curve along which price × quantity is always the same"], "answer": 0, "marks": 1, "mark_scheme": "A: any quantity is bought at one price, none above it.", "explanation": "B is perfectly inelastic; D is unitary elasticity along its whole length; C has a PED that changes along the line.", "atom_ids": ["0455-2.6-A09", "0455-2.6-A08"], "template_id": "T9", "misconception_id": null, "difficulty": 1}, {"id": "q02", "type": "numeric", "prompt": "Sign convention for this self-check: write PED with its minus sign. When the price of a carton of mango juice rises from $2.50 to $3.00, weekly sales fall from 1,200 to 1,050 cartons. Calculate the PED (two decimal places).", "answer": -0.625, "tolerance": 0.02, "unit": "", "marks": 1, "mark_scheme": "%ΔQd = -12.5%; %ΔP = +20%; PED = -12.5 ÷ 20 = -0.625 (accept −0.62 or −0.63)", "explanation": "Quantity on top: -12.5 ÷ 20 = −0.625, so demand is price-inelastic. −1.6 would mean the formula was inverted; −300 (that is −150 ÷ 0.50) would mean raw changes were used instead of percentages.", "atom_ids": ["0455-2.6-A02", "0455-2.6-A05", "0455-2.6-A06"], "template_id": "T1", "misconception_id": null, "difficulty": 1}, {"id": "q03", "type": "mcq", "prompt": "Sign convention: PED is written with its minus sign (for example −0.5) and classified by its size. Calculate percentage changes from the original (starting) value. A snack's demand schedule is: at $6, 20 thousand a week; at $5, 22 thousand; at $4, 30 thousand; at $3, 42 thousand; at $2, 66 thousand. Over which price cut is demand price-inelastic?", "options": ["$6 to $5", "$5 to $4", "$4 to $3", "$3 to $2"], "answer": 0, "marks": 1, "mark_scheme": "A: %ΔQd = 10%, %ΔP = -16.7%, PED = -0.6; size below 1.", "explanation": "PEDs for the four cuts are -0.6, -1.82, -1.6 and -1.71. Only the first has a size below 1. Revenue confirms it: $120 thousand falls to $110 thousand, while every other cut raises revenue ($110k to $120k, $120k to $126k, $126k to $132k).", "atom_ids": ["0455-2.6-A05", "0455-2.6-A02", "0455-2.6-A07"], "template_id": "T1", "misconception_id": null, "difficulty": 2}, {"id": "q04", "type": "numeric", "prompt": "Sign convention: PED is written with its minus sign (for example −0.5) and classified by its size. A cricket ground sells 20,000 tickets per match. Demand for tickets has a PED of -1.5. The ground cuts its ticket price by 8%. How many tickets per match will it now sell?", "answer": 22400, "tolerance": 0.005, "unit": "tickets", "marks": 1, "mark_scheme": "%ΔQd = −1.5 × −8 = +12%; 20,000 × 1.12 = 22,400", "explanation": "A price cut raises quantity demanded (a movement along the curve). 12% of 20,000 is 2,400 extra tickets. An answer of 17,600 has the direction wrong.", "atom_ids": ["0455-2.6-A11", "0455-2.6-A13"], "template_id": "T2", "misconception_id": null, "difficulty": 2}, {"id": "q05", "type": "numeric", "prompt": "Sign convention: PED is written with its minus sign (for example −0.5) and classified by its size. The PED for sugary soft drinks is -0.6. The government wants to cut the quantity bought by 15%. By what percentage must the price rise?", "answer": 25.0, "tolerance": 0.01, "unit": "%", "marks": 1, "mark_scheme": "%ΔP = %ΔQd ÷ PED = −15 ÷ −0.6 = 25%", "explanation": "Divide, do not multiply: 15 × 0.6 = 9% is the inverted-formula trap. Check: inelastic demand means the price change must be bigger than the quantity change.", "atom_ids": ["0455-2.6-A12", "0455-2.6-A04"], "template_id": "T2", "misconception_id": "0455-2.6-M01", "difficulty": 2}, {"id": "q06", "type": "mcq", "prompt": "Along a single downward-sloping straight-line demand curve, where is demand more price-elastic?", "options": ["At higher prices", "At lower prices", "Nowhere: PED is the same everywhere because the slope is constant", "Only where the line meets the quantity axis"], "answer": 0, "marks": 1, "mark_scheme": "A: the same price change is a small % at high prices but the quantity change is a large % of a small quantity.", "explanation": "For example, with Q = 100 − 10P, PED is -4 at P = 8 but -0.25 at P = 2. C confuses elasticity with slope.", "atom_ids": ["0455-2.6-A10"], "template_id": "T9", "misconception_id": null, "difficulty": 3}], "atoms": {"0455-2.6-A09": "Perfectly inelastic demand is a vertical curve, perfectly elastic a horizontal one; of two curves through the same point, the steeper is less elastic.", "0455-2.6-A08": "Perfectly inelastic demand has PED = 0 (quantity does not change); perfectly elastic demand has PED = infinity (any quantity at one price, none above it).", "0455-2.6-A02": "Percentage change = (new value - original value) / original value x 100.", "0455-2.6-A05": "Calculate PED from two price-quantity pairs, a schedule or given percentages, then classify the result.", "0455-2.6-A06": "PED is normally negative and is a pure number, not a percentage; Cambridge accepts it with or without the minus sign and it is classified by its size.", "0455-2.6-A07": "Size of PED above 1 is elastic, between 0 and 1 inelastic, exactly 1 unitary.", "0455-2.6-A11": "Run PED forwards: % change in quantity demanded = PED x % change in price.", "0455-2.6-A13": "Turn a percentage change into a new quantity: new quantity = original x (1 + % change / 100).", "0455-2.6-A12": "Run PED backwards: % change in price needed = target % change in quantity / PED.", "0455-2.6-A04": "PED = % change in quantity demanded / % change in price.", "0455-2.6-M01": "PED (or PES) = % change in price / % change in quantity.", "0455-2.6-A10": "PED is not slope: along one straight-line demand curve PED falls as price falls."}}

data/sets/0455_ch2_elasticity_and_market_system_q03.json

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-{"id": "0455_ch2_elasticity_and_market_system_q03", "chapterId": "0455_ch2_elasticity_and_market_system", "subject": "Economics", "code": "0455", "chapter": "PED, PES and the market economic system", "topicId": "t3", "topicTitle": "PED, revenue and decisions", "quizNo": 3, "quizCount": 7, "order": 3, "title": "PED and revenue", "kind": "topic", "intro": "Use PED to predict revenue and to judge pricing, wage and tax decisions.", "minutes": 9, "active": true, "questions": [{"id": "q14", "type": "mcq", "prompt": "A bakery estimates the PED for its bread at −0.3. It cuts the price of a loaf. What happens to its total revenue from bread?", "options": ["It rises, because more loaves are sold", "It falls, because quantity rises by a smaller percentage than price falls", "It stays the same", "It cannot change, because PED is less than 1"], "answer": 1, "marks": 1, "mark_scheme": "B: inelastic demand, price cut → revenue falls.", "explanation": "With inelastic demand the price effect outweighs the quantity effect. A is the 'more sales always means more revenue' slip; C would need PED of −1.", "atom_ids": ["0455-2.6-A22", "0455-2.6-A21"], "template_id": "T4", "misconception_id": null, "difficulty": 1}, {"id": "q15", "type": "mcq", "prompt": "After a cinema raised its ticket price, its total revenue from tickets rose. What can be concluded?", "options": ["Demand for its tickets is price-inelastic", "Demand for its tickets is price-elastic", "Its profit must have risen", "Demand for its tickets must have increased"], "answer": 0, "marks": 1, "mark_scheme": "A: revenue moved in the same direction as price, so the % fall in quantity was smaller than the % rise in price.", "explanation": "A price rise raises revenue only when demand is inelastic; with elastic demand it would lower revenue. Profit also depends on costs, which PED says nothing about. A price rise causes a contraction along the demand curve, not an increase in demand.", "atom_ids": ["0455-2.6-A22", "0455-2.6-A23"], "template_id": "T4", "misconception_id": "0455-2.6-M06", "difficulty": 2}, {"id": "q16", "type": "numeric", "prompt": "On a demand diagram, a firm cuts its price from $8 to $6 and its sales rise from 50 to 80 units. The revenue rectangle lost because each original unit now sells for less is $100; the rectangle gained from the extra units is added. By how many dollars does total revenue change? (Give a negative number for a fall.)", "answer": 80, "tolerance": 0.001, "unit": "$", "marks": 2, "mark_scheme": "1 mark: gained rectangle = $6 × (80 − 50) = $180; 1 mark: net change = 180 − 100 = +$80 (or 6×80 − 8×50 = 480 − 400)", "explanation": "Revenue rises from $400 to $480. The gain beats the loss because demand here is elastic (PED = -2.4), so a price cut raises revenue.", "atom_ids": ["0455-2.6-A26", "0455-2.6-A25", "0455-2.6-A23"], "template_id": "T20", "misconception_id": null, "difficulty": 2}, {"id": "q17", "type": "mcq", "prompt": "A government places an indirect tax on petrol. The PED for petrol is −0.2. What is the most likely result?", "options": ["Large tax revenue and only a small fall in the quantity bought", "Small tax revenue and a large fall in the quantity bought", "Producers pay most of the tax because they cannot raise the price", "Quantity bought falls by a greater percentage than the price rises"], "answer": 0, "marks": 1, "mark_scheme": "A: inelastic demand → sales fall little, so revenue is large and most of the tax is passed on.", "explanation": "B and D describe elastic demand. C reverses the incidence: with inelastic demand producers can pass the tax on to buyers.", "atom_ids": ["0455-2.6-A29", "0455-2.6-A30"], "template_id": "T8", "misconception_id": null, "difficulty": 2}, {"id": "q18", "type": "mcq", "prompt": "Demand for hand-made furniture is price-elastic. The furniture makers' trade union wins a wage rise, and firms pass the higher cost on in a higher price. What is the most likely result?", "options": ["Many sales and jobs are lost, because quantity demanded falls by a larger percentage than the price rises", "Few jobs are lost, because demand for furniture is elastic", "Firms hire more workers, because revenue rises after the price rise", "Jobs are unaffected, because the demand for labour does not depend on the demand for furniture"], "answer": 0, "marks": 1, "mark_scheme": "A: demand for labour is derived from demand for the product; elastic product demand means a large fall in output.", "explanation": "With elastic demand a price rise loses many sales and lowers revenue, so fewer workers are needed. That weakens the union's bargaining power. Workers making a product with inelastic demand are in a stronger position.", "atom_ids": ["0455-2.6-A28", "0455-2.6-A23"], "template_id": "T19", "misconception_id": null, "difficulty": 2}, {"id": "q19", "type": "mcq", "prompt": "A bus company finds that peak-time commuters have few alternatives, while off-peak shoppers can easily walk or postpone their trips. Which fare plan is most likely to raise its total revenue?", "options": ["Raise peak fares and cut off-peak fares", "Cut peak fares and raise off-peak fares", "Raise all fares by the same percentage", "Cut all fares by the same percentage"], "answer": 0, "marks": 1, "mark_scheme": "A: peak demand is inelastic (price rise raises revenue); off-peak demand is elastic (price cut raises revenue).", "explanation": "Charging different prices to groups with different PEDs is price discrimination. Raising the inelastic fare loses few passengers; cutting the elastic fare attracts proportionately more.", "atom_ids": ["0455-2.6-A27", "0455-2.6-A22", "0455-2.6-A23"], "template_id": "T7", "misconception_id": null, "difficulty": 2}, {"id": "q20", "type": "numeric", "prompt": "Sign convention: PED is written with its minus sign (for example −0.5) and classified by its size. A tax raises the price of a packet of cigarettes by 15%, from $8.00 to $9.20. The PED for cigarettes is −0.4, and 50 million packets a year were sold before the rise. Assuming the whole $1.20 rise is tax, calculate the extra tax revenue the government collects, in millions of dollars.", "answer": 56.4, "tolerance": 0.01, "unit": "$ million", "marks": 2, "mark_scheme": "1 mark: %ΔQd = −0.4 × 15 = -6%, so sales = 50 × 0.94 = 47 million; 1 mark: 47 × $1.20 = $56.4 million.", "explanation": "Sales fall only 6%, so most smokers keep buying and pay the tax. $60 million would forget that sales fall at all. Inelastic demand makes this tax good at raising revenue but weak at reducing smoking.", "atom_ids": ["0455-2.6-A29", "0455-2.6-A30", "0455-2.6-A11"], "template_id": "T8", "misconception_id": null, "difficulty": 3}], "atoms": {"0455-2.6-A22": "With inelastic demand, a price rise raises total revenue and a price cut lowers it.", "0455-2.6-A21": "Total revenue = price x quantity sold, and it equals consumer expenditure on the product.", "0455-2.6-A23": "With elastic demand, a price rise lowers total revenue and a price cut raises it.", "0455-2.6-M06": "A price rise always raises revenue; revenue and profit are the same thing.", "0455-2.6-A26": "On a demand diagram revenue is the rectangle P x Q; a price change swaps one rectangle for another, and the larger one shows the revenue direction.", "0455-2.6-A25": "Calculate total revenue before and after a price change and state the change.", "0455-2.6-A29": "An indirect tax on a product with inelastic demand raises much revenue and is largely passed on to consumers.", "0455-2.6-A30": "A tax on a harmful product with inelastic demand cuts consumption only a little; a tariff protects domestic producers most when demand for the import is elastic.", "0455-2.6-A28": "Consumers are hit hardest by price rises on inelastic necessities; workers' jobs are most at risk when demand for their product is elastic, because demand for labour is derived.", "0455-2.6-A27": "Firms raise price when demand is inelastic and cut it when demand is elastic; they charge more in the market where demand is less elastic, and advertise to build brand loyalty that makes demand less elastic.", "0455-2.6-A11": "Run PED forwards: % change in quantity demanded = PED x % change in price."}}
+{"id": "0455_ch2_elasticity_and_market_system_q03", "chapterId": "0455_ch2_elasticity_and_market_system", "subject": "Economics", "code": "0455", "chapter": "PED, PES and the market economic system", "topicId": "t3", "topicTitle": "PED, revenue and decisions", "quizNo": 3, "quizCount": 7, "order": 3, "title": "PED and revenue", "kind": "topic", "intro": "Use PED to predict revenue and to judge pricing, wage and tax decisions.", "minutes": 9, "active": true, "questions": [{"id": "q14", "type": "mcq", "prompt": "A bakery estimates the PED for its bread at −0.3. It cuts the price of a loaf. What happens to its total revenue from bread?", "options": ["It rises, because more loaves are sold", "It falls, because quantity rises by a smaller percentage than price falls", "It stays the same", "It cannot change, because PED is less than 1"], "answer": 1, "marks": 1, "mark_scheme": "B: inelastic demand, price cut → revenue falls.", "explanation": "With inelastic demand the price effect outweighs the quantity effect. A is the 'more sales always means more revenue' slip; C would need PED of −1.", "atom_ids": ["0455-2.6-A22", "0455-2.6-A21"], "template_id": "T4", "misconception_id": null, "difficulty": 1}, {"id": "q15", "type": "mcq", "prompt": "After a cinema raised its ticket price, its total revenue from tickets rose. What can be concluded?", "options": ["Demand for its tickets is price-inelastic", "Demand for its tickets is price-elastic", "Its profit must have risen", "Demand for its tickets must have increased"], "answer": 0, "marks": 1, "mark_scheme": "A: revenue moved in the same direction as price, so the % fall in quantity was smaller than the % rise in price.", "explanation": "A price rise raises revenue only when demand is inelastic; with elastic demand it would lower revenue. Profit also depends on costs, which PED says nothing about. A price rise causes a contraction along the demand curve, not an increase in demand.", "atom_ids": ["0455-2.6-A22", "0455-2.6-A23"], "template_id": "T4", "misconception_id": "0455-2.6-M06", "difficulty": 2}, {"id": "q16", "type": "numeric", "prompt": "On a demand diagram, a firm cuts its price from $8 to $6 and its sales rise from 50 to 80 units. The revenue rectangle lost because each original unit now sells for less is $100; the rectangle gained from the extra units is added. By how many dollars does total revenue change? (Give a negative number for a fall.)", "answer": 80, "tolerance": 0.001, "unit": "$", "marks": 2, "mark_scheme": "1 mark: gained rectangle = $6 × (80 − 50) = $180; 1 mark: net change = 180 − 100 = +$80 (or 6×80 − 8×50 = 480 − 400)", "explanation": "Revenue rises from $400 to $480. The gain beats the loss because demand here is elastic (PED = -2.4), so a price cut raises revenue.", "atom_ids": ["0455-2.6-A26", "0455-2.6-A25", "0455-2.6-A23"], "template_id": "T20", "misconception_id": null, "difficulty": 2}, {"id": "q17", "type": "mcq", "prompt": "A government places an indirect tax on petrol. The PED for petrol is −0.2. What is the most likely result?", "options": ["Large tax revenue and only a small fall in the quantity bought", "Small tax revenue and a large fall in the quantity bought", "Producers pay most of the tax because they cannot raise the price", "Quantity bought falls by a greater percentage than the price rises"], "answer": 0, "marks": 1, "mark_scheme": "A: inelastic demand → sales fall little, so revenue is large and most of the tax is passed on.", "explanation": "B and D describe elastic demand. C reverses the incidence: with inelastic demand producers can pass the tax on to buyers.", "atom_ids": ["0455-2.6-A29", "0455-2.6-A30"], "template_id": "T8", "misconception_id": null, "difficulty": 2}, {"id": "q18", "type": "mcq", "prompt": "Demand for hand-made furniture is price-elastic. The furniture makers' trade union wins a wage rise, and firms pass the higher cost on in a higher price. What is the most likely result?", "options": ["Many sales and jobs are lost, because quantity demanded falls by a larger percentage than the price rises", "Few jobs are lost, because demand for furniture is elastic", "Firms hire more workers, because revenue rises after the price rise", "Jobs are unaffected, because the demand for labour does not depend on the demand for furniture"], "answer": 0, "marks": 1, "mark_scheme": "A: demand for labour is derived from demand for the product; elastic product demand means a large fall in output.", "explanation": "With elastic demand a price rise loses many sales and lowers revenue, so fewer workers are needed. That weakens the union's bargaining power. Workers making a product with inelastic demand are in a stronger position.", "atom_ids": ["0455-2.6-A28", "0455-2.6-A23"], "template_id": "T19", "misconception_id": null, "difficulty": 2}, {"id": "q19", "type": "mcq", "prompt": "A bus company finds that peak-time commuters have few alternatives, while off-peak shoppers can easily walk or postpone their trips. Which fare plan is most likely to raise its total revenue?", "options": ["Raise peak fares and cut off-peak fares", "Cut peak fares and raise off-peak fares", "Raise all fares by the same percentage", "Cut all fares by the same percentage"], "answer": 0, "marks": 1, "mark_scheme": "A: peak demand is inelastic (price rise raises revenue); off-peak demand is elastic (price cut raises revenue).", "explanation": "Charging different prices to groups with different PEDs is price discrimination. Raising the inelastic fare loses few passengers; cutting the elastic fare attracts proportionately more.", "atom_ids": ["0455-2.6-A27", "0455-2.6-A22", "0455-2.6-A23"], "template_id": "T7", "misconception_id": null, "difficulty": 2}, {"id": "q20", "type": "numeric", "prompt": "Sign convention: PED is written with its minus sign (for example −0.5) and classified by its size. A tax raises the price of a packet of cigarettes by 15%, from $8.00 to $9.20. The PED for cigarettes is −0.4, and 50 million packets a year were sold before the rise. There was no tax on cigarettes before. Assuming the whole $1.20 rise is the new tax, calculate the revenue the government collects from this tax, in millions of dollars.", "answer": 56.4, "tolerance": 0.01, "unit": "$ million", "marks": 2, "mark_scheme": "1 mark: %ΔQd = −0.4 × 15 = -6%, so sales = 50 × 0.94 = 47 million; 1 mark: 47 × $1.20 = $56.4 million.", "explanation": "Sales fall only 6%, so most smokers keep buying and pay the tax. $60 million would forget that sales fall at all. Inelastic demand makes this tax good at raising revenue but weak at reducing smoking.", "atom_ids": ["0455-2.6-A29", "0455-2.6-A30", "0455-2.6-A11"], "template_id": "T8", "misconception_id": null, "difficulty": 3}], "atoms": {"0455-2.6-A22": "With inelastic demand, a price rise raises total revenue and a price cut lowers it.", "0455-2.6-A21": "Total revenue = price x quantity sold, and it equals consumer expenditure on the product.", "0455-2.6-A23": "With elastic demand, a price rise lowers total revenue and a price cut raises it.", "0455-2.6-M06": "A price rise always raises revenue; revenue and profit are the same thing.", "0455-2.6-A26": "On a demand diagram revenue is the rectangle P x Q; a price change swaps one rectangle for another, and the larger one shows the revenue direction.", "0455-2.6-A25": "Calculate total revenue before and after a price change and state the change.", "0455-2.6-A29": "An indirect tax on a product with inelastic demand raises much revenue and is largely passed on to consumers.", "0455-2.6-A30": "A tax on a harmful product with inelastic demand cuts consumption only a little; a tariff protects domestic producers most when demand for the import is elastic.", "0455-2.6-A28": "Consumers are hit hardest by price rises on inelastic necessities; workers' jobs are most at risk when demand for their product is elastic, because demand for labour is derived.", "0455-2.6-A27": "Firms raise price when demand is inelastic and cut it when demand is elastic; they charge more in the market where demand is less elastic, and advertise to build brand loyalty that makes demand less elastic.", "0455-2.6-A11": "Run PED forwards: % change in quantity demanded = PED x % change in price."}}

