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while near the sodium ion we are more likely to ñnd the oxygen end, because the
sodium is positive and the oxygen end of the water is negative, and they attract
electrically. Can we tell from this picture whether the salt is đ/ssolưing ín water
or crstallizing out of water? Of course we cønnot tell, because while some of
the atoms are leaving the crystal other atoms are rejoining it. The process is a
đụngmïc one, just as in the case of evaporation, and it depends on whether there
is more or less salt in the water than the amount needed for equilibrium. By
cquilibrium we mean that situation in which the rate at which atoms are leaving
Just matches the rate at which they are coming back. If there is almost no salt in
the water, more atoms leave than return, and the salt dissolves. If, on the other
hand, there are too many “salt atoms,” more return than leave, and the salt is
crystallizing.
In passing, we mention that the concept of a rmmolecule oŸ a substanece is onÌy
approximate and exists only for a certain class of substances. It is clear in the
case of water that the three atoms are actually stuck together. lt is not so clear in
the case of sodium chloride in the solid. 'Phere is just an arrangement of sodiun
and chlorine Ions in a cubic pattern. There is no natural way to group them as
“molecules of salt.”
Returning to our discussion of solution and precipitation, if we increase the
temperature of the salt solution, then the rate at which atoms are taken away
1s increased, and so is the rate at which atoms are brought back. It turns out
to be very diflcult, in general, to predict which way it is going to go, whether
more or less of the solid will dissolve. Most substances dissolve more, but some
substances dissolve less, as the temperature increases.
1-4 Chemical reactions
In all of the processes which have been described so far, the atoms and the
lons have not changed partners, but of course there are cireumstances in which
the atoms do change combinations, forming new molecules. 'This is ilustrated in
Eig. I-8. Á process in which the rearrangement of the atomic partners OcCUTS is
what we call a chemjcal reaction. The other processes so far described are called
physical processes, but there is no sharp distinction bebween the bwo. (Nature
does not care what we call it, she just keeps on doïng it.) Thịs figure is supposed
to represent carbon burning in oxygen. In the case oŸ oxygen, #o oxygen atoms
sbick together very stronply. (Why do not #hree or even ƒour stick together? That
is one of the very peculiar characteristics of such atomic processes. Atoms are
--- Trang 46 ---
`/{Š Ý
¬ K.ey 0559555 956
CARBON BURNING IN OXYGEN
Figure 1-8
very special: they like certain particular partners, certain particular directions,
and so on. lt is the job of physics to analyze why each one wants what it wants.
At any rate, two oxygen atoms form, saturated and happy, a molecule.)
The carbon atoms are supposed to be in a solid crystal (which could be
graphite or diamond*). Now, for example, one of the oxygen molecules can come
over to the carbon, and each atom can pick up a carbon atom and go fying of
in a new combination—“carbon-oxygen”—which is a molecule of the gas called
carbon monoxide. It is given the chemical name CO. It is very simple: the letters
“CO” are practically a picbure of that molecule. But carbon attracts oxygen
much more than oxygen attracts oxygen or carbon attracts carbon. 'Pherefore
in this process the oxygen may arrive with only a little energy, but the oxygen
and carbon will snap together with a tremendous vengeance and commotion, and
everything near them will pick up the energy. A large amount of motion energy,
kinetic energy, is thus generated. This of course 1s burnzng; we are getting hea
from the combination oŸ oxygen and carbon. The heat is ordinarily in the form
of the molecular motion of the hot gas, but in certain circumstances it can be so
enormous that it generates /2gh. That is how one gets fiames.
In addition, the carbon monoxide is not quite satisfed. It is possible for it to
attach another oxygen, so that we might have a much more complicated reaction
in which the oxygen is combining with the carbon, while at the same time there
happens to be a collision with a carbon monoxide molecule. Ône oxygen atom
could attach itself to the CO and ultimately form a molecule, composed of one
carbon and two oxygens, which is designated COsa and called carbon dioxide. lf
we burn the carbon with very little oxygen in a very rapid reaction (for example,
in an automobile engine, where the explosion 1s so fast that there is not time
* One can burn a diamond in air.
--- Trang 47 ---
for it to make carbon dioxide) a considerable amount of carbon monoxide is
formed. In many such rearrangements, a very large amount oŸ energy is released,
forming explosions, Ñames, etc., depending on the reactions. Chemists have
studied these arrangements of the atoms, and found that every substance is some
type OŸ arrangement oƒ atoms.
To illustrate thịs idea, let us consider another example. lIf we go into a fñeld
of small violets, we know what “that smell” is. It is some kind of molecule, or
arrangement of atoms, that has worked Its way into our noses. First of all, hou
dịd it work its way in? That is rather easy. If the smell is some kind of molecule
in the aïr, jiggling around and being knocked every which way, it might have
accidentallu worked its way into the nose. Certainly it has no particular desire to
get into our nose. lt is merely one helpless part of a jostling crowd of molecules,
and in its aimless wanderings this particular chunk of matter happens to fñnd
1tself in the nose.
Now chemists can take special molecules like the odor of violets, and analyze
them and tell us the ezøc# arrangement of the atoms in space. We know that
the carbon dioxide molecule is straight and symmetrical: O—C——O. (That can
be determined easily, too, by physical methods.) However, even for the vastly
more complicated arrangements of atoms that there are in chemistry, one can,
by a long, remarkable process of detective work, fnd the arrangements of the
atoms. Figure l-9 is a picture of the air in the neighborhood of a violet; again
we find nitrogen and oxygen in the air, and water vapor. (Why is there water
vapor? Because the violet is œef. AII plants transpire.) However, we also see
a “monster” composed of carbon atoms, hydrogen atoms, and oxygen atoms,
which have picked a certain particular pattern in which to be arranged. It is
a mụch more complicated arrangement than that of carbon dioxide; in fact, 1%
2 ©° 42
4JDD4Đ
ODOR OF VIOLETS
Figure 1-9
--- Trang 48 ---
CHa: CHs
N >c< HN ọ