data/sets/0610_ch05_q03.json

@@ -1 +1 @@
-{"id": "0610_ch05_q03", "chapterId": "0610_ch05", "subject": "Biology", "code": "0610", "chapter": "Enzymes (with enzyme practical skills)", "topicId": "t3", "topicTitle": "pH and enzyme concentration", "quizNo": 3, "quizCount": 7, "order": 3, "title": "pH and enzyme concentration", "kind": "topic", "intro": "Practise explaining pH by shape and fit, gut pH, and what more enzyme or more substrate does.", "minutes": 6, "active": true, "questions": [{"id": "q14", "type": "mcq", "prompt": "A graph of pepsin activity against pH has a single peak. At about which pH is the peak?", "options": ["pH 2", "pH 7", "pH 8", "pH 12"], "answer": 0, "marks": 1, "mark_scheme": "Optimum about pH 2 (pepsin works in the acidic stomach).", "explanation": "Optimum pH matches where an enzyme works. The stomach is acidic, so pepsin peaks near pH 2; amylase, trypsin and lipase peak near neutral to slightly alkaline pH. The graph falls on both sides of the peak.", "atom_ids": ["0610-5-A21", "0610-5-A22"], "template_id": "T5", "misconception_id": null, "difficulty": 1}, {"id": "q15", "type": "mcq", "prompt": "Amylase from saliva is swallowed with food. Why does it stop digesting starch in the stomach?", "options": ["No starch reaches the stomach", "The stomach is colder than the mouth", "Hydrochloric acid makes the stomach contents about pH 2, far from amylase's optimum; the shape of its active site changes and it is denatured", "Pepsin breaks down starch before amylase can"], "answer": 2, "marks": 1, "mark_scheme": "Stomach acid / low pH; active site changes shape / amylase denatured.", "explanation": "The stomach is at body temperature, so temperature is not the cause. Pepsin is a protease and cannot act on starch. The low pH changes the shape of amylase's active site.", "atom_ids": ["0610-5-A23", "0610-5-A22"], "template_id": "T4", "misconception_id": null, "difficulty": 2}, {"id": "q16", "type": "mcq", "prompt": "How does bile help amylase, trypsin and lipase to work in the duodenum?", "options": ["Bile digests fat into fatty acids and glycerol", "Bile is acidic and lowers the pH to the enzymes' optimum", "Bile is alkaline and neutralises the acidic mixture from the stomach, giving a pH that suits these enzymes", "Bile is an enzyme that breaks down protein"], "answer": 2, "marks": 1, "mark_scheme": "Bile is alkaline; neutralises stomach acid; suitable pH for intestinal enzymes.", "explanation": "Bile is not an enzyme and does not digest anything: it is alkaline and emulsifies fat. Lipase does the digesting, and the fatty acids it makes lower the pH.", "atom_ids": ["0610-5-A23"], "template_id": "T13", "misconception_id": "0610-5-M08", "difficulty": 2}, {"id": "q17", "type": "mcq", "prompt": "Times for starch to disappear at 35 °C were 240 s with 0.5% amylase, 120 s with 1.0%, 60 s with 2.0% and 30 s with 4.0%. Which explains why each doubling of amylase concentration halves the time?", "options": ["Twice as many active sites are available, so twice as many enzyme-substrate complexes form per second", "The amylase molecules gain more kinetic energy", "More amylase raises the optimum temperature", "Amylase molecules collide with each other more often"], "answer": 0, "marks": 1, "mark_scheme": "More enzyme = more active sites; more enzyme-substrate complexes per second.", "explanation": "Temperature is the same in every tube, so kinetic energy does not change. What matters is collisions between enzyme and substrate; doubling the enzyme doubles the active sites available.", "atom_ids": ["0610-5-A24", "0610-5-A08"], "template_id": "P14", "misconception_id": null, "difficulty": 2}, {"id": "q18", "type": "mcq", "prompt": "An enzyme is most active at pH 7. Which explanation of its lower activity at pH 9 would earn credit?", "options": ["At pH 9 the molecules have less kinetic energy, so they collide less often", "At pH 9 the shape of the active site changes, so fewer substrate molecules fit and fewer enzyme-substrate complexes form", "At pH 9 the enzyme is killed", "At pH 9 the substrate is used up faster"], "answer": 1, "marks": 1, "mark_scheme": "Shape of active site changes; poorer fit; fewer ESCs.", "explanation": "pH acts on shape and fit. Kinetic energy depends on temperature, which is held constant in a pH experiment. Enzymes are denatured, not killed.", "atom_ids": ["0610-5-A19", "0610-5-A20", "0610-5-M03"], "template_id": "T5", "misconception_id": "0610-5-M03", "difficulty": 3}, {"id": "q19", "type": "mcq", "prompt": "The same mass of potato (catalase) was added to hydrogen peroxide of different concentrations. Oxygen produced per minute: 2.1 cm³ at 2 volume, 5.0 at 5, 10.0 at 10, 13.5 at 15 and 15.0 at 20 volume. Why does the rate rise less and less above 10 volume?", "options": ["The catalase is being used up", "Most active sites are already occupied, so extra substrate cannot form many more complexes; the amount of enzyme is becoming the limiting factor", "Concentrated hydrogen peroxide has less kinetic energy", "The catalase has reached its optimum temperature"], "answer": 1, "marks": 1, "mark_scheme": "Active sites (nearly) all busy / saturated; enzyme now limiting; rate levels off.", "explanation": "Up to 10 volume the rate is roughly proportional to concentration because active sites are spare. As they fill, the line levels off towards a plateau: a plateau means something other than the x-axis variable is limiting. Enzymes are not used up.", "atom_ids": ["0610-5-A25", "0610-5-A08"], "template_id": "P14", "misconception_id": null, "difficulty": 3}], "atoms": {"0610-5-A21": "A rate-pH graph has a peak at the optimum pH and falls on both sides; different enzymes peak at different pH values.", "0610-5-A22": "Optimum pH matches where the enzyme works: pepsin about pH 2 in the stomach; amylase, trypsin and lipase near neutral to slightly alkaline in the mouth and small intestine.", "0610-5-A23": "Stomach hydrochloric acid denatures amylase; alkaline bile neutralises the acidic mixture so the duodenum has a suitable pH for amylase, trypsin and lipase.", "0610-5-M08": "bile digests fat, or bile and lipase are acidic.", "0610-5-A24": "More enzyme means more active sites, so more complexes form per second and the rate rises.", "0610-5-A08": "The rate of an enzyme reaction is set by how many enzyme-substrate complexes form per second, i.e. by the frequency of effective collisions between enzyme and substrate.", "0610-5-A19": "Moving pH away from the optimum changes the shape of the active site, so the fit with the substrate is poorer and fewer complexes form; extremes of pH denature the enzyme.", "0610-5-A20": "pH effects are explained by shape and fit, not by kinetic energy; temperature is held constant in a pH experiment.", "0610-5-M03": "pH changes enzyme activity through kinetic energy or collision frequency.", "0610-5-A25": "When every active site is busy (or another factor limits), the rate stops rising and the graph levels off; a plateau means something else is limiting."}}
+{"id": "0610_ch05_q03", "chapterId": "0610_ch05", "subject": "Biology", "code": "0610", "chapter": "Enzymes (with enzyme practical skills)", "topicId": "t3", "topicTitle": "pH and enzyme concentration", "quizNo": 3, "quizCount": 7, "order": 3, "title": "pH and enzyme concentration", "kind": "topic", "intro": "Practise explaining pH by shape and fit, gut pH, denaturation by extreme pH, and what more enzyme does.", "minutes": 6, "active": true, "questions": [{"id": "q14", "type": "mcq", "prompt": "A graph of pepsin activity against pH has a single peak. At about which pH is the peak?", "options": ["pH 2", "pH 7", "pH 8", "pH 12"], "answer": 0, "marks": 1, "mark_scheme": "Optimum about pH 2 (pepsin works in the acidic stomach).", "explanation": "Optimum pH matches where an enzyme works. The stomach is acidic, so pepsin peaks near pH 2; amylase, trypsin and lipase peak near neutral to slightly alkaline pH. The graph falls on both sides of the peak.", "atom_ids": ["0610-5-A21", "0610-5-A22"], "template_id": "T5", "misconception_id": null, "difficulty": 1}, {"id": "q15", "type": "mcq", "prompt": "Amylase from saliva is swallowed with food. Why does it stop digesting starch in the stomach?", "options": ["No starch reaches the stomach", "The stomach is colder than the mouth", "Hydrochloric acid makes the stomach contents about pH 2, far from amylase's optimum; the shape of its active site changes and it is denatured", "Pepsin breaks down starch before amylase can"], "answer": 2, "marks": 1, "mark_scheme": "Stomach acid / low pH; active site changes shape / amylase denatured.", "explanation": "The stomach is at body temperature, so temperature is not the cause. Pepsin is a protease and cannot act on starch. The low pH changes the shape of amylase's active site.", "atom_ids": ["0610-5-A23", "0610-5-A22"], "template_id": "T4", "misconception_id": null, "difficulty": 2}, {"id": "q16", "type": "mcq", "prompt": "How does bile help amylase, trypsin and lipase to work in the duodenum?", "options": ["Bile digests fat into fatty acids and glycerol", "Bile is acidic and lowers the pH to the enzymes' optimum", "Bile is alkaline and neutralises the acidic mixture from the stomach, giving a pH that suits these enzymes", "Bile is an enzyme that breaks down protein"], "answer": 2, "marks": 1, "mark_scheme": "Bile is alkaline; neutralises stomach acid; suitable pH for intestinal enzymes.", "explanation": "Bile is not an enzyme and does not digest anything: it is alkaline and emulsifies fat. Lipase does the digesting, and the fatty acids it makes lower the pH.", "atom_ids": ["0610-5-A23"], "template_id": "T13", "misconception_id": "0610-5-M08", "difficulty": 2}, {"id": "q17", "type": "mcq", "prompt": "Times for starch to disappear at 35 °C were 240 s with 0.5% amylase, 120 s with 1.0%, 60 s with 2.0% and 30 s with 4.0%. Which explains why each doubling of amylase concentration halves the time?", "options": ["Twice as many active sites are available, so twice as many enzyme-substrate complexes form per second", "The amylase molecules gain more kinetic energy", "More amylase raises the optimum temperature", "Amylase molecules collide with each other more often"], "answer": 0, "marks": 1, "mark_scheme": "More enzyme = more active sites; more enzyme-substrate complexes per second.", "explanation": "Temperature is the same in every tube, so kinetic energy does not change. What matters is collisions between enzyme and substrate; doubling the enzyme doubles the active sites available.", "atom_ids": ["0610-5-A24", "0610-5-A08"], "template_id": "P14", "misconception_id": null, "difficulty": 2}, {"id": "q18", "type": "mcq", "prompt": "An enzyme is most active at pH 7. Which explanation of its lower activity at pH 9 would earn credit?", "options": ["At pH 9 the molecules have less kinetic energy, so they collide less often", "At pH 9 the shape of the active site changes, so fewer substrate molecules fit and fewer enzyme-substrate complexes form", "At pH 9 the enzyme is killed", "At pH 9 the substrate is used up faster"], "answer": 1, "marks": 1, "mark_scheme": "Shape of active site changes; poorer fit; fewer ESCs.", "explanation": "pH acts on shape and fit. Kinetic energy depends on temperature, which is held constant in a pH experiment. Enzymes are denatured, not killed.", "atom_ids": ["0610-5-A19", "0610-5-A20", "0610-5-M03"], "template_id": "T5", "misconception_id": "0610-5-M03", "difficulty": 3}, {"id": "q19", "type": "mcq", "prompt": "An enzyme has an optimum pH of 8. At pH 8 it digested 3.0 units of substrate per minute. A second sample of the same enzyme was kept at pH 2 for 30 minutes, then returned to pH 8 and given fresh substrate at the same temperature. It now digested only 0.2 units per minute. Which explanation would earn credit?", "options": ["The extreme pH denatured the enzyme: the shape of its active site changed permanently, so the substrate no longer fits and few enzyme-substrate complexes form", "At pH 2 the enzyme molecules lost kinetic energy, so they now collide with substrate molecules less often", "The enzyme was used up while it was kept at pH 2", "The acid changed the substrate rather than the enzyme, so there was less substrate left to digest"], "answer": 0, "marks": 1, "mark_scheme": "A: extreme pH denatured the enzyme / changed the shape of the active site, so the substrate is no longer complementary and fewer enzyme-substrate complexes form.", "explanation": "Activity stayed low even after the pH was put back to 8, so the change was permanent: the extreme pH denatured the enzyme by changing the shape of its active site. pH acts on shape and fit, not on kinetic energy (B); the temperature was the same throughout. Enzymes are not used up (C), and fresh substrate was given at pH 8, so D cannot explain it.", "atom_ids": ["0610-5-A19", "0610-5-A20"], "template_id": "T4", "misconception_id": "0610-5-M03", "difficulty": 3}], "atoms": {"0610-5-A21": "A rate-pH graph has a peak at the optimum pH and falls on both sides; different enzymes peak at different pH values.", "0610-5-A22": "Optimum pH matches where the enzyme works: pepsin about pH 2 in the stomach; amylase, trypsin and lipase near neutral to slightly alkaline in the mouth and small intestine.", "0610-5-A23": "Stomach hydrochloric acid denatures amylase; alkaline bile neutralises the acidic mixture so the duodenum has a suitable pH for amylase, trypsin and lipase.", "0610-5-M08": "bile digests fat, or bile and lipase are acidic.", "0610-5-A24": "More enzyme means more active sites, so more complexes form per second and the rate rises.", "0610-5-A08": "The rate of an enzyme reaction is set by how many enzyme-substrate complexes form per second, i.e. by the frequency of effective collisions between enzyme and substrate.", "0610-5-A19": "Moving pH away from the optimum changes the shape of the active site, so the fit with the substrate is poorer and fewer complexes form; extremes of pH denature the enzyme.", "0610-5-A20": "pH effects are explained by shape and fit, not by kinetic energy; temperature is held constant in a pH experiment.", "0610-5-M03": "pH changes enzyme activity through kinetic energy or collision frequency."}}

data/sets/0620_ch04_q03.json

@@ -1 +1 @@
-{"id": "0620_ch04_q03", "chapterId": "0620_ch04", "subject": "Chemistry", "code": "0620", "chapter": "Chemical formulae and equations", "topicId": "t3", "topicTitle": "Ions and their charges", "quizNo": 3, "quizCount": 7, "order": 3, "title": "Ions and their charges", "kind": "topic", "intro": "Practise working out ion charges from the Periodic Table and recalling the compound ions.", "minutes": 6, "active": true, "questions": [{"id": "q13", "type": "mcq", "prompt": "Selenium is in Group VI of the Periodic Table. What is the formula of the selenide ion?", "options": ["Se⁶⁺", "Se²⁺", "Se²⁻", "Se⁶⁻"], "answer": 2, "marks": 1, "mark_scheme": "C (Se²⁻)", "explanation": "A Group VI atom has six outer electrons and gains two to fill its outer shell, giving a 2− ion, just like oxide O²⁻ and sulfide S²⁻. Non-metals form negative ions, and losing or gaining six electrons is far too costly.", "atom_ids": ["0620-3.1-A18"], "template_id": "T25", "misconception_id": null, "difficulty": 1}, {"id": "q14", "type": "mcq", "prompt": "Which pair of names and formulae are BOTH correct?", "options": ["Na₂S sodium sulfate; Na₂SO₄ sodium sulfide", "Na₂S sodium sulfide; Na₂SO₄ sodium sulfate", "Na₂SO₄ sodium sulfide; NaS sodium sulfate", "Na₂S sodium sulfide; NaSO₄ sodium sulfate"], "answer": 1, "marks": 1, "mark_scheme": "B", "explanation": "-ide means the single element (S²⁻); -ate means an oxygen-containing ion (SO₄²⁻). Both ions are 2−, so both salts need two Na⁺.", "atom_ids": ["0620-3.1-A24", "0620-3.1-A22"], "template_id": "T24", "misconception_id": null, "difficulty": 1}, {"id": "q15", "type": "multi", "prompt": "Which of these elements do NOT normally form simple ions? Select all that apply.", "options": ["carbon", "sodium", "argon", "oxygen", "silicon", "calcium"], "answer": [0, 2, 4], "marks": 1, "mark_scheme": "carbon, argon and silicon only", "explanation": "Group IV atoms (C, Si) would have to gain or lose four electrons, which costs too much energy, and noble gases (Ar) already have full outer shells. Sodium forms Na⁺, oxygen O²⁻ and calcium Ca²⁺.", "atom_ids": ["0620-3.1-A19", "0620-3.1-A18"], "template_id": null, "misconception_id": null, "difficulty": 2}, {"id": "q16", "type": "mcq", "prompt": "What does the (II) in the name copper(II) chloride tell you?", "options": ["The copper ion has a charge of 2+, so the formula is CuCl₂.", "The chloride ion has a charge of 2−.", "The compound has an overall charge of 2+.", "There are two copper atoms in the formula."], "answer": 0, "marks": 1, "mark_scheme": "A: the charge (oxidation number) of the copper ion is 2+", "explanation": "The Roman numeral always refers to the metal and gives its positive ion charge. Chloride is always Cl⁻ (B), a compound is always neutral (C), and the numeral is not a count of atoms (D). Cu²⁺ needs two Cl⁻, so CuCl₂.", "atom_ids": ["0620-3.1-A20"], "template_id": "T24", "misconception_id": "0620-3.1-M08", "difficulty": 2}, {"id": "q17", "type": "mcq", "prompt": "Which is the formula of the nitrate ion?", "options": ["NO₃²⁻", "NO₃⁻", "N³⁻", "NH₄⁺"], "answer": 1, "marks": 1, "mark_scheme": "B (NO₃⁻)", "explanation": "Nitrate is NO₃⁻ with a single negative charge; giving it 2− (A) is a common slip from confusing it with carbonate and sulfate. N³⁻ is the nitride ion (C) and NH₄⁺ is ammonium (D).", "atom_ids": ["0620-3.1-A22", "0620-3.1-A24"], "template_id": "T24", "misconception_id": "0620-3.1-M06", "difficulty": 2}, {"id": "q18", "type": "mcq", "prompt": "Ethanoic acid, CH₃COOH, forms the ethanoate ion. Which is the formula of calcium ethanoate?", "options": ["CaCH₃COO", "(CH₃COO)₂Ca", "CH₃COOCa₂", "(CH₃COOH)₂Ca"], "answer": 1, "marks": 1, "mark_scheme": "B ((CH₃COO)₂Ca)", "explanation": "The acid loses the H⁺ from its COOH group, giving CH₃COO⁻. Calcium is Ca²⁺, so two ethanoate ions are needed and they go in a bracket. D keeps the acidic hydrogen; A and C are not neutral.", "atom_ids": ["0620-3.1-A23", "0620-3.1-A18"], "template_id": "T26", "misconception_id": null, "difficulty": 3}], "atoms": {"0620-3.1-A18": "Main-group ion charge follows the group: I 1+, II 2+, Al 3+, V 3-, VI 2-, VII 1-.", "0620-3.1-A24": "Name endings: -ide means a single element (or hydroxide), -ate means the ion contains oxygen; sulfide S2- and sulfate SO4 2- are different ions.", "0620-3.1-A22": "Compound ions to know: NH4+, OH-, NO3-, CO3 2-, SO4 2-, HCO3-, PO4 3-.", "0620-3.1-A19": "Group IV and Group VIII elements do not normally form simple ions; hydrogen forms H+.", "0620-3.1-A20": "A Roman numeral in a name gives the oxidation number of the metal, equal to its ion charge: iron(II) Fe2+, iron(III) Fe3+, copper(II) Cu2+; it is always positive and refers to the metal only.", "0620-3.1-M08": "the Roman numeral is the charge of the compound, of the non-metal, a valency, or just the number 2.", "0620-3.1-M06": "compound ions can be guessed: nitrate with a 2- charge, ammonium as NH3, sulfide and sulfate interchangeable, nitric acid as H2NO4.", "0620-3.1-A23": "A carboxylic acid RCOOH gives the anion RCOO- (ethanoate CH3COO-), so its salts are written like other salts: (CH3COO)2Ca."}}
+{"id": "0620_ch04_q03", "chapterId": "0620_ch04", "subject": "Chemistry", "code": "0620", "chapter": "Chemical formulae and equations", "topicId": "t3", "topicTitle": "Ions and their charges", "quizNo": 3, "quizCount": 7, "order": 3, "title": "Ions and their charges", "kind": "topic", "intro": "Practise working out ion charges from the Periodic Table and recalling the compound ions.", "minutes": 6, "active": true, "questions": [{"id": "q13", "type": "mcq", "prompt": "Selenium is in Group VI of the Periodic Table. What is the formula of the selenide ion?", "options": ["Se⁶⁺", "Se²⁺", "Se²⁻", "Se⁶⁻"], "answer": 2, "marks": 1, "mark_scheme": "C (Se²⁻)", "explanation": "A Group VI atom has six outer electrons and gains two to fill its outer shell, giving a 2− ion, just like oxide O²⁻ and sulfide S²⁻. Non-metals form negative ions, and losing or gaining six electrons is far too costly.", "atom_ids": ["0620-3.1-A18"], "template_id": "T25", "misconception_id": null, "difficulty": 1}, {"id": "q14", "type": "mcq", "prompt": "Which pair of names and formulae are BOTH correct?", "options": ["Na₂S sodium sulfate; Na₂SO₄ sodium sulfide", "Na₂S sodium sulfide; Na₂SO₄ sodium sulfate", "Na₂SO₄ sodium sulfide; NaS sodium sulfate", "Na₂S sodium sulfide; NaSO₄ sodium sulfate"], "answer": 1, "marks": 1, "mark_scheme": "B", "explanation": "-ide means the single element (S²⁻); -ate means an oxygen-containing ion (SO₄²⁻). Both ions are 2−, so both salts need two Na⁺.", "atom_ids": ["0620-3.1-A24", "0620-3.1-A22"], "template_id": "T24", "misconception_id": null, "difficulty": 1}, {"id": "q15", "type": "multi", "prompt": "Which of these elements do NOT normally form simple ions? Select all that apply.", "options": ["carbon", "sodium", "argon", "oxygen", "silicon", "calcium"], "answer": [0, 2, 4], "marks": 1, "mark_scheme": "carbon, argon and silicon only", "explanation": "Group IV atoms (C, Si) would have to gain or lose four electrons, which costs too much energy, and noble gases (Ar) already have full outer shells. Sodium forms Na⁺, oxygen O²⁻ and calcium Ca²⁺.", "atom_ids": ["0620-3.1-A19", "0620-3.1-A18"], "template_id": null, "misconception_id": null, "difficulty": 2}, {"id": "q16", "type": "mcq", "prompt": "What does the (II) in the name copper(II) chloride tell you?", "options": ["The copper ion has a charge of 2+, so the formula is CuCl₂.", "The chloride ion has a charge of 2−.", "The compound has an overall charge of 2+.", "There are two copper atoms in the formula."], "answer": 0, "marks": 1, "mark_scheme": "A: the charge (oxidation number) of the copper ion is 2+", "explanation": "In copper(II) chloride the Roman numeral is the oxidation number of copper, which for a metal ion equals its charge: Cu²⁺. Chloride is always Cl⁻ (B), a compound is always neutral (C), and the numeral is not a count of atoms (D). Cu²⁺ needs two Cl⁻, so CuCl₂.", "atom_ids": ["0620-3.1-A20"], "template_id": "T24", "misconception_id": "0620-3.1-M08", "difficulty": 2}, {"id": "q17", "type": "mcq", "prompt": "Which is the formula of the nitrate ion?", "options": ["NO₃²⁻", "NO₃⁻", "N³⁻", "NH₄⁺"], "answer": 1, "marks": 1, "mark_scheme": "B (NO₃⁻)", "explanation": "Nitrate is NO₃⁻ with a single negative charge; giving it 2− (A) is a common slip from confusing it with carbonate and sulfate. N³⁻ is the nitride ion (C) and NH₄⁺ is ammonium (D).", "atom_ids": ["0620-3.1-A22", "0620-3.1-A24"], "template_id": "T24", "misconception_id": "0620-3.1-M06", "difficulty": 2}, {"id": "q18", "type": "mcq", "prompt": "Ethanoic acid, CH₃COOH, forms the ethanoate ion. Which is the formula of calcium ethanoate?", "options": ["CaCH₃COO", "(CH₃COO)₂Ca", "CH₃COOCa₂", "(CH₃COOH)₂Ca"], "answer": 1, "marks": 1, "mark_scheme": "B ((CH₃COO)₂Ca)", "explanation": "The acid loses the H⁺ from its COOH group, giving CH₃COO⁻. Calcium is Ca²⁺, so two ethanoate ions are needed and they go in a bracket. D keeps the acidic hydrogen; A and C are not neutral.", "atom_ids": ["0620-3.1-A23", "0620-3.1-A18"], "template_id": "T26", "misconception_id": null, "difficulty": 3}], "atoms": {"0620-3.1-A18": "Main-group ion charge follows the group: I 1+, II 2+, Al 3+, V 3-, VI 2-, VII 1-.", "0620-3.1-A24": "Name endings: -ide means a single element (or hydroxide), -ate means the ion contains oxygen; sulfide S2- and sulfate SO4 2- are different ions.", "0620-3.1-A22": "Compound ions to know: NH4+, OH-, NO3-, CO3 2-, SO4 2-, HCO3-, PO4 3-.", "0620-3.1-A19": "Group IV and Group VIII elements do not normally form simple ions; hydrogen forms H+.", "0620-3.1-A20": "A Roman numeral in a name gives the oxidation number of the element it follows; for a metal in a simple ionic compound this equals the charge on the metal ion, e.g. iron(III) = Fe3+, copper(II) = Cu2+ (but silicon(IV) oxide is not ionic).", "0620-3.1-M08": "the Roman numeral is the charge of the compound, of the non-metal, a valency, or just the number 2.", "0620-3.1-M06": "compound ions can be guessed: nitrate with a 2- charge, ammonium as NH3, sulfide and sulfate interchangeable, nitric acid as H2NO4.", "0620-3.1-A23": "A carboxylic acid RCOOH gives the anion RCOO- (ethanoate CH3COO-), so its salts are written like other salts: (CH3COO)2Ca."}}

data/sets/0620_ch04_q04.json

@@ -1 +1 @@
-{"id": "0620_ch04_q04", "chapterId": "0620_ch04", "subject": "Chemistry", "code": "0620", "chapter": "Chemical formulae and equations", "topicId": "t4", "topicTitle": "Formulae of ionic compounds", "quizNo": 4, "quizCount": 7, "order": 4, "title": "Formulae of ionic compounds", "kind": "topic", "intro": "Practise building neutral formulae from ion charges, and reading charges back from a formula.", "minutes": 8, "active": true, "questions": [{"id": "q19", "type": "mcq", "prompt": "What is the formula of iron(III) sulfate?", "options": ["FeSO₄", "Fe₂(SO₄)₃", "Fe₃(SO₄)₂", "Fe(SO₄)₃"], "answer": 1, "marks": 1, "mark_scheme": "Fe₂(SO₄)₃", "explanation": "Fe³⁺ and SO₄²⁻: the LCM of 3 and 2 is 6, so two Fe³⁺ (6+) balance three SO₄²⁻ (6−). FeSO₄ is iron(II) sulfate; the other two are not neutral.", "atom_ids": ["0620-3.1-A20", "0620-3.1-A22", "0620-3.1-A26"], "template_id": "T24", "misconception_id": null, "difficulty": 1}, {"id": "q20", "type": "mcq", "prompt": "Which is the formula of the pink crystals of hydrated cobalt(II) chloride?", "options": ["CoCl₂", "CoCl₂·5H₂O", "CoCl₂·6H₂O", "Co₂Cl·6H₂O"], "answer": 2, "marks": 1, "mark_scheme": "C", "explanation": "Hydrated cobalt(II) chloride has six waters of crystallisation (copper(II) sulfate has five). CoCl₂ alone is the anhydrous blue solid; Co₂Cl ignores the 2+ charge.", "atom_ids": ["0620-3.1-A35", "0620-3.1-A09"], "template_id": "T28", "misconception_id": null, "difficulty": 1}, {"id": "q21", "type": "mcq", "prompt": "Element X is in Group II and element Y is in Group V of the Periodic Table. They form an ionic compound. What is its formula?", "options": ["X₂Y₅", "XY", "X₃Y₂", "X₂Y₃"], "answer": 2, "marks": 1, "mark_scheme": "C", "explanation": "Group II gives X²⁺ and Group V gives Y³⁻; three X²⁺ (6+) balance two Y³⁻ (6−). X₂Y₅ uses the group numbers as subscripts.", "atom_ids": ["0620-3.1-A18", "0620-3.1-A26"], "template_id": "T25", "misconception_id": "0620-3.1-M09", "difficulty": 2}, {"id": "q22", "type": "mcq", "prompt": "Ammonium phosphate contains NH₄⁺ and PO₄³⁻ ions. What is its formula?", "options": ["NH₄PO₄", "(NH₄)₃PO₄", "NH₄₃PO₄", "NH₄(PO₄)₃"], "answer": 1, "marks": 1, "mark_scheme": "B ((NH₄)₃PO₄)", "explanation": "Three NH₄⁺ (3+) balance one PO₄³⁻ (3−). Because the ammonium ion is used more than once it needs a bracket: NH₄₃ in C reads as 43 hydrogen atoms. A is not neutral (1+ and 3−), and D gets the ratio upside down.", "atom_ids": ["0620-3.1-A26", "0620-3.1-A25", "0620-3.1-A07"], "template_id": "T25", "misconception_id": null, "difficulty": 2}, {"id": "q23", "type": "mcq", "prompt": "Calcium (Group II) forms a compound with the formula Ca₃(XO₄)₂, where XO₄ is a compound ion. What is the charge on the XO₄ ion?", "options": ["1−", "2−", "3−", "6−"], "answer": 2, "marks": 1, "mark_scheme": "C", "explanation": "Three Ca²⁺ give 6+, which must be balanced by two XO₄ ions, so each is 3− (like phosphate, PO₄³⁻). 6− is the total, not the charge on one ion.", "atom_ids": ["0620-3.1-A29", "0620-3.1-A25"], "template_id": "T26", "misconception_id": null, "difficulty": 2}, {"id": "q24", "type": "mcq", "prompt": "A model of part of an ionic lattice contains 12 ions of a Group II metal X and 8 ions of a non-metal Y. What is the formula of the compound, and what is the charge on one Y ion?", "options": ["X₁₂Y₈; Y is 3−", "X₃Y₂; Y is 3−", "X₃Y₂; Y is 2−", "X₂Y₃; Y is 2−"], "answer": 1, "marks": 1, "mark_scheme": "B: ratio 12 : 8 = 3 : 2, so X₃Y₂; 3 × 2+ = 6+ shared by two Y, so Y is 3−", "explanation": "A formula gives the simplest whole-number ratio, so 12 : 8 cancels to 3 : 2 (A is not simplified). Three X²⁺ give 6+, so the two Y ions carry 6− between them: 3− each, a Group V ion like N³⁻ in Mg₃N₂.", "atom_ids": ["0620-3.1-A31", "0620-3.1-A27", "0620-3.1-A25"], "template_id": "T27", "misconception_id": null, "difficulty": 2}, {"id": "q25", "type": "mcq", "prompt": "Sodium sulfite has the formula Na₂SO₃. Which gives the formula of the sulfite ion and of the acid it comes from?", "options": ["SO₃²⁻ and H₂SO₃", "SO₃⁻ and HSO₃", "SO₄²⁻ and H₂SO₄", "SO₃²⁻ and H₂SO₄"], "answer": 0, "marks": 1, "mark_scheme": "A", "explanation": "Two Na⁺ give 2+, so the anion is SO₃²⁻. The parent acid has each Na⁺ replaced by H⁺: H₂SO₃. C and D swap in sulfate, but the formula printed in the question has only three oxygen atoms.", "atom_ids": ["0620-3.1-A30", "0620-3.1-A29"], "template_id": "T26", "misconception_id": null, "difficulty": 2}, {"id": "q26", "type": "mcq", "prompt": "An oxide of iron contains both Fe²⁺ and Fe³⁺ ions, together with O²⁻ ions. Which formula fits this description?", "options": ["FeO", "Fe₂O₃", "Fe₃O₄", "Fe₃O₂"], "answer": 2, "marks": 1, "mark_scheme": "C: one Fe²⁺ + two Fe³⁺ = 8+, balanced by four O²⁻ = 8−", "explanation": "FeO and Fe₂O₃ are neutral, but each contains only one kind of iron ion, so they do not fit the description. Fe₃O₄ is neutral with one Fe²⁺ and two Fe³⁺. Fe₃O₂ cannot be neutral: three iron ions give at least 6+, but two O²⁻ give only 4−.", "atom_ids": ["0620-3.1-A32", "0620-3.1-A25"], "template_id": "T25", "misconception_id": "0620-3.1-M10", "difficulty": 3}], "atoms": {"0620-3.1-A20": "A Roman numeral in a name gives the oxidation number of the metal, equal to its ion charge: iron(II) Fe2+, iron(III) Fe3+, copper(II) Cu2+; it is always positive and refers to the metal only.", "0620-3.1-A22": "Compound ions to know: NH4+, OH-, NO3-, CO3 2-, SO4 2-, HCO3-, PO4 3-.", "0620-3.1-A26": "Formula from charges: write both ions, find the lowest common multiple of the charge sizes, take enough of each ion to reach it, bracket a compound ion used more than once (Al2(SO4)3).", "0620-3.1-A35": "Hydrated copper(II) sulfate is CuSO4.5H2O (blue) and hydrated cobalt(II) chloride is CoCl2.6H2O (pink).", "0620-3.1-A09": "The raised dot in a hydrated salt (CuSO4.5H2O) means together with: whole water molecules are added, not multiplied.", "0620-3.1-A18": "Main-group ion charge follows the group: I 1+, II 2+, Al 3+, V 3-, VI 2-, VII 1-.", "0620-3.1-M09": "the group numbers of two elements are the subscripts of their compound (Group V + Group VII gives P5Q7).", "0620-3.1-A25": "An ionic compound is neutral: total positive charge equals total negative charge.", "0620-3.1-A07": "A subscript after a bracket multiplies everything inside; brackets are used only when a compound ion appears more than once (Ca(NO3)2, but NaNO3).", "0620-3.1-A29": "Inverse: find an unknown ion charge from a formula and the known ion (Cr2(SO4)3 gives Cr3+).", "0620-3.1-A31": "Deduce a formula from a model or lattice diagram by counting the relative numbers of each particle, checking neutrality when charges are shown.", "0620-3.1-A27": "The swap-the-charges shortcut must be followed by simplifying (Mg2O2 becomes MgO).", "0620-3.1-A30": "Read the anion from a salt formula, and the parent acid by replacing the metal ions with H+ (Na2SO3 gives SO3 2- and H2SO3).", "0620-3.1-A32": "For a compound containing two ions of one element (mixed valence), choose the formula that is both neutral and contains every stated ion.", "0620-3.1-M10": "any neutral formula fits a description of the ions present."}}
+{"id": "0620_ch04_q04", "chapterId": "0620_ch04", "subject": "Chemistry", "code": "0620", "chapter": "Chemical formulae and equations", "topicId": "t4", "topicTitle": "Formulae of ionic compounds", "quizNo": 4, "quizCount": 7, "order": 4, "title": "Formulae of ionic compounds", "kind": "topic", "intro": "Practise building neutral formulae from ion charges, and reading charges back from a formula.", "minutes": 8, "active": true, "questions": [{"id": "q19", "type": "mcq", "prompt": "What is the formula of iron(III) sulfate?", "options": ["FeSO₄", "Fe₂(SO₄)₃", "Fe₃(SO₄)₂", "Fe(SO₄)₃"], "answer": 1, "marks": 1, "mark_scheme": "Fe₂(SO₄)₃", "explanation": "Fe³⁺ and SO₄²⁻: the LCM of 3 and 2 is 6, so two Fe³⁺ (6+) balance three SO₄²⁻ (6−). FeSO₄ is iron(II) sulfate; the other two are not neutral.", "atom_ids": ["0620-3.1-A20", "0620-3.1-A22", "0620-3.1-A26"], "template_id": "T24", "misconception_id": null, "difficulty": 1}, {"id": "q20", "type": "mcq", "prompt": "Which is the formula of the pink crystals of hydrated cobalt(II) chloride?", "options": ["CoCl₂", "CoCl₂·5H₂O", "CoCl₂·6H₂O", "Co₂Cl·6H₂O"], "answer": 2, "marks": 1, "mark_scheme": "C", "explanation": "Hydrated cobalt(II) chloride has six waters of crystallisation (copper(II) sulfate has five). CoCl₂ alone is the anhydrous blue solid; Co₂Cl ignores the 2+ charge.", "atom_ids": ["0620-3.1-A35", "0620-3.1-A09"], "template_id": "T28", "misconception_id": null, "difficulty": 1}, {"id": "q21", "type": "mcq", "prompt": "Element X is in Group II and element Y is in Group V of the Periodic Table. They form an ionic compound. What is its formula?", "options": ["X₂Y₅", "XY", "X₃Y₂", "X₂Y₃"], "answer": 2, "marks": 1, "mark_scheme": "C", "explanation": "Group II gives X²⁺ and Group V gives Y³⁻; three X²⁺ (6+) balance two Y³⁻ (6−). X₂Y₅ uses the group numbers as subscripts.", "atom_ids": ["0620-3.1-A18", "0620-3.1-A26"], "template_id": "T25", "misconception_id": "0620-3.1-M09", "difficulty": 2}, {"id": "q22", "type": "mcq", "prompt": "Ammonium phosphate contains NH₄⁺ and PO₄³⁻ ions. What is its formula?", "options": ["NH₄PO₄", "(NH₄)₃PO₄", "NH₄₃PO₄", "NH₄(PO₄)₃"], "answer": 1, "marks": 1, "mark_scheme": "B ((NH₄)₃PO₄)", "explanation": "Three NH₄⁺ (3+) balance one PO₄³⁻ (3−). Because the ammonium ion is used more than once it needs a bracket: NH₄₃ in C reads as 43 hydrogen atoms. A is not neutral (1+ and 3−), and D gets the ratio upside down.", "atom_ids": ["0620-3.1-A26", "0620-3.1-A25", "0620-3.1-A07"], "template_id": "T25", "misconception_id": null, "difficulty": 2}, {"id": "q23", "type": "mcq", "prompt": "Calcium (Group II) forms a compound with the formula Ca₃(XO₄)₂, where XO₄ is a compound ion. What is the charge on the XO₄ ion?", "options": ["1−", "2−", "3−", "6−"], "answer": 2, "marks": 1, "mark_scheme": "C", "explanation": "Three Ca²⁺ give 6+, which must be balanced by two XO₄ ions, so each is 3− (like phosphate, PO₄³⁻). 6− is the total, not the charge on one ion.", "atom_ids": ["0620-3.1-A29", "0620-3.1-A25"], "template_id": "T26", "misconception_id": null, "difficulty": 2}, {"id": "q24", "type": "mcq", "prompt": "A model of part of an ionic lattice contains 12 ions of a Group II metal X and 8 ions of a non-metal Y. What is the formula of the compound, and what is the charge on one Y ion?", "options": ["X₁₂Y₈; Y is 3−", "X₃Y₂; Y is 3−", "X₃Y₂; Y is 2−", "X₂Y₃; Y is 2−"], "answer": 1, "marks": 1, "mark_scheme": "B: ratio 12 : 8 = 3 : 2, so X₃Y₂; 3 × 2+ = 6+ shared by two Y, so Y is 3−", "explanation": "A formula gives the simplest whole-number ratio, so 12 : 8 cancels to 3 : 2 (A is not simplified). Three X²⁺ give 6+, so the two Y ions carry 6− between them: 3− each, a Group V ion like N³⁻ in Mg₃N₂.", "atom_ids": ["0620-3.1-A31", "0620-3.1-A27", "0620-3.1-A25"], "template_id": "T27", "misconception_id": null, "difficulty": 2}, {"id": "q25", "type": "mcq", "prompt": "Sodium sulfite has the formula Na₂SO₃. Which gives the formula of the sulfite ion and of the acid it comes from?", "options": ["SO₃²⁻ and H₂SO₃", "SO₃⁻ and HSO₃", "SO₄²⁻ and H₂SO₄", "SO₃²⁻ and H₂SO₄"], "answer": 0, "marks": 1, "mark_scheme": "A", "explanation": "Two Na⁺ give 2+, so the anion is SO₃²⁻. The parent acid has each Na⁺ replaced by H⁺: H₂SO₃. C and D swap in sulfate, but the formula printed in the question has only three oxygen atoms.", "atom_ids": ["0620-3.1-A30", "0620-3.1-A29"], "template_id": "T26", "misconception_id": null, "difficulty": 2}, {"id": "q26", "type": "mcq", "prompt": "An oxide of iron contains both Fe²⁺ and Fe³⁺ ions, together with O²⁻ ions. Which formula fits this description?", "options": ["FeO", "Fe₂O₃", "Fe₃O₄", "Fe₃O₂"], "answer": 2, "marks": 1, "mark_scheme": "C: one Fe²⁺ + two Fe³⁺ = 8+, balanced by four O²⁻ = 8−", "explanation": "FeO and Fe₂O₃ are neutral, but each contains only one kind of iron ion, so they do not fit the description. Fe₃O₄ is neutral with one Fe²⁺ and two Fe³⁺. Fe₃O₂ cannot be neutral: three iron ions give at least 6+, but two O²⁻ give only 4−.", "atom_ids": ["0620-3.1-A32", "0620-3.1-A25"], "template_id": "T25", "misconception_id": "0620-3.1-M10", "difficulty": 3}], "atoms": {"0620-3.1-A20": "A Roman numeral in a name gives the oxidation number of the element it follows; for a metal in a simple ionic compound this equals the charge on the metal ion, e.g. iron(III) = Fe3+, copper(II) = Cu2+ (but silicon(IV) oxide is not ionic).", "0620-3.1-A22": "Compound ions to know: NH4+, OH-, NO3-, CO3 2-, SO4 2-, HCO3-, PO4 3-.", "0620-3.1-A26": "Formula from charges: write both ions, find the lowest common multiple of the charge sizes, take enough of each ion to reach it, bracket a compound ion used more than once (Al2(SO4)3).", "0620-3.1-A35": "Hydrated copper(II) sulfate is CuSO4.5H2O (blue) and hydrated cobalt(II) chloride is CoCl2.6H2O (pink).", "0620-3.1-A09": "The raised dot in a hydrated salt (CuSO4.5H2O) means together with: whole water molecules are added, not multiplied.", "0620-3.1-A18": "Main-group ion charge follows the group: I 1+, II 2+, Al 3+, V 3-, VI 2-, VII 1-.", "0620-3.1-M09": "the group numbers of two elements are the subscripts of their compound (Group V + Group VII gives P5Q7).", "0620-3.1-A25": "An ionic compound is neutral: total positive charge equals total negative charge.", "0620-3.1-A07": "A subscript after a bracket multiplies everything inside; brackets are used only when a compound ion appears more than once (Ca(NO3)2, but NaNO3).", "0620-3.1-A29": "Inverse: find an unknown ion charge from a formula and the known ion (Cr2(SO4)3 gives Cr3+).", "0620-3.1-A31": "Deduce a formula from a model or lattice diagram by counting the relative numbers of each particle, checking neutrality when charges are shown.", "0620-3.1-A27": "The swap-the-charges shortcut must be followed by simplifying (Mg2O2 becomes MgO).", "0620-3.1-A30": "Read the anion from a salt formula, and the parent acid by replacing the metal ions with H+ (Na2SO3 gives SO3 2- and H2SO3).", "0620-3.1-A32": "For a compound containing two ions of one element (mixed valence), choose the formula that is both neutral and contains every stated ion.", "0620-3.1-M10": "any neutral formula fits a description of the ions present."}}

data/sets/0620_ch04_q07.json

@@ -1 +1 @@
-{"id": "0620_ch04_q07", "chapterId": "0620_ch04", "subject": "Chemistry", "code": "0620", "chapter": "Chemical formulae and equations", "topicId": null, "topicTitle": "Mixed review", "quizNo": 7, "quizCount": 7, "order": 7, "title": "Mixed review", "kind": "mixed", "intro": "One question or more from every topic, including the traps examiners see most often.", "minutes": 8, "active": true, "questions": [{"id": "q42", "type": "mcq", "prompt": "A student writes 2Na(s) + Cl(g) → 2NaCl(s) for sodium burning in chlorine. What is the error?", "options": ["Chlorine must be written as Cl₂ because it is diatomic: 2Na(s) + Cl₂(g) → 2NaCl(s).", "Sodium chloride should be written NaCl₂.", "Sodium should be written as Na₂.", "There is no error."], "answer": 0, "marks": 1, "mark_scheme": "A", "explanation": "Chlorine exists as Cl₂ molecules, so the free element is always written Cl₂. With Cl₂ the equation also balances. Na⁺ and Cl⁻ give NaCl (B is not neutral), and sodium is a metal, written as single atoms (C).", "atom_ids": ["0620-3.1-A04", "0620-3.1-A40"], "template_id": "T29", "misconception_id": "0620-3.1-M01", "difficulty": 1}, {"id": "q43", "type": "mcq", "prompt": "Hydrazine has the molecular formula N₂H₄. What is its empirical formula?", "options": ["N₂H₄", "NH₂", "NH₄", "N₂H₂"], "answer": 1, "marks": 1, "mark_scheme": "B (NH₂)", "explanation": "The highest common factor of 2 and 4 is 2, so N₂H₄ cancels to NH₂. Repeating N₂H₄ gives the molecular formula, not the simplest ratio; NH₄ and N₂H₂ divide only one subscript.", "atom_ids": ["0620-3.1-A16", "0620-3.1-A14"], "template_id": "T16", "misconception_id": "0620-3.1-M15", "difficulty": 1}, {"id": "q44", "type": "mcq", "prompt": "A question asks: Write a chemical equation for the reaction of zinc with dilute hydrochloric acid. Which answer earns the marks?", "options": ["zinc + hydrochloric acid → zinc chloride + hydrogen", "Zn + 2HCl → ZnCl₂ + H₂", "Zn + HCl → ZnCl + H", "Zinc reacts with the acid to give a salt and hydrogen gas."], "answer": 1, "marks": 1, "mark_scheme": "B", "explanation": "A chemical equation means a balanced symbol equation. A word equation (A) or a sentence (D) scores nothing here. C has two wrong formulae: zinc forms Zn²⁺, so ZnCl₂, and hydrogen is H₂.", "atom_ids": ["0620-3.1-A37", "0620-3.1-A36", "0620-3.1-A48"], "template_id": "T29", "misconception_id": "0620-3.1-M18", "difficulty": 1}, {"id": "q45", "type": "mcq", "prompt": "Titanium forms the titanium(IV) ion. What is the formula of titanium(IV) sulfate?", "options": ["TiSO₄", "Ti(SO₄)₂", "Ti₂(SO₄)₄", "Ti₂(SO₄)₃"], "answer": 1, "marks": 1, "mark_scheme": "B (Ti(SO₄)₂)", "explanation": "Ti⁴⁺ needs two SO₄²⁻ to be neutral. TiSO₄ treats titanium as 2+; Ti₂(SO₄)₄ is neutral but not the simplest ratio (the swap-the-charges shortcut must be cancelled); Ti₂(SO₄)₃ treats titanium as 3+.", "atom_ids": ["0620-3.1-A20", "0620-3.1-A26", "0620-3.1-A27"], "template_id": "T24", "misconception_id": null, "difficulty": 2}, {"id": "q46", "type": "mcq", "prompt": "The molybdate ion has the formula MoO₄²⁻. What is the formula of aluminium molybdate?", "options": ["AlMoO₄", "Al₂(MoO₄)₃", "Al₃(MoO₄)₂", "Al(MoO₄)₃"], "answer": 1, "marks": 1, "mark_scheme": "Al₂(MoO₄)₃", "explanation": "Treat the unfamiliar ion like sulfate: Al³⁺ and MoO₄²⁻ need 6+ and 6−, so two Al and three MoO₄. Al₃(MoO₄)₂ has swapped the numbers the wrong way (9+ and 4−).", "atom_ids": ["0620-3.1-A28", "0620-3.1-A26", "0620-3.1-A18"], "template_id": "T25", "misconception_id": null, "difficulty": 2}, {"id": "q47", "type": "mcq", "prompt": "Chlorine gas is bubbled through aqueous potassium bromide and the solution turns orange. Which is the ionic equation?", "options": ["Cl₂(g) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)", "Cl₂²⁻(aq) + Br₂(aq) → Cl₂(g) + Br₂²⁻(aq)", "2Cl⁻(aq) + Br₂(aq) → Cl₂(g) + 2Br⁻(aq)", "Cl₂(g) + Br⁻(aq) → Cl⁻(aq) + Br₂(aq)"], "answer": 0, "marks": 1, "mark_scheme": "A", "explanation": "The more reactive chlorine takes electrons from bromide ions; K⁺ is a spectator. The halogens stay as neutral molecules (B puts charges on them), bromide is the reactant, not the product (C), and D is not balanced for Cl or Br atoms.", "atom_ids": ["0620-3.1-A54", "0620-3.1-A55"], "template_id": "T36", "misconception_id": "0620-3.1-M14", "difficulty": 2}, {"id": "q48", "type": "short", "prompt": "Magnesium burns in nitrogen to form magnesium nitride, which contains Mg²⁺ and N³⁻ ions. Write the balanced symbol equation for this reaction.", "answer": null, "marks": 2, "mark_scheme": "Mg₃N₂ as the only product; balanced with N₂: 3Mg + N₂ → Mg₃N₂", "explanation": "Charges first: three Mg²⁺ balance two N³⁻, so Mg₃N₂. Nitrogen is diatomic, so one N₂ supplies both N atoms and three Mg atoms are needed.", "atom_ids": ["0620-3.1-A47", "0620-3.1-A26", "0620-3.1-A04", "0620-3.1-A40"], "template_id": "T30", "misconception_id": null, "difficulty": 3}], "atoms": {"0620-3.1-A04": "Seven elements exist as diatomic molecules and are written H2, N2, O2, F2, Cl2, Br2, I2 whenever they appear as the element.", "0620-3.1-A40": "Balancing order: an element found in one substance each side first (largest formula first), compound ions as units, free elements last; recount everything and use the smallest whole numbers.", "0620-3.1-M01": "the free element can be written as a single atom (O, H, N, Cl, Br) in an equation.", "0620-3.1-A16": "Find an empirical formula from a molecular formula by dividing every subscript by their highest common factor; if it is 1 the two formulae are the same.", "0620-3.1-A14": "Empirical formula: the simplest whole-number ratio of the different atoms or ions in a compound.", "0620-3.1-M15": "the molecular formula is the empirical formula.", "0620-3.1-A37": "A word equation names reactants and products (reactants -> products); it is not accepted where a chemical or symbol equation is asked for.", "0620-3.1-A36": "When a formula is asked for, give a formula, not a name; when a formula is printed in the question, copy it exactly.", "0620-3.1-A48": "Acid patterns: acid + metal -> salt + hydrogen; acid + base -> salt + water; acid + carbonate -> salt + water + carbon dioxide; reactive metal + water -> hydroxide + hydrogen.", "0620-3.1-M18": "a name or word equation answers a request for a formula or chemical equation.", "0620-3.1-A20": "A Roman numeral in a name gives the oxidation number of the metal, equal to its ion charge: iron(II) Fe2+, iron(III) Fe3+, copper(II) Cu2+; it is always positive and refers to the metal only.", "0620-3.1-A26": "Formula from charges: write both ions, find the lowest common multiple of the charge sizes, take enough of each ion to reach it, bracket a compound ion used more than once (Al2(SO4)3).", "0620-3.1-A27": "The swap-the-charges shortcut must be followed by simplifying (Mg2O2 becomes MgO).", "0620-3.1-A28": "Apply the charge method to supplied or unfamiliar ions, and to compounds with three kinds of ion, by totalling all positive and all negative charges.", "0620-3.1-A18": "Main-group ion charge follows the group: I 1+, II 2+, Al 3+, V 3-, VI 2-, VII 1-.", "0620-3.1-A54": "Displacement ionic equations: metal + metal ion (Fe + Cu2+ -> Fe2+ + Cu) or halogen molecule + halide ion (Cl2 + 2I- -> 2Cl- + I2); the halogens stay diatomic and the Group I cations are spectators.", "0620-3.1-A55": "Balance an ionic equation for charge as well as atoms (2Al + 3Cu2+ -> 2Al3+ + 3Cu has 6+ on each side).", "0620-3.1-M14": "in a halogen displacement the halogen molecules carry charges, the ions swap sides, or the reaction is written as a precipitation or half-equation.", "0620-3.1-A47": "Deduce an equation from information: list the named species, write formulae (copy given ones), deduce unnamed products from a reaction pattern or the atom count, balance, add state symbols if asked."}}
+{"id": "0620_ch04_q07", "chapterId": "0620_ch04", "subject": "Chemistry", "code": "0620", "chapter": "Chemical formulae and equations", "topicId": null, "topicTitle": "Mixed review", "quizNo": 7, "quizCount": 7, "order": 7, "title": "Mixed review", "kind": "mixed", "intro": "One question or more from every topic, including the traps examiners see most often.", "minutes": 8, "active": true, "questions": [{"id": "q42", "type": "mcq", "prompt": "A student writes 2Na(s) + Cl(g) → 2NaCl(s) for sodium burning in chlorine. What is the error?", "options": ["Chlorine must be written as Cl₂ because it is diatomic: 2Na(s) + Cl₂(g) → 2NaCl(s).", "Sodium chloride should be written NaCl₂.", "Sodium should be written as Na₂.", "There is no error."], "answer": 0, "marks": 1, "mark_scheme": "A", "explanation": "Chlorine exists as Cl₂ molecules, so the free element is always written Cl₂. With Cl₂ the equation also balances. Na⁺ and Cl⁻ give NaCl (B is not neutral), and sodium is a metal, written as single atoms (C).", "atom_ids": ["0620-3.1-A04", "0620-3.1-A40"], "template_id": "T29", "misconception_id": "0620-3.1-M01", "difficulty": 1}, {"id": "q43", "type": "mcq", "prompt": "Hydrazine has the molecular formula N₂H₄. What is its empirical formula?", "options": ["N₂H₄", "NH₂", "NH₄", "N₂H₂"], "answer": 1, "marks": 1, "mark_scheme": "B (NH₂)", "explanation": "The highest common factor of 2 and 4 is 2, so N₂H₄ cancels to NH₂. Repeating N₂H₄ gives the molecular formula, not the simplest ratio; NH₄ and N₂H₂ divide only one subscript.", "atom_ids": ["0620-3.1-A16", "0620-3.1-A14"], "template_id": "T16", "misconception_id": "0620-3.1-M15", "difficulty": 1}, {"id": "q44", "type": "mcq", "prompt": "A question asks: Write a chemical equation for the reaction of zinc with dilute hydrochloric acid. Which answer earns the marks?", "options": ["zinc + hydrochloric acid → zinc chloride + hydrogen", "Zn + 2HCl → ZnCl₂ + H₂", "Zn + HCl → ZnCl + H", "Zinc reacts with the acid to give a salt and hydrogen gas."], "answer": 1, "marks": 1, "mark_scheme": "B", "explanation": "A chemical equation means a balanced symbol equation. A word equation (A) or a sentence (D) scores nothing here. C has two wrong formulae: zinc forms Zn²⁺, so ZnCl₂, and hydrogen is H₂.", "atom_ids": ["0620-3.1-A37", "0620-3.1-A36", "0620-3.1-A48"], "template_id": "T29", "misconception_id": "0620-3.1-M18", "difficulty": 1}, {"id": "q45", "type": "mcq", "prompt": "Titanium forms the titanium(IV) ion. What is the formula of titanium(IV) sulfate?", "options": ["TiSO₄", "Ti(SO₄)₂", "Ti₂(SO₄)₄", "Ti₂(SO₄)₃"], "answer": 1, "marks": 1, "mark_scheme": "B (Ti(SO₄)₂)", "explanation": "Ti⁴⁺ needs two SO₄²⁻ to be neutral. TiSO₄ treats titanium as 2+; Ti₂(SO₄)₄ is neutral but not the simplest ratio (the swap-the-charges shortcut must be cancelled); Ti₂(SO₄)₃ treats titanium as 3+.", "atom_ids": ["0620-3.1-A20", "0620-3.1-A26", "0620-3.1-A27"], "template_id": "T24", "misconception_id": null, "difficulty": 2}, {"id": "q46", "type": "mcq", "prompt": "The molybdate ion has the formula MoO₄²⁻. What is the formula of aluminium molybdate?", "options": ["AlMoO₄", "Al₂(MoO₄)₃", "Al₃(MoO₄)₂", "Al(MoO₄)₃"], "answer": 1, "marks": 1, "mark_scheme": "Al₂(MoO₄)₃", "explanation": "Treat the unfamiliar ion like sulfate: Al³⁺ and MoO₄²⁻ need 6+ and 6−, so two Al and three MoO₄. Al₃(MoO₄)₂ has swapped the numbers the wrong way (9+ and 4−).", "atom_ids": ["0620-3.1-A28", "0620-3.1-A26", "0620-3.1-A18"], "template_id": "T25", "misconception_id": null, "difficulty": 2}, {"id": "q47", "type": "mcq", "prompt": "Chlorine gas is bubbled through aqueous potassium bromide and the solution turns orange. Which is the ionic equation?", "options": ["Cl₂(g) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)", "Cl₂²⁻(aq) + Br₂(aq) → Cl₂(g) + Br₂²⁻(aq)", "2Cl⁻(aq) + Br₂(aq) → Cl₂(g) + 2Br⁻(aq)", "Cl₂(g) + Br⁻(aq) → Cl⁻(aq) + Br₂(aq)"], "answer": 0, "marks": 1, "mark_scheme": "A", "explanation": "The more reactive chlorine takes electrons from bromide ions; K⁺ is a spectator. The halogens stay as neutral molecules (B puts charges on them), bromide is the reactant, not the product (C), and D is not balanced for Cl or Br atoms.", "atom_ids": ["0620-3.1-A54", "0620-3.1-A55"], "template_id": "T36", "misconception_id": "0620-3.1-M14", "difficulty": 2}, {"id": "q48", "type": "short", "prompt": "Magnesium burns in nitrogen to form magnesium nitride, which contains Mg²⁺ and N³⁻ ions. Write the balanced symbol equation for this reaction.", "answer": null, "marks": 2, "mark_scheme": "Mg₃N₂ as the only product; balanced with N₂: 3Mg + N₂ → Mg₃N₂", "explanation": "Charges first: three Mg²⁺ balance two N³⁻, so Mg₃N₂. Nitrogen is diatomic, so one N₂ supplies both N atoms and three Mg atoms are needed.", "atom_ids": ["0620-3.1-A47", "0620-3.1-A26", "0620-3.1-A04", "0620-3.1-A40"], "template_id": "T30", "misconception_id": null, "difficulty": 3}], "atoms": {"0620-3.1-A04": "Seven elements exist as diatomic molecules and are written H2, N2, O2, F2, Cl2, Br2, I2 whenever they appear as the element.", "0620-3.1-A40": "Balancing order: an element found in one substance each side first (largest formula first), compound ions as units, free elements last; recount everything and use the smallest whole numbers.", "0620-3.1-M01": "the free element can be written as a single atom (O, H, N, Cl, Br) in an equation.", "0620-3.1-A16": "Find an empirical formula from a molecular formula by dividing every subscript by their highest common factor; if it is 1 the two formulae are the same.", "0620-3.1-A14": "Empirical formula: the simplest whole-number ratio of the different atoms or ions in a compound.", "0620-3.1-M15": "the molecular formula is the empirical formula.", "0620-3.1-A37": "A word equation names reactants and products (reactants -> products); it is not accepted where a chemical or symbol equation is asked for.", "0620-3.1-A36": "When a formula is asked for, give a formula, not a name; when a formula is printed in the question, copy it exactly.", "0620-3.1-A48": "Acid patterns: acid + metal -> salt + hydrogen; acid + base -> salt + water; acid + carbonate -> salt + water + carbon dioxide; reactive metal + water -> hydroxide + hydrogen.", "0620-3.1-M18": "a name or word equation answers a request for a formula or chemical equation.", "0620-3.1-A20": "A Roman numeral in a name gives the oxidation number of the element it follows; for a metal in a simple ionic compound this equals the charge on the metal ion, e.g. iron(III) = Fe3+, copper(II) = Cu2+ (but silicon(IV) oxide is not ionic).", "0620-3.1-A26": "Formula from charges: write both ions, find the lowest common multiple of the charge sizes, take enough of each ion to reach it, bracket a compound ion used more than once (Al2(SO4)3).", "0620-3.1-A27": "The swap-the-charges shortcut must be followed by simplifying (Mg2O2 becomes MgO).", "0620-3.1-A28": "Apply the charge method to supplied or unfamiliar ions, and to compounds with three kinds of ion, by totalling all positive and all negative charges.", "0620-3.1-A18": "Main-group ion charge follows the group: I 1+, II 2+, Al 3+, V 3-, VI 2-, VII 1-.", "0620-3.1-A54": "Displacement ionic equations: metal + metal ion (Fe + Cu2+ -> Fe2+ + Cu) or halogen molecule + halide ion (Cl2 + 2I- -> 2Cl- + I2); the halogens stay diatomic and the Group I cations are spectators.", "0620-3.1-A55": "Balance an ionic equation for charge as well as atoms (2Al + 3Cu2+ -> 2Al3+ + 3Cu has 6+ on each side).", "0620-3.1-M14": "in a halogen displacement the halogen molecules carry charges, the ions swap sides, or the reaction is written as a precipitation or half-equation.", "0620-3.1-A47": "Deduce an equation from information: list the named species, write formulae (copy given ones), deduce unnamed products from a reaction pattern or the atom count, balance, add state symbols if asked."}}

data/sets/0620_ch05_q03.json

@@ -1 +1 @@
-{"id": "0620_ch05_q03", "chapterId": "0620_ch05", "subject": "Chemistry", "code": "0620", "chapter": "Chemical calculations", "topicId": "t3", "topicTitle": "Reacting masses and limits", "quizNo": 3, "quizCount": 7, "order": 3, "title": "Reacting masses and limits", "kind": "topic", "intro": "Practise using equation ratios to predict masses and spotting which reactant runs out first.", "minutes": 9, "active": true, "questions": [{"id": "q13", "type": "numeric", "prompt": "Magnesium burns in oxygen: 2Mg + O₂ → 2MgO. The equation shows that 48 g of magnesium makes 80 g of magnesium oxide. What mass of magnesium oxide is made from 12 g of magnesium?", "answer": 20, "tolerance": 0.01, "unit": "g", "marks": 1, "mark_scheme": "20 g: 12 g is a quarter of 48 g, so a quarter of 80 g.", "explanation": "The mass ratio of substances in a reaction is fixed by the equation, so scale it. Check with conservation of mass: 48 g Mg + 32 g O₂ = 80 g MgO.", "atom_ids": ["0620-3.2-A12", "0620-3.2-A13"], "template_id": "T6", "misconception_id": null, "difficulty": 1}, {"id": "q14", "type": "mcq", "prompt": "A student adds zinc carbonate powder to dilute sulfuric acid until the zinc carbonate is in excess. Which observation shows that it is in excess?", "options": ["Bubbling stops and some solid stays undissolved", "All of the solid dissolves", "The bubbling becomes faster", "The solution turns from blue to colourless"], "answer": 0, "marks": 1, "mark_scheme": "Effervescence stops and solid remains.", "explanation": "When the acid (the limiting reactant) is used up, no more gas can form and the extra carbonate has nothing left to react with, so it stays as solid.", "atom_ids": ["0620-3.3-A27", "0620-3.3-A24"], "template_id": null, "misconception_id": null, "difficulty": 1}, {"id": "q15", "type": "mcq", "prompt": "Nitrogen reacts with hydrogen: N₂ + 3H₂ → 2NH₃. What is the maximum mass of ammonia made from 5.6 g of nitrogen? (Mr: N₂ = 28, NH₃ = 17)", "options": ["3.4 g", "6.8 g", "11.2 g", "13.6 g"], "answer": 1, "marks": 1, "mark_scheme": "6.8 g: moles N₂ = 0.200; moles NH₃ = 0.400; × 17.", "explanation": "Coefficients are mole ratios, not mass ratios. 11.2 g applies the 1 : 2 ratio straight to the mass; 3.4 g ignores the ratio.", "atom_ids": ["0620-3.2-A12", "0620-3.3-A23", "0620-3.3-A22"], "template_id": "T6", "misconception_id": "0620-3.2-M16", "difficulty": 2}, {"id": "q16", "type": "numeric", "prompt": "Iron(III) oxide is reduced by carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Calculate the mass of iron made from 40.0 g of iron(III) oxide. Use the following steps: moles of Fe₂O₃ (Mr = 160); moles of Fe; mass of Fe (Ar = 56).", "answer": 28.0, "tolerance": 0.01, "unit": "g", "marks": 2, "mark_scheme": "M1 moles Fe₂O₃ = 40.0 ÷ 160 = 0.250 mol; M2 moles Fe = 2 × 0.250 = 0.500 mol, mass = 0.500 × 56 = 28.0 g", "explanation": "Convert to moles, use the ratio (1 Fe₂O₃ : 2 Fe), convert back. 14.0 g means the ×2 ratio step was missed: the middle step is where most marks are lost.", "atom_ids": ["0620-3.3-A23", "0620-3.3-A22", "0620-3.3-A56"], "template_id": "T6", "misconception_id": null, "difficulty": 2}, {"id": "q17", "type": "short", "prompt": "0.50 g of magnesium is added to 20.0 cm³ of 1.00 mol/dm³ hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. Determine which reactant is limiting. Show your working and explain.", "answer": null, "marks": 2, "mark_scheme": "moles Mg = 0.50 ÷ 24 = 0.0208 and moles HCl = 0.0200 (both needed); 0.0208 mol Mg needs 0.0417 mol HCl (1 : 2 ratio), so HCl is limiting (Mg in excess)", "explanation": "Comparing 0.0208 with 0.0200 alone suggests magnesium is limiting; the 1 : 2 ratio means the acid runs out first.", "atom_ids": ["0620-3.3-A25", "0620-3.3-A24"], "template_id": "T13", "misconception_id": null, "difficulty": 3}, {"id": "q18", "type": "numeric", "prompt": "Aluminium reacts with iodine: 2Al + 3I₂ → 2AlI₃. 5.40 g of aluminium is mixed with 50.8 g of iodine and the reaction goes to completion. Calculate the mass of aluminium iodide formed. (Ar: Al = 27, I = 127)", "answer": 54.4, "tolerance": 0.01, "unit": "g", "marks": 2, "mark_scheme": "M1 moles Al = 5.40 ÷ 27 = 0.200; moles I₂ = 50.8 ÷ 254 = 0.200; 0.200 mol Al needs 0.300 mol I₂, so iodine is limiting; M2 moles AlI₃ = 0.200 × 2/3 = 0.133 mol; Mr AlI₃ = 408; mass = 54.4 g", "explanation": "Equal moles do not mean neither is limiting: the 2 : 3 ratio means iodine runs out first, so the product is worked out from iodine. 81.6 g comes from using aluminium, the reactant in excess.", "atom_ids": ["0620-3.3-A25", "0620-3.3-A26", "0620-3.3-A23"], "template_id": "T13", "misconception_id": null, "difficulty": 3}], "atoms": {"0620-3.2-A12": "Reacting masses by simple proportion: coefficient x Mr gives the mass of each substance; scale to the given mass.", "0620-3.2-A13": "Total mass of reactants equals total mass of products; use it as a check on a mass statement.", "0620-3.3-A27": "Excess of a solid reactant is shown by solid remaining undissolved and effervescence stopping.", "0620-3.3-A24": "The limiting reactant is the one used up first; the other is in excess; the product is fixed by the limiting reactant.", "0620-3.3-A23": "Three-step mole method: convert the given quantity to moles, apply the coefficient ratio (wanted / given), convert the wanted moles back.", "0620-3.3-A22": "The coefficients of a balanced equation are mole ratios.", "0620-3.2-M16": "apply the equation coefficients directly to masses (4 : 8 means mass doubles).", "0620-3.3-A56": "Paper 4 presents chains as 'Use the following steps' bullets with an answer line and unit for each step; each bullet is one mark.", "0620-3.3-A25": "Identify the limiting reactant: convert both reactants to moles and compare with the equation ratio (e.g. moles / coefficient; smaller is limiting).", "0620-3.3-A26": "Theoretical product (and hence % yield) must be calculated from the limiting reactant, never from the one in excess."}}
+{"id": "0620_ch05_q03", "chapterId": "0620_ch05", "subject": "Chemistry", "code": "0620", "chapter": "Chemical calculations", "topicId": "t3", "topicTitle": "Reacting masses and limits", "quizNo": 3, "quizCount": 7, "order": 3, "title": "Reacting masses and limits", "kind": "topic", "intro": "Practise using equation ratios to predict masses and spotting which reactant runs out first.", "minutes": 9, "active": true, "questions": [{"id": "q13", "type": "numeric", "prompt": "Magnesium burns in oxygen: 2Mg + O₂ → 2MgO. The equation shows that 48 g of magnesium makes 80 g of magnesium oxide. What mass of magnesium oxide is made from 12 g of magnesium?", "answer": 20, "tolerance": 0.01, "unit": "g", "marks": 1, "mark_scheme": "20 g: 12 g is a quarter of 48 g, so a quarter of 80 g.", "explanation": "The mass ratio of substances in a reaction is fixed by the equation, so scale it. Check with conservation of mass: 48 g Mg + 32 g O₂ = 80 g MgO.", "atom_ids": ["0620-3.2-A12", "0620-3.2-A13"], "template_id": "T6", "misconception_id": null, "difficulty": 1}, {"id": "q14", "type": "mcq", "prompt": "A student adds zinc carbonate powder to dilute sulfuric acid until the zinc carbonate is in excess. Which observation shows that it is in excess?", "options": ["Bubbling stops and some solid stays undissolved", "All of the solid dissolves", "The bubbling becomes faster", "The solution turns from blue to colourless"], "answer": 0, "marks": 1, "mark_scheme": "Effervescence stops and solid remains.", "explanation": "When the acid (the limiting reactant) is used up, no more gas can form and the extra carbonate has nothing left to react with, so it stays as solid.", "atom_ids": ["0620-3.3-A27", "0620-3.3-A24"], "template_id": null, "misconception_id": null, "difficulty": 1}, {"id": "q15", "type": "mcq", "prompt": "Nitrogen reacts with hydrogen: N₂ + 3H₂ → 2NH₃. What is the maximum mass of ammonia made from 5.6 g of nitrogen? (Mr: N₂ = 28, NH₃ = 17)", "options": ["3.4 g", "6.8 g", "11.2 g", "13.6 g"], "answer": 1, "marks": 1, "mark_scheme": "6.8 g: moles N₂ = 0.200; moles NH₃ = 0.400; × 17.", "explanation": "Coefficients are mole ratios, not mass ratios. 11.2 g applies the 1 : 2 ratio straight to the mass; 3.4 g ignores the ratio.", "atom_ids": ["0620-3.2-A12", "0620-3.3-A23", "0620-3.3-A22"], "template_id": "T6", "misconception_id": "0620-3.2-M16", "difficulty": 2}, {"id": "q16", "type": "numeric", "prompt": "Iron(III) oxide is reduced by carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Calculate the mass of iron made from 40.0 g of iron(III) oxide. Use the following steps: moles of Fe₂O₃ (Mr = 160); moles of Fe; mass of Fe (Ar = 56).", "answer": 28.0, "tolerance": 0.01, "unit": "g", "marks": 2, "mark_scheme": "M1 moles Fe₂O₃ = 40.0 ÷ 160 = 0.250 mol; M2 moles Fe = 2 × 0.250 = 0.500 mol, mass = 0.500 × 56 = 28.0 g", "explanation": "Convert to moles, use the ratio (1 Fe₂O₃ : 2 Fe), convert back. 14.0 g means the ×2 ratio step was missed: the middle step is where most marks are lost.", "atom_ids": ["0620-3.3-A23", "0620-3.3-A22", "0620-3.3-A56"], "template_id": "T6", "misconception_id": null, "difficulty": 2}, {"id": "q17", "type": "short", "prompt": "0.30 g of magnesium is added to dilute hydrochloric acid containing 0.0200 mol of HCl: Mg + 2HCl → MgCl₂ + H₂. Determine which reactant is limiting. Show your working and explain. (Ar: Mg = 24)", "answer": null, "marks": 2, "mark_scheme": "M1 moles Mg = 0.30 ÷ 24 = 0.0125 mol; M2 0.0125 mol Mg needs 2 × 0.0125 = 0.0250 mol HCl but only 0.0200 mol is present, so HCl is limiting (Mg in excess). Accept the reverse comparison: 0.0200 mol HCl reacts with only 0.0100 mol Mg, less than the 0.0125 mol present, so Mg is in excess.", "explanation": "Comparing 0.0125 with 0.0200 on their own suggests magnesium is limiting. But each Mg needs two HCl, so the magnesium would need 0.0250 mol of acid and the acid runs out first.", "atom_ids": ["0620-3.3-A25", "0620-3.3-A24"], "template_id": "T13", "misconception_id": "0620-3.3-M07", "difficulty": 3}, {"id": "q18", "type": "numeric", "prompt": "Aluminium reacts with iodine: 2Al + 3I₂ → 2AlI₃. 5.40 g of aluminium is mixed with 50.8 g of iodine and the reaction goes to completion. Calculate the mass of aluminium iodide formed. (Ar: Al = 27, I = 127)", "answer": 54.4, "tolerance": 0.01, "unit": "g", "marks": 2, "mark_scheme": "M1 moles Al = 5.40 ÷ 27 = 0.200; moles I₂ = 50.8 ÷ 254 = 0.200; 0.200 mol Al needs 0.300 mol I₂, so iodine is limiting; M2 moles AlI₃ = 0.200 × 2/3 = 0.133 mol; Mr AlI₃ = 408; mass = 54.4 g", "explanation": "Equal moles do not mean neither is limiting: the 2 : 3 ratio means iodine runs out first, so the product is worked out from iodine. 81.6 g comes from using aluminium, the reactant in excess.", "atom_ids": ["0620-3.3-A25", "0620-3.3-A26", "0620-3.3-A23"], "template_id": "T13", "misconception_id": null, "difficulty": 3}], "atoms": {"0620-3.2-A12": "Reacting masses by simple proportion: coefficient x Mr gives the mass of each substance; scale to the given mass.", "0620-3.2-A13": "Total mass of reactants equals total mass of products; use it as a check on a mass statement.", "0620-3.3-A27": "Excess of a solid reactant is shown by solid remaining undissolved and effervescence stopping.", "0620-3.3-A24": "The limiting reactant is the one used up first; the other is in excess; the product is fixed by the limiting reactant.", "0620-3.3-A23": "Three-step mole method: convert the given quantity to moles, apply the coefficient ratio (wanted / given), convert the wanted moles back.", "0620-3.3-A22": "The coefficients of a balanced equation are mole ratios.", "0620-3.2-M16": "apply the equation coefficients directly to masses (4 : 8 means mass doubles).", "0620-3.3-A56": "Paper 4 presents chains as 'Use the following steps' bullets with an answer line and unit for each step; each bullet is one mark.", "0620-3.3-A25": "Identify the limiting reactant: convert both reactants to moles and compare with the equation ratio (e.g. moles / coefficient; smaller is limiting).", "0620-3.3-M07": "equal moles of two reactants means neither is limiting.", "0620-3.3-A26": "Theoretical product (and hence % yield) must be calculated from the limiting reactant, never from the one in excess."}}

data/sets/0625_ch03_q04.json

@@ -1 +1 @@
-{"id": "0625_ch03_q04", "chapterId": "0625_ch03", "subject": "Physics", "code": "0625", "chapter": "Forces and motion", "topicId": "t4", "topicTitle": "Motion in a circle", "quizNo": 4, "quizCount": 7, "order": 4, "title": "Motion in a circle", "kind": "topic", "intro": "Practise why a force towards the centre is needed to move in a circle.", "minutes": 7, "active": true, "questions": [{"id": "q22", "type": "mcq", "prompt": "A car drives round a roundabout at a constant speed of 8 m/s. Which statement about the resultant force on the car is correct?", "options": ["There is no resultant force because the speed is constant.", "It points outwards, away from the centre of the roundabout.", "It points along the direction in which the car is moving.", "It points towards the centre of the roundabout."], "answer": 3, "marks": 1, "mark_scheme": "B1 option D", "explanation": "The direction of the velocity keeps changing, so the car is accelerating and a resultant force acts, perpendicular to the motion and towards the centre (supplied by friction on the tyres).", "atom_ids": ["0625-1.5-A24", "0625-1.5-A25"], "template_id": "T14", "misconception_id": null, "difficulty": 1}, {"id": "q23", "type": "mcq", "prompt": "Which of these objects has a resultant force acting on it?", "options": ["a parachutist falling at terminal velocity", "a car moving at a constant speed along a straight road", "a satellite moving at a constant speed in a circular orbit", "a book resting on a shelf"], "answer": 2, "marks": 1, "mark_scheme": "B1 option C", "explanation": "The satellite's direction of motion keeps changing, so its velocity changes and a resultant force (gravity, towards the centre of the orbit) must act. The other three have constant velocity (or are at rest), so their forces are balanced.", "atom_ids": ["0625-1.5-A24", "0625-1.5-A25"], "template_id": "T14", "misconception_id": "0625-1.5-M08", "difficulty": 1}, {"id": "q24", "type": "mcq", "prompt": "A ball on a string is whirled in a horizontal circle. The radius of the circle stays the same, but the tension in the string is increased. What happens?", "options": ["The speed of the ball increases.", "The speed of the ball decreases.", "The speed stays the same and the ball moves outwards.", "The mass of the ball increases."], "answer": 0, "marks": 1, "mark_scheme": "B1 option A", "explanation": "With mass and radius fixed, a larger force towards the centre means a higher speed. (With speed fixed instead, a larger force would give a smaller radius.)", "atom_ids": ["0625-1.5-A26"], "template_id": "T14", "misconception_id": null, "difficulty": 2}, {"id": "q25", "type": "short", "prompt": "A car travels round a circular bend at a constant speed of 15 m/s. Explain why the car is accelerating even though its speed does not change.", "answer": null, "marks": 2, "mark_scheme": "the direction of motion changes, so the velocity changes (velocity has direction); acceleration is change in velocity per unit time, so the car accelerates (a resultant force acts towards the centre of the bend)", "explanation": "Speed is only the size of the velocity. On a bend the direction changes all the time, so the velocity changes, and any change of velocity is an acceleration, caused by a resultant force towards the centre (friction on the tyres).", "atom_ids": ["0625-1.5-A24", "0625-1.5-A06"], "template_id": "T14", "misconception_id": null, "difficulty": 2}, {"id": "q26", "type": "multi", "prompt": "A car goes round a level circular bend. The greatest friction force the tyres can provide stays the same. Select all the changes that would make the car more likely to slide off the bend.", "options": ["entering the bend at a higher speed", "taking a tighter bend (smaller radius) at the same speed", "carrying a heavier load at the same speed, with the same greatest friction force", "entering the bend at a lower speed", "taking a wider bend (larger radius) at the same speed"], "answer": [0, 1, 2], "marks": 2, "mark_scheme": "2 marks for A, B and C only; 1 mark for two correct and no wrong", "explanation": "The car slides off when the force needed towards the centre is more than friction can give. A larger force is needed for a higher speed, a smaller radius, or a larger mass. A lower speed or a wider bend needs less force.", "atom_ids": ["0625-1.5-A26", "0625-1.5-A25"], "template_id": "T14", "misconception_id": null, "difficulty": 3}], "atoms": {"0625-1.5-A24": "Motion in a circle at constant speed is accelerated motion because the direction of velocity changes, so a resultant force must act.", "0625-1.5-A25": "The resultant force on an object moving in a circle is perpendicular to its motion, towards the centre; friction, tension or gravity supplies it.", "0625-1.5-M08": "An object moving in a circle at constant speed needs no resultant force, or the force points outward / along the motion.", "0625-1.5-A26": "Circular motion, qualitatively: larger force -> higher speed (m, r fixed); larger force -> smaller radius (m, v fixed); larger mass needs larger force (v, r fixed); too little friction -> the object slides off.", "0625-1.5-A06": "A resultant force changes velocity: it changes the speed, the direction of motion, or both."}}
+{"id": "0625_ch03_q04", "chapterId": "0625_ch03", "subject": "Physics", "code": "0625", "chapter": "Forces and motion", "topicId": "t4", "topicTitle": "Motion in a circle", "quizNo": 4, "quizCount": 7, "order": 4, "title": "Motion in a circle", "kind": "topic", "intro": "Practise why a force towards the centre is needed to move in a circle.", "minutes": 7, "active": true, "questions": [{"id": "q22", "type": "mcq", "prompt": "A car drives round a roundabout at a constant speed of 8 m/s. Which statement about the resultant force on the car is correct?", "options": ["There is no resultant force because the speed is constant.", "It points outwards, away from the centre of the roundabout.", "It points along the direction in which the car is moving.", "It points towards the centre of the roundabout."], "answer": 3, "marks": 1, "mark_scheme": "B1 option D", "explanation": "The direction of the velocity keeps changing, so the car is accelerating and a resultant force acts, perpendicular to the motion and towards the centre (supplied by friction on the tyres).", "atom_ids": ["0625-1.5-A24", "0625-1.5-A25"], "template_id": "T14", "misconception_id": null, "difficulty": 1}, {"id": "q23", "type": "mcq", "prompt": "Which of these objects has a resultant force acting on it?", "options": ["a parachutist falling at terminal velocity", "a car moving at a constant speed along a straight road", "a satellite moving at a constant speed in a circular orbit", "a book resting on a shelf"], "answer": 2, "marks": 1, "mark_scheme": "B1 option C", "explanation": "The satellite's direction of motion keeps changing, so its velocity changes and a resultant force (gravity, towards the centre of the orbit) must act. The other three have constant velocity (or are at rest), so their forces are balanced.", "atom_ids": ["0625-1.5-A24", "0625-1.5-A25"], "template_id": "T14", "misconception_id": "0625-1.5-M08", "difficulty": 1}, {"id": "q24", "type": "mcq", "prompt": "A ball on a string is whirled in a horizontal circle. The radius of the circle stays the same, but the tension in the string is increased. What happens?", "options": ["The speed of the ball increases.", "The speed of the ball decreases.", "The speed stays the same and the ball moves outwards.", "The mass of the ball increases."], "answer": 0, "marks": 1, "mark_scheme": "B1 option A", "explanation": "With mass and radius fixed, a larger force towards the centre means a higher speed. (With speed fixed instead, a larger force would give a smaller radius.)", "atom_ids": ["0625-1.5-A26"], "template_id": "T14", "misconception_id": null, "difficulty": 2}, {"id": "q25", "type": "short", "prompt": "A car travels round a circular bend at a constant speed of 15 m/s. Explain why the car is accelerating even though its speed does not change.", "answer": null, "marks": 2, "mark_scheme": "the direction of motion changes, so the velocity changes (velocity has direction); acceleration is change in velocity per unit time, so the car accelerates (a resultant force acts towards the centre of the bend)", "explanation": "Speed is only the size of the velocity. On a bend the direction changes all the time, so the velocity changes, and any change of velocity is an acceleration, caused by a resultant force towards the centre (friction on the tyres).", "atom_ids": ["0625-1.5-A24", "0625-1.5-A06"], "template_id": "T14", "misconception_id": null, "difficulty": 2}, {"id": "q26", "type": "multi", "prompt": "A ball on a string is whirled in a horizontal circle. The string breaks if the tension in it is greater than a fixed value. Select all the changes that would make the string more likely to break.", "options": ["whirling the ball at a higher speed, with the same radius", "using a smaller radius, at the same speed", "using a heavier ball, at the same speed and radius", "whirling the ball at a lower speed, with the same radius", "using a larger radius, at the same speed"], "answer": [0, 1, 2], "marks": 2, "mark_scheme": "2 marks for A, B and C only; 1 mark for two correct and no wrong", "explanation": "The tension provides the force towards the centre. A larger force is needed for a higher speed, a smaller radius or a larger mass (with the other two kept the same), so the tension rises towards its breaking value. A lower speed or a larger radius needs less force.", "atom_ids": ["0625-1.5-A26", "0625-1.5-A25"], "template_id": "T14", "misconception_id": null, "difficulty": 3}], "atoms": {"0625-1.5-A24": "Motion in a circle at constant speed is accelerated motion because the direction of velocity changes, so a resultant force must act.", "0625-1.5-A25": "The resultant force on an object moving in a circle is perpendicular to its motion, towards the centre; friction, tension or gravity supplies it.", "0625-1.5-M08": "An object moving in a circle at constant speed needs no resultant force, or the force points outward / along the motion.", "0625-1.5-A26": "Circular motion, qualitatively: larger force -> higher speed (m, r fixed); larger force -> smaller radius (m, v fixed); larger mass needs larger force (v, r fixed); too little friction -> the object slides off.", "0625-1.5-A06": "A resultant force changes velocity: it changes the speed, the direction of motion, or both."}}

data/sets/0625_ch03_q06.json

@@ -1 +1 @@
-{"id": "0625_ch03_q06", "chapterId": "0625_ch03", "subject": "Physics", "code": "0625", "chapter": "Forces and motion", "topicId": "t6", "topicTitle": "Momentum and impulse", "quizNo": 6, "quizCount": 7, "order": 6, "title": "Momentum and impulse", "kind": "topic", "intro": "Practise momentum, impulse with signs, F = Δp/Δt and conservation.", "minutes": 12, "active": true, "questions": [{"id": "q33", "type": "mcq", "prompt": "Which is the definition of momentum?", "options": ["mass × velocity", "force × perpendicular distance from a pivot", "force × time for which the force acts", "mass × acceleration"], "answer": 0, "marks": 1, "mark_scheme": "B1 option A", "explanation": "Momentum p = mv, in kg m/s, in the direction of the velocity. B is a moment (a different topic with a similar name), C is impulse and D is force.", "atom_ids": ["0625-1.6-A01"], "template_id": "T18", "misconception_id": "0625-1.6-M08", "difficulty": 1}, {"id": "q34", "type": "mcq", "prompt": "Which is the definition of the impulse of a force?", "options": ["force per unit time", "the rate of change of momentum", "force × time for which the force acts", "mass × velocity"], "answer": 2, "marks": 1, "mark_scheme": "B1 option C", "explanation": "Impulse = FΔt, force multiplied by the time it acts, and it equals the change in momentum (unit N s). The rate of change of momentum is the force itself, not the impulse; mass × velocity is momentum.", "atom_ids": ["0625-1.6-A04", "0625-1.6-A03"], "template_id": "T19", "misconception_id": "0625-1.6-M03", "difficulty": 1}, {"id": "q35", "type": "numeric", "prompt": "A ball of mass 0.16 kg hits a wall at 12 m/s and rebounds along the same line at 9.0 m/s. Calculate the size of the change in momentum of the ball.", "answer": 3.36, "tolerance": 0.02, "unit": "kg m/s", "marks": 2, "mark_scheme": "C1 Δp = m(v − u) with opposite signs: 0.16 × (9.0 − (−12)); A1 3.4 kg m/s (N s)", "explanation": "The velocity reverses, so the speeds add: 0.16 × 21 = 3.36 kg m/s. Subtracting the speeds (0.16 × 3.0 = 0.48) ignores the change of direction, the error examiner reports flag most often in this chapter.", "atom_ids": ["0625-1.6-A05", "0625-1.6-A01"], "template_id": "T19", "misconception_id": "0625-1.6-M01", "difficulty": 2}, {"id": "q36", "type": "numeric", "prompt": "The ball in the previous question is in contact with the wall for 0.024 s. Calculate the average force the wall exerts on the ball.", "answer": 140.0, "tolerance": 0.03, "unit": "N", "marks": 2, "mark_scheme": "C1 F = Δp ÷ Δt = 3.36 ÷ 0.024; A1 140 N (ecf from the change in momentum)", "explanation": "Force = change in momentum per unit time: 3.36 ÷ 0.024 = 140 N. Using 0.48 kg m/s gives 20 N.", "atom_ids": ["0625-1.6-A06", "0625-1.6-A03"], "template_id": "T20", "misconception_id": null, "difficulty": 2}, {"id": "q37", "type": "mcq", "prompt": "An object of mass m changes velocity from u to v in time t because of a resultant force F. Which expression is equal to F?", "options": ["mv ÷ t", "(mv − mu) ÷ t", "m(v − u) × t", "(mv − mu) × t"], "answer": 1, "marks": 1, "mark_scheme": "B1 option B", "explanation": "F = ma and a = (v − u)/t, so F = (mv − mu)/t: force is the change in momentum per unit time. (mv − mu) × t has no meaning; the change in momentum itself is the impulse F × t.", "atom_ids": ["0625-1.6-A02", "0625-1.6-A03"], "template_id": "T20", "misconception_id": null, "difficulty": 2}, {"id": "q38", "type": "mcq", "prompt": "A railway wagon of mass 6000 kg moving east at 3.0 m/s collides with a wagon of mass 4000 kg moving west at 2.0 m/s. They couple together. What is their velocity just after the collision?", "options": ["1.0 m/s east", "1.0 m/s west", "2.6 m/s east", "0.5 m/s east"], "answer": 0, "marks": 2, "mark_scheme": "C1 total momentum before = 18 000 − 8000 = 10 000 kg m/s east; A1 v = 10 000 ÷ 10 000 = 1.0 m/s east", "explanation": "Take east as positive: 6000 × 3.0 − 4000 × 2.0 = 10 000 kg m/s, shared by 10 000 kg, so 1.0 m/s east. 2.6 m/s adds the momenta as if both moved east.", "atom_ids": ["0625-1.6-A09", "0625-1.6-A05"], "template_id": "T21", "misconception_id": null, "difficulty": 2}, {"id": "q39", "type": "short", "prompt": "A gymnast bends her knees as she lands. Explain, using ideas about momentum, why this reduces the force on her legs.", "answer": null, "marks": 2, "mark_scheme": "her change in momentum (impulse) is the same either way; it takes place over a longer time, so since F = Δp/Δt the force is smaller", "explanation": "The momentum change is fixed by her mass and landing speed; bending the knees only stretches the time. Saying it 'reduces the impulse' is a common wrong answer.", "atom_ids": ["0625-1.6-A07"], "template_id": "T22", "misconception_id": null, "difficulty": 2}, {"id": "q40", "type": "mcq", "prompt": "A cannon of mass 850 kg stands at rest on smooth ground and is free to roll. It fires a 12 kg cannonball horizontally at 95 m/s. What is the velocity of the cannon just after firing?", "options": ["1.3 m/s in the opposite direction to the ball", "1.3 m/s in the same direction as the ball", "95 m/s in the opposite direction to the ball", "0 m/s, because the cannon is much heavier than the ball"], "answer": 0, "marks": 1, "mark_scheme": "B1 option A (0 = 12 × 95 + 850v, so v = −1.3 m/s)", "explanation": "The cannon and ball push on each other with equal and opposite forces for the same time, so they get equal and opposite impulses and the total momentum, zero before firing, is still zero after: the ball's 12 × 95 = 1140 kg m/s forward must be matched by 1140 kg m/s backward, giving 1140 ÷ 850 = 1.3 m/s backwards. B treats the two as moving off together, as if they had stuck.", "atom_ids": ["0625-1.6-A10", "0625-1.6-A09", "0625-1.6-A08"], "template_id": "T21", "misconception_id": "0625-1.6-M07", "difficulty": 3}], "atoms": {"0625-1.6-A01": "Momentum p = mv (mass x velocity), unit kg m/s; it is a vector in the direction of the velocity.", "0625-1.6-M08": "Momentum and moment are the same idea.", "0625-1.6-A04": "Impulse = force x time for which it acts = change in momentum: FΔt = Δ(mv); unit N s (= kg m/s).", "0625-1.6-A03": "Resultant force F = Δp/Δt (change in momentum per unit time).", "0625-1.6-M03": "Impulse is force per unit time, or the rate of change of momentum.", "0625-1.6-A05": "Change of momentum with direction: fix a positive direction, Δp = m(v - u) with signs; on a rebound the speeds add; a reverse impulse first cancels the initial momentum.", "0625-1.6-M01": "Change in momentum = mass x (difference of speeds), even when the direction reverses.", "0625-1.6-A06": "Force from a momentum change: F = Δp/Δt, converting ms to s; or initial velocity from FΔt/m.", "0625-1.6-A02": "F = ma rewritten: F = m(v - u)/t = (mv - mu)/t, so resultant force = change in momentum per unit time.", "0625-1.6-A09": "Conservation method: positive direction; total momentum before (with signs) = total after; solve for the unknown velocity or mass; the sign gives the direction.", "0625-1.6-A07": "For a fixed change in momentum, a longer contact time gives a smaller force; the impulse itself is not reduced (airbags, crumple zones, bending knees, moving hands back, soft floors).", "0625-1.6-A10": "Explosions and recoil from rest: total momentum stays zero, so the parts move apart with equal and opposite momenta.", "0625-1.6-A08": "In an interaction the two objects receive equal and opposite impulses, so with no external resultant force the total momentum is conserved.", "0625-1.6-M07": "Objects pushed apart from rest move off together, as if they had collided and stuck."}}
+{"id": "0625_ch03_q06", "chapterId": "0625_ch03", "subject": "Physics", "code": "0625", "chapter": "Forces and motion", "topicId": "t6", "topicTitle": "Momentum and impulse", "quizNo": 6, "quizCount": 7, "order": 6, "title": "Momentum and impulse", "kind": "topic", "intro": "Practise momentum, impulse with signs, F = Δp/Δt and conservation.", "minutes": 12, "active": true, "questions": [{"id": "q33", "type": "mcq", "prompt": "Which is the definition of momentum?", "options": ["mass × velocity", "force × perpendicular distance from a pivot", "force × time for which the force acts", "mass × acceleration"], "answer": 0, "marks": 1, "mark_scheme": "B1 option A", "explanation": "Momentum p = mv, in kg m/s, in the direction of the velocity. B is a moment (a different topic with a similar name), C is impulse and D is force.", "atom_ids": ["0625-1.6-A01"], "template_id": "T18", "misconception_id": "0625-1.6-M08", "difficulty": 1}, {"id": "q34", "type": "mcq", "prompt": "Which is the definition of the impulse of a force?", "options": ["force per unit time", "the rate of change of momentum", "force × time for which the force acts", "mass × velocity"], "answer": 2, "marks": 1, "mark_scheme": "B1 option C", "explanation": "Impulse = FΔt, force multiplied by the time it acts, and it equals the change in momentum (unit N s). The rate of change of momentum is the force itself, not the impulse; mass × velocity is momentum.", "atom_ids": ["0625-1.6-A04", "0625-1.6-A03"], "template_id": "T19", "misconception_id": "0625-1.6-M03", "difficulty": 1}, {"id": "q35", "type": "numeric", "prompt": "A ball of mass 0.16 kg hits a wall at 12 m/s and rebounds along the same line at 9.0 m/s. Calculate the size of the change in momentum of the ball.", "answer": 3.36, "tolerance": 0.02, "unit": "kg m/s", "marks": 2, "mark_scheme": "C1 Δp = m(v − u) with opposite signs: 0.16 × (9.0 − (−12)); A1 3.4 kg m/s (N s)", "explanation": "The velocity reverses, so the speeds add: 0.16 × 21 = 3.36 kg m/s. Subtracting the speeds (0.16 × 3.0 = 0.48) ignores the change of direction, the error examiner reports flag most often in this chapter.", "atom_ids": ["0625-1.6-A05", "0625-1.6-A01"], "template_id": "T19", "misconception_id": "0625-1.6-M01", "difficulty": 2}, {"id": "q36", "type": "numeric", "prompt": "The ball in the previous question is in contact with the wall for 0.024 s. Calculate the average force the wall exerts on the ball.", "answer": 140.0, "tolerance": 0.03, "unit": "N", "marks": 2, "mark_scheme": "C1 F = Δp ÷ Δt = 3.36 ÷ 0.024; A1 140 N (ecf from the change in momentum)", "explanation": "Force = change in momentum per unit time: 3.36 ÷ 0.024 = 140 N. Using 0.48 kg m/s gives 20 N.", "atom_ids": ["0625-1.6-A06", "0625-1.6-A03"], "template_id": "T20", "misconception_id": null, "difficulty": 2}, {"id": "q37", "type": "mcq", "prompt": "An object of mass m changes velocity from u to v in time t because of a resultant force F. Which expression is equal to F?", "options": ["mv ÷ t", "(mv − mu) ÷ t", "m(v − u) × t", "(mv − mu) × t"], "answer": 1, "marks": 1, "mark_scheme": "B1 option B", "explanation": "F = ma and a = (v − u)/t, so F = (mv − mu)/t: force is the change in momentum per unit time. (mv − mu) × t has no meaning; the change in momentum itself is the impulse F × t.", "atom_ids": ["0625-1.6-A02", "0625-1.6-A03"], "template_id": "T20", "misconception_id": null, "difficulty": 2}, {"id": "q38", "type": "mcq", "prompt": "A railway wagon of mass 6000 kg moving east at 3.0 m/s collides with a wagon of mass 4000 kg moving west at 2.0 m/s. They couple together. What is their velocity just after the collision?", "options": ["1.0 m/s east", "1.0 m/s west", "2.6 m/s east", "0.5 m/s east"], "answer": 0, "marks": 2, "mark_scheme": "C1 total momentum before = 18 000 − 8000 = 10 000 kg m/s east; A1 v = 10 000 ÷ 10 000 = 1.0 m/s east", "explanation": "Take east as positive: 6000 × 3.0 − 4000 × 2.0 = 10 000 kg m/s, shared by 10 000 kg, so 1.0 m/s east. 2.6 m/s adds the momenta as if both moved east.", "atom_ids": ["0625-1.6-A09", "0625-1.6-A05"], "template_id": "T21", "misconception_id": null, "difficulty": 2}, {"id": "q39", "type": "short", "prompt": "A gymnast bends her knees as she lands. Explain, using ideas about momentum, why this reduces the force on her legs.", "answer": null, "marks": 2, "mark_scheme": "her change in momentum (impulse) is the same either way; it takes place over a longer time, so since F = Δp/Δt the force is smaller", "explanation": "The momentum change is fixed by her mass and landing speed; bending the knees only stretches the time. Saying it 'reduces the impulse' is a common wrong answer.", "atom_ids": ["0625-1.6-A07"], "template_id": "T22", "misconception_id": null, "difficulty": 2}, {"id": "q40", "type": "mcq", "prompt": "A cannon of mass 850 kg stands at rest on smooth ground and is free to roll. It fires a 12 kg cannonball horizontally at 95 m/s. What is the velocity of the cannon just after firing?", "options": ["1.3 m/s in the opposite direction to the ball", "1.3 m/s in the same direction as the ball", "95 m/s in the opposite direction to the ball", "0 m/s, because the cannon is much heavier than the ball"], "answer": 0, "marks": 1, "mark_scheme": "B1 option A (0 = 12 × 95 + 850v, so v = −1.3 m/s)", "explanation": "The cannon and ball push on each other with equal and opposite forces for the same time, so they get equal and opposite impulses and the total momentum, zero before firing, is still zero after: the ball's 12 × 95 = 1140 kg m/s forward must be matched by 1140 kg m/s backward, giving 1140 ÷ 850 = 1.3 m/s backwards. B has the right size but the wrong direction: with zero total momentum the cannon must move opposite to the ball. D is wrong because a heavy object still recoils, just slowly; 0 m/s is also what you get if you treat the cannon and ball as moving off together, as if they had stuck.", "atom_ids": ["0625-1.6-A10", "0625-1.6-A09", "0625-1.6-A08"], "template_id": "T21", "misconception_id": "0625-1.6-M07", "difficulty": 3}], "atoms": {"0625-1.6-A01": "Momentum p = mv (mass x velocity), unit kg m/s; it is a vector in the direction of the velocity.", "0625-1.6-M08": "Momentum and moment are the same idea.", "0625-1.6-A04": "Impulse = force x time for which it acts = change in momentum: FΔt = Δ(mv); unit N s (= kg m/s).", "0625-1.6-A03": "Resultant force F = Δp/Δt (change in momentum per unit time).", "0625-1.6-M03": "Impulse is force per unit time, or the rate of change of momentum.", "0625-1.6-A05": "Change of momentum with direction: fix a positive direction, Δp = m(v - u) with signs; on a rebound the speeds add; a reverse impulse first cancels the initial momentum.", "0625-1.6-M01": "Change in momentum = mass x (difference of speeds), even when the direction reverses.", "0625-1.6-A06": "Force from a momentum change: F = Δp/Δt, converting ms to s; or initial velocity from FΔt/m.", "0625-1.6-A02": "F = ma rewritten: F = m(v - u)/t = (mv - mu)/t, so resultant force = change in momentum per unit time.", "0625-1.6-A09": "Conservation method: positive direction; total momentum before (with signs) = total after; solve for the unknown velocity or mass; the sign gives the direction.", "0625-1.6-A07": "For a fixed change in momentum, a longer contact time gives a smaller force; the impulse itself is not reduced (airbags, crumple zones, bending knees, moving hands back, soft floors).", "0625-1.6-A10": "Explosions and recoil from rest: total momentum stays zero, so the parts move apart with equal and opposite momenta.", "0625-1.6-A08": "In an interaction the two objects receive equal and opposite impulses, so with no external resultant force the total momentum is conserved.", "0625-1.6-M07": "Objects pushed apart from rest move off together, as if they had collided and stuck."}}

data/topics/0620_ch04.json

@@ -1 +1 @@
-{"chapterId": "0620_ch04", "subject": "Chemistry", "code": "0620", "chapter": "Chemical formulae and equations", "mixedQuizId": "0620_ch04_q07", "topics": [{"id": "t1", "order": 1, "title": "Symbols and reading formulae", "goal": "Read symbols, subscripts, brackets and coefficients to count every atom in a formula.", "atom_ids": ["0620-3.1-A01", "0620-3.1-A03", "0620-3.1-A04", "0620-3.1-A05", "0620-3.1-A06", "0620-3.1-A07", "0620-3.1-A08", "0620-3.1-A09", "0620-3.1-A10"], "paragraphs": ["P1", "P2", "P3"], "quizId": "0620_ch04_q01", "atoms": {"0620-3.1-A01": "Atoms are conserved in a chemical reaction: they are regrouped, never created, destroyed or turned into another element.", "0620-3.1-A03": "Element symbols start with a capital letter; a second letter is lower case, and case carries meaning (Co is cobalt, CO is carbon monoxide).", "0620-3.1-A04": "Seven elements exist as diatomic molecules and are written H2, N2, O2, F2, Cl2, Br2, I2 whenever they appear as the element.", "0620-3.1-A05": "Other elements are written as single atoms in equations: metals, carbon, silicon, sulfur and the noble gases.", "0620-3.1-A06": "A subscript counts the atom (or bracketed group) immediately before it: H2SO4 has 2 H, 1 S, 4 O.", "0620-3.1-A07": "A subscript after a bracket multiplies everything inside; brackets are used only when a compound ion appears more than once (Ca(NO3)2, but NaNO3).", "0620-3.1-A08": "A coefficient in front of a formula multiplies the whole formula: 3H2SO4 contains 6 H, 3 S, 12 O.", "0620-3.1-A09": "The raised dot in a hydrated salt (CuSO4.5H2O) means together with: whole water molecules are added, not multiplied.", "0620-3.1-A10": "A compound formula never shows the ion charges: MgCl2, not Mg2+Cl2-."}}, {"id": "t2", "order": 2, "title": "Molecular and empirical formulae", "goal": "Deduce molecular and empirical formulae and explain when only an empirical formula exists.", "atom_ids": ["0620-3.1-A11", "0620-3.1-A12", "0620-3.1-A13", "0620-3.1-A14", "0620-3.1-A15", "0620-3.1-A16", "0620-3.1-A17"], "paragraphs": ["P4", "P5"], "quizId": "0620_ch04_q02", "atoms": {"0620-3.1-A11": "Molecular formula: the number and type of different atoms in one molecule.", "0620-3.1-A12": "Deduce a molecular formula from a model, displayed or structural formula by counting each kind of atom, including atoms inside groups (CH3, OH, COOH).", "0620-3.1-A13": "A molecular formula loses the arrangement of atoms, so different compounds (isomers) can share one molecular formula; organic formulae are written C, then H, then other elements.", "0620-3.1-A14": "Empirical formula: the simplest whole-number ratio of the different atoms or ions in a compound.", "0620-3.1-A15": "Ionic lattices and giant covalent substances (NaCl, SiO2) have only an empirical formula, because there is no separate molecule.", "0620-3.1-A16": "Find an empirical formula from a molecular formula by dividing every subscript by their highest common factor; if it is 1 the two formulae are the same.", "0620-3.1-A17": "A polymer has no single molecular formula because its chains are of indefinite and varying length; only the repeat unit can be written."}}, {"id": "t3", "order": 3, "title": "Ions and their charges", "goal": "Predict ion charges from the Periodic Table and recall the compound ions and their names.", "atom_ids": ["0620-3.1-A18", "0620-3.1-A19", "0620-3.1-A20", "0620-3.1-A21", "0620-3.1-A22", "0620-3.1-A23", "0620-3.1-A24"], "paragraphs": ["P6", "P7"], "quizId": "0620_ch04_q03", "atoms": {"0620-3.1-A18": "Main-group ion charge follows the group: I 1+, II 2+, Al 3+, V 3-, VI 2-, VII 1-.", "0620-3.1-A19": "Group IV and Group VIII elements do not normally form simple ions; hydrogen forms H+.", "0620-3.1-A20": "A Roman numeral in a name gives the oxidation number of the metal, equal to its ion charge: iron(II) Fe2+, iron(III) Fe3+, copper(II) Cu2+; it is always positive and refers to the metal only.", "0620-3.1-A21": "Zinc forms only Zn2+ and silver only Ag+, so their names usually carry no numeral.", "0620-3.1-A22": "Compound ions to know: NH4+, OH-, NO3-, CO3 2-, SO4 2-, HCO3-, PO4 3-.", "0620-3.1-A23": "A carboxylic acid RCOOH gives the anion RCOO- (ethanoate CH3COO-), so its salts are written like other salts: (CH3COO)2Ca.", "0620-3.1-A24": "Name endings: -ide means a single element (or hydroxide), -ate means the ion contains oxygen; sulfide S2- and sulfate SO4 2- are different ions."}}, {"id": "t4", "order": 4, "title": "Formulae of ionic compounds", "goal": "Build neutral formulae from ion charges, and work backwards from a formula to an ion.", "atom_ids": ["0620-3.1-A02", "0620-3.1-A25", "0620-3.1-A26", "0620-3.1-A27", "0620-3.1-A28", "0620-3.1-A29", "0620-3.1-A30", "0620-3.1-A31", "0620-3.1-A32", "0620-3.1-A33", "0620-3.1-A34", "0620-3.1-A35", "0620-3.1-A36"], "paragraphs": ["P8", "P9", "P10"], "quizId": "0620_ch04_q04", "atoms": {"0620-3.1-A02": "Charge is conserved: the total charge on each side of an equation is the same.", "0620-3.1-A25": "An ionic compound is neutral: total positive charge equals total negative charge.", "0620-3.1-A26": "Formula from charges: write both ions, find the lowest common multiple of the charge sizes, take enough of each ion to reach it, bracket a compound ion used more than once (Al2(SO4)3).", "0620-3.1-A27": "The swap-the-charges shortcut must be followed by simplifying (Mg2O2 becomes MgO).", "0620-3.1-A28": "Apply the charge method to supplied or unfamiliar ions, and to compounds with three kinds of ion, by totalling all positive and all negative charges.", "0620-3.1-A29": "Inverse: find an unknown ion charge from a formula and the known ion (Cr2(SO4)3 gives Cr3+).", "0620-3.1-A30": "Read the anion from a salt formula, and the parent acid by replacing the metal ions with H+ (Na2SO3 gives SO3 2- and H2SO3).", "0620-3.1-A31": "Deduce a formula from a model or lattice diagram by counting the relative numbers of each particle, checking neutrality when charges are shown.", "0620-3.1-A32": "For a compound containing two ions of one element (mixed valence), choose the formula that is both neutral and contains every stated ion.", "0620-3.1-A33": "Formulae of named molecular compounds: H2O, CO2, CO, NH3, CH4, SO2, SO3, NO2, NO.", "0620-3.1-A34": "Formulae of the common acids: HCl, H2SO4, HNO3, CH3COOH.", "0620-3.1-A35": "Hydrated copper(II) sulfate is CuSO4.5H2O (blue) and hydrated cobalt(II) chloride is CoCl2.6H2O (pink).", "0620-3.1-A36": "When a formula is asked for, give a formula, not a name; when a formula is printed in the question, copy it exactly."}}, {"id": "t5", "order": 5, "title": "Writing and balancing equations", "goal": "Write balanced symbol equations with correct state symbols, including for reactions described in words.", "atom_ids": ["0620-3.1-A37", "0620-3.1-A38", "0620-3.1-A39", "0620-3.1-A40", "0620-3.1-A41", "0620-3.1-A42", "0620-3.1-A43", "0620-3.1-A44", "0620-3.1-A45", "0620-3.1-A46", "0620-3.1-A47", "0620-3.1-A48", "0620-3.1-A49"], "paragraphs": ["P11", "P12", "P13", "P14", "P15"], "quizId": "0620_ch04_q05", "atoms": {"0620-3.1-A37": "A word equation names reactants and products (reactants -> products); it is not accepted where a chemical or symbol equation is asked for.", "0620-3.1-A38": "First step of any symbol equation: write the correct formula of every species; this is usually the first mark (M1).", "0620-3.1-A39": "Balance by changing coefficients only, never subscripts or formulae.", "0620-3.1-A40": "Balancing order: an element found in one substance each side first (largest formula first), compound ions as units, free elements last; recount everything and use the smallest whole numbers.", "0620-3.1-A41": "Combustion: balance C into CO2, then H into H2O, count the O needed, supply it as O2 (subtract any O in the fuel); if the O2 count is odd or half, double all coefficients.", "0620-3.1-A42": "Deduce an unknown formula in an equation from the atom count (missing product; cracking products from a given ratio).", "0620-3.1-A43": "A reversible reaction is written with a double arrow; balancing rules are unchanged.", "0620-3.1-A44": "State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water; aqueous is not the same as liquid.", "0620-3.1-A45": "Assigning states: metals, oxides, powdered carbonates and precipitates (s); acids, alkalis and soluble salts in solution (aq); water (l); gases at r.t.p. (g); a halogen formed in solution (aq).", "0620-3.1-A46": "The state depends on the conditions stated: water from a hot combustion is (g); a liquid product at r.t.p. is (l).", "0620-3.1-A47": "Deduce an equation from information: list the named species, write formulae (copy given ones), deduce unnamed products from a reaction pattern or the atom count, balance, add state symbols if asked.", "0620-3.1-A48": "Acid patterns: acid + metal -> salt + hydrogen; acid + base -> salt + water; acid + carbonate -> salt + water + carbon dioxide; reactive metal + water -> hydroxide + hydrogen.", "0620-3.1-A49": "Other patterns: element + oxygen -> oxide; a more reactive metal or halogen displaces a less reactive one; carbonate on heating -> oxide + carbon dioxide; complete combustion -> carbon dioxide + water."}}, {"id": "t6", "order": 6, "title": "Ionic equations", "goal": "Write ionic equations for precipitation, neutralisation and displacement, balanced for atoms and charge.", "atom_ids": ["0620-3.1-A50", "0620-3.1-A51", "0620-3.1-A52", "0620-3.1-A53", "0620-3.1-A54", "0620-3.1-A55", "0620-3.1-A56", "0620-3.1-A57"], "paragraphs": ["P16", "P17", "P18"], "quizId": "0620_ch04_q06", "atoms": {"0620-3.1-A50": "A dissolved ionic compound exists as separate ions; ions present unchanged before and after a reaction are spectator ions.", "0620-3.1-A51": "Ionic equation recipe: full equation; split aqueous ionic substances into ions; do not split solids, water, gases, covalent molecules or metals; cancel spectators; check atoms and charge.", "0620-3.1-A52": "Precipitation ionic equation: two aqueous ions -> one solid, (aq) + (aq) -> (s); take the precipitate formula and split it into its ions to get the left-hand side (Pb2+ + 2I- -> PbI2).", "0620-3.1-A53": "Every acid-alkali neutralisation has the ionic equation H+(aq) + OH-(aq) -> H2O(l).", "0620-3.1-A54": "Displacement ionic equations: metal + metal ion (Fe + Cu2+ -> Fe2+ + Cu) or halogen molecule + halide ion (Cl2 + 2I- -> 2Cl- + I2); the halogens stay diatomic and the Group I cations are spectators.", "0620-3.1-A55": "Balance an ionic equation for charge as well as atoms (2Al + 3Cu2+ -> 2Al3+ + 3Cu has 6+ on each side).", "0620-3.1-A56": "An ionic half-equation is a different object: one electrode process with electrons written explicitly; it belongs to electrolysis and redox.", "0620-3.1-A57": "Marking pattern: formula items one mark, exact; equations M1 species, M2 balanced, M3 state symbols; precipitation ionic M1 precipitate only, M2 its two ions only, M3 (aq)+(aq)->(s); extra species lose M1."}}]}
+{"chapterId": "0620_ch04", "subject": "Chemistry", "code": "0620", "chapter": "Chemical formulae and equations", "mixedQuizId": "0620_ch04_q07", "topics": [{"id": "t1", "order": 1, "title": "Symbols and reading formulae", "goal": "Read symbols, subscripts, brackets and coefficients to count every atom in a formula.", "atom_ids": ["0620-3.1-A01", "0620-3.1-A03", "0620-3.1-A04", "0620-3.1-A05", "0620-3.1-A06", "0620-3.1-A07", "0620-3.1-A08", "0620-3.1-A09", "0620-3.1-A10"], "paragraphs": ["P1", "P2", "P3"], "quizId": "0620_ch04_q01", "atoms": {"0620-3.1-A01": "Atoms are conserved in a chemical reaction: they are regrouped, never created, destroyed or turned into another element.", "0620-3.1-A03": "Element symbols start with a capital letter; a second letter is lower case, and case carries meaning (Co is cobalt, CO is carbon monoxide).", "0620-3.1-A04": "Seven elements exist as diatomic molecules and are written H2, N2, O2, F2, Cl2, Br2, I2 whenever they appear as the element.", "0620-3.1-A05": "Other elements are written as single atoms in equations: metals, carbon, silicon, sulfur and the noble gases.", "0620-3.1-A06": "A subscript counts the atom (or bracketed group) immediately before it: H2SO4 has 2 H, 1 S, 4 O.", "0620-3.1-A07": "A subscript after a bracket multiplies everything inside; brackets are used only when a compound ion appears more than once (Ca(NO3)2, but NaNO3).", "0620-3.1-A08": "A coefficient in front of a formula multiplies the whole formula: 3H2SO4 contains 6 H, 3 S, 12 O.", "0620-3.1-A09": "The raised dot in a hydrated salt (CuSO4.5H2O) means together with: whole water molecules are added, not multiplied.", "0620-3.1-A10": "A compound formula never shows the ion charges: MgCl2, not Mg2+Cl2-."}}, {"id": "t2", "order": 2, "title": "Molecular and empirical formulae", "goal": "Deduce molecular and empirical formulae and explain when only an empirical formula exists.", "atom_ids": ["0620-3.1-A11", "0620-3.1-A12", "0620-3.1-A13", "0620-3.1-A14", "0620-3.1-A15", "0620-3.1-A16", "0620-3.1-A17"], "paragraphs": ["P4", "P5"], "quizId": "0620_ch04_q02", "atoms": {"0620-3.1-A11": "Molecular formula: the number and type of different atoms in one molecule.", "0620-3.1-A12": "Deduce a molecular formula from a model, displayed or structural formula by counting each kind of atom, including atoms inside groups (CH3, OH, COOH).", "0620-3.1-A13": "A molecular formula loses the arrangement of atoms, so different compounds (isomers) can share one molecular formula; organic formulae are written C, then H, then other elements.", "0620-3.1-A14": "Empirical formula: the simplest whole-number ratio of the different atoms or ions in a compound.", "0620-3.1-A15": "Ionic lattices and giant covalent substances (NaCl, SiO2) have only an empirical formula, because there is no separate molecule.", "0620-3.1-A16": "Find an empirical formula from a molecular formula by dividing every subscript by their highest common factor; if it is 1 the two formulae are the same.", "0620-3.1-A17": "A polymer has no single molecular formula because its chains are of indefinite and varying length; only the repeat unit can be written."}}, {"id": "t3", "order": 3, "title": "Ions and their charges", "goal": "Predict ion charges from the Periodic Table and recall the compound ions and their names.", "atom_ids": ["0620-3.1-A18", "0620-3.1-A19", "0620-3.1-A20", "0620-3.1-A21", "0620-3.1-A22", "0620-3.1-A23", "0620-3.1-A24"], "paragraphs": ["P6", "P7"], "quizId": "0620_ch04_q03", "atoms": {"0620-3.1-A18": "Main-group ion charge follows the group: I 1+, II 2+, Al 3+, V 3-, VI 2-, VII 1-.", "0620-3.1-A19": "Group IV and Group VIII elements do not normally form simple ions; hydrogen forms H+.", "0620-3.1-A20": "A Roman numeral in a name gives the oxidation number of the element it follows; for a metal in a simple ionic compound this equals the charge on the metal ion, e.g. iron(III) = Fe3+, copper(II) = Cu2+ (but silicon(IV) oxide is not ionic).", "0620-3.1-A21": "Zinc forms only Zn2+ and silver only Ag+, so their names usually carry no numeral.", "0620-3.1-A22": "Compound ions to know: NH4+, OH-, NO3-, CO3 2-, SO4 2-, HCO3-, PO4 3-.", "0620-3.1-A23": "A carboxylic acid RCOOH gives the anion RCOO- (ethanoate CH3COO-), so its salts are written like other salts: (CH3COO)2Ca.", "0620-3.1-A24": "Name endings: -ide means a single element (or hydroxide), -ate means the ion contains oxygen; sulfide S2- and sulfate SO4 2- are different ions."}}, {"id": "t4", "order": 4, "title": "Formulae of ionic compounds", "goal": "Build neutral formulae from ion charges, and work backwards from a formula to an ion.", "atom_ids": ["0620-3.1-A02", "0620-3.1-A25", "0620-3.1-A26", "0620-3.1-A27", "0620-3.1-A28", "0620-3.1-A29", "0620-3.1-A30", "0620-3.1-A31", "0620-3.1-A32", "0620-3.1-A33", "0620-3.1-A34", "0620-3.1-A35", "0620-3.1-A36"], "paragraphs": ["P8", "P9", "P10"], "quizId": "0620_ch04_q04", "atoms": {"0620-3.1-A02": "Charge is conserved: the total charge on each side of an equation is the same.", "0620-3.1-A25": "An ionic compound is neutral: total positive charge equals total negative charge.", "0620-3.1-A26": "Formula from charges: write both ions, find the lowest common multiple of the charge sizes, take enough of each ion to reach it, bracket a compound ion used more than once (Al2(SO4)3).", "0620-3.1-A27": "The swap-the-charges shortcut must be followed by simplifying (Mg2O2 becomes MgO).", "0620-3.1-A28": "Apply the charge method to supplied or unfamiliar ions, and to compounds with three kinds of ion, by totalling all positive and all negative charges.", "0620-3.1-A29": "Inverse: find an unknown ion charge from a formula and the known ion (Cr2(SO4)3 gives Cr3+).", "0620-3.1-A30": "Read the anion from a salt formula, and the parent acid by replacing the metal ions with H+ (Na2SO3 gives SO3 2- and H2SO3).", "0620-3.1-A31": "Deduce a formula from a model or lattice diagram by counting the relative numbers of each particle, checking neutrality when charges are shown.", "0620-3.1-A32": "For a compound containing two ions of one element (mixed valence), choose the formula that is both neutral and contains every stated ion.", "0620-3.1-A33": "Formulae of named molecular compounds: H2O, CO2, CO, NH3, CH4, SO2, SO3, NO2, NO.", "0620-3.1-A34": "Formulae of the common acids: HCl, H2SO4, HNO3, CH3COOH.", "0620-3.1-A35": "Hydrated copper(II) sulfate is CuSO4.5H2O (blue) and hydrated cobalt(II) chloride is CoCl2.6H2O (pink).", "0620-3.1-A36": "When a formula is asked for, give a formula, not a name; when a formula is printed in the question, copy it exactly."}}, {"id": "t5", "order": 5, "title": "Writing and balancing equations", "goal": "Write balanced symbol equations with correct state symbols, including for reactions described in words.", "atom_ids": ["0620-3.1-A37", "0620-3.1-A38", "0620-3.1-A39", "0620-3.1-A40", "0620-3.1-A41", "0620-3.1-A42", "0620-3.1-A43", "0620-3.1-A44", "0620-3.1-A45", "0620-3.1-A46", "0620-3.1-A47", "0620-3.1-A48", "0620-3.1-A49"], "paragraphs": ["P11", "P12", "P13", "P14", "P15"], "quizId": "0620_ch04_q05", "atoms": {"0620-3.1-A37": "A word equation names reactants and products (reactants -> products); it is not accepted where a chemical or symbol equation is asked for.", "0620-3.1-A38": "First step of any symbol equation: write the correct formula of every species; this is usually the first mark (M1).", "0620-3.1-A39": "Balance by changing coefficients only, never subscripts or formulae.", "0620-3.1-A40": "Balancing order: an element found in one substance each side first (largest formula first), compound ions as units, free elements last; recount everything and use the smallest whole numbers.", "0620-3.1-A41": "Combustion: balance C into CO2, then H into H2O, count the O needed, supply it as O2 (subtract any O in the fuel); if the O2 count is odd or half, double all coefficients.", "0620-3.1-A42": "Deduce an unknown formula in an equation from the atom count (missing product; cracking products from a given ratio).", "0620-3.1-A43": "A reversible reaction is written with a double arrow; balancing rules are unchanged.", "0620-3.1-A44": "State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water; aqueous is not the same as liquid.", "0620-3.1-A45": "Assigning states: metals, oxides, powdered carbonates and precipitates (s); acids, alkalis and soluble salts in solution (aq); water (l); gases at r.t.p. (g); a halogen formed in solution (aq).", "0620-3.1-A46": "The state depends on the conditions stated: water from a hot combustion is (g); a liquid product at r.t.p. is (l).", "0620-3.1-A47": "Deduce an equation from information: list the named species, write formulae (copy given ones), deduce unnamed products from a reaction pattern or the atom count, balance, add state symbols if asked.", "0620-3.1-A48": "Acid patterns: acid + metal -> salt + hydrogen; acid + base -> salt + water; acid + carbonate -> salt + water + carbon dioxide; reactive metal + water -> hydroxide + hydrogen.", "0620-3.1-A49": "Other patterns: element + oxygen -> oxide; a more reactive metal or halogen displaces a less reactive one; carbonate on heating -> oxide + carbon dioxide; complete combustion -> carbon dioxide + water."}}, {"id": "t6", "order": 6, "title": "Ionic equations", "goal": "Write ionic equations for precipitation, neutralisation and displacement, balanced for atoms and charge.", "atom_ids": ["0620-3.1-A50", "0620-3.1-A51", "0620-3.1-A52", "0620-3.1-A53", "0620-3.1-A54", "0620-3.1-A55", "0620-3.1-A56", "0620-3.1-A57"], "paragraphs": ["P16", "P17", "P18"], "quizId": "0620_ch04_q06", "atoms": {"0620-3.1-A50": "A dissolved ionic compound exists as separate ions; ions present unchanged before and after a reaction are spectator ions.", "0620-3.1-A51": "Ionic equation recipe: full equation; split aqueous ionic substances into ions; do not split solids, water, gases, covalent molecules or metals; cancel spectators; check atoms and charge.", "0620-3.1-A52": "Precipitation ionic equation: two aqueous ions -> one solid, (aq) + (aq) -> (s); take the precipitate formula and split it into its ions to get the left-hand side (Pb2+ + 2I- -> PbI2).", "0620-3.1-A53": "Every acid-alkali neutralisation has the ionic equation H+(aq) + OH-(aq) -> H2O(l).", "0620-3.1-A54": "Displacement ionic equations: metal + metal ion (Fe + Cu2+ -> Fe2+ + Cu) or halogen molecule + halide ion (Cl2 + 2I- -> 2Cl- + I2); the halogens stay diatomic and the Group I cations are spectators.", "0620-3.1-A55": "Balance an ionic equation for charge as well as atoms (2Al + 3Cu2+ -> 2Al3+ + 3Cu has 6+ on each side).", "0620-3.1-A56": "An ionic half-equation is a different object: one electrode process with electrons written explicitly; it belongs to electrolysis and redox.", "0620-3.1-A57": "Marking pattern: formula items one mark, exact; equations M1 species, M2 balanced, M3 state symbols; precipitation ionic M1 precipitate only, M2 its two ions only, M3 (aq)+(aq)->(s); extra species lose M1."}}]}

static/index.html

@@ -401,6 +401,15 @@ body.in-chat main { height: 100%; padding-block: 12px; display: block; }
     const exp = m[2] != null ? m[2] : m[3];
     return parseFloat(m[1]) * (exp != null ? Math.pow(10, parseInt(exp, 10)) : 1);
   }
+  // Significant figures in what the student typed (mantissa only; trailing zeros of a whole number don't count).
+  function sigFigs(raw) {
+    if (raw == null) return 0;
+    let s = String(raw).replace(/\s+/g, "").replace(/,/g, "").replace(/−/g, "-");
+    const m = s.match(/^[-+]?(\d*\.?\d+|\d+\.)/); if (!m) return 0;
+    let t = m[1];
+    if (t.includes(".")) { t = t.replace(".", "").replace(/^0+/, ""); return t.length; }
+    t = t.replace(/^0+/, "").replace(/0+$/, ""); return t.length;
+  }
   function markQuestion(q, resp) {
     const max = Number(q.marks) || 1;
     if (q.type === "mcq") {
@@ -421,7 +430,12 @@ body.in-chat main { height: 100%; padding-block: 12px; display: block; }
       const v = parseNumber(resp);
       const a = Number(q.answer);
       const tol = q.tolerance != null ? Number(q.tolerance) : 0.01;
-      const right = Number.isFinite(v) && (a === 0 ? Math.abs(v) < 1e-9 : Math.abs(v - a) <= Math.abs(a) * tol + 1e-12);
+      const within = Number.isFinite(v) && (a === 0 ? Math.abs(v) < 1e-9 : Math.abs(v - a) <= Math.abs(a) * tol + 1e-12);
+      // Also right: the correct answer rounded to the precision the student used (at least 2 s.f.),
+      // e.g. 25 for 24.5 N, or 10.0 for 10.048 cm³.
+      const sf = sigFigs(resp);
+      const rounded = Number.isFinite(v) && a !== 0 && sf >= 2 && Math.abs(Number(a.toPrecision(sf)) - v) <= Math.abs(v) * 1e-9;
+      const right = within || rounded;
       return { got: right ? max : 0, max, auto: true, right, parsed: Number.isFinite(v) ? v : null };
     }
     return { got: null, max, auto: false, right: null }; // short: self-marked