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of view. However, the atomic hypothesis also describes ørocesses, and so we shall
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°Ồ s
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WATER EVAPORATING IN AIR
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©XYGEN HYDROGEN NITROGEN
Figure 1-5
now look at a number of processes from an atomie standpoint. The first process
that we shall look at is associated with the surface of the water. What happens at
the surface of the water? We shall now make the picture more complieated—=and
more realistic—by imagining that the surface is in air. Eigure I-5 shows the
surface of water in air. We see the water molecules as before, forming a body of
liquid water, but now we also see the surface of the water. Above the surface
we fnd a number of things: First of all there are water molecules, as in steam.
Thìs is 0øfer 0apor, which is always found above liquid water. (There is an
cquilibrium between the steam vapor and the water which will be described later.)
Tn addition we ñnd some other molecules—here two oxygen atoms stuck together
by themselves, forming an ozgen rnolecule, there two nitrogen atoms also stuck
together to make a nitrogen molecule. Air consists almost entirely of nitrogen,
oxygen, some water vapor, and lesser amounts of carbon dioxide, argon, and
other things. So above the water surface is the air, a gas, containing some water
vapor. Now what is happening in this picture? 'Phe molecules in the water are
always jiggling around. Erom time to time, one on the surface happens to be hit a
little harder than usual, and gets knocked away. It is hard to see that happening
in the picture because ït is a sfZll picture. But we can imagine that one molecule
near the surface has Just been hit and is Ñying out, or perhaps another one has
been hit and is fying out. Thus, molecule by molecule, the water disappears——1t
evaporates. But if we ciose the vessel above, after a while we shall fnd a large
number of molecules of water amongst the air molecules. From tỉme to time, one
of these vapor molecules comes fÑying down to the water and gets sbuck again.
So we see that what looks like a dead, uninteresting thing—a glass of water with
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a cover, that has been sitting there for perhaps twenty years—really contains
a dynamic and interesting phenomenon which is goïng on all the time. 'To our
eyes, our crude eyes, nothing 1s changing, but if we could see it a billion times
magnifed, we would see that from its own point oŸ view it is always changing:
mmolecules are leaving the surface, molecules are coming back.
Why do +0e see no change? Because just as many molecules are leaving as are
coming backl In the long run “nothing happens.” If we then take the top of the
vessel of and blow the moist air away, replacing it with dry air, then the number
of molecules leaving is just the same as it was before, because this depends on
the jiggling of the water, but the number coming back is greatly reduced because
there are so many fewer water molecules above the water. Thherefore there are
more going out than coming in, and the water evaporates. Hence, If you wish to
evaporate water turn on the fanl
Here is something else: Which molecules leave? When a molecule leaves 1t
is due to an accidental, extra accumulation of a little bit more than ordinary
energy, which it needs iÝ it is to break away from the attractions of its neighbors.
'Therefore, since those that leave have more energy than the average, the ones that
are left have iess average motion than they had before. 5o the liquid gradually
cools 1ƒ it evaporates. Of course, when a molecule of vapor comes from the air to
the water below there is a sudden great attraction as the molecule approaches the
surface. 'Phis speeds up the incoming molecule and results in generation of heat.
So when they leave they take away heat; when they come back they generate
heat. Of course when there is no net evaporation the result is nothing—the
water is not changing temperature. lf we blow on the water so as to maintain a
continuous preponderance in the number evaporating, then the water is cooled.
Hence, blow on soup to cool it†
Of course you should realize that the processes just described are more
complicated than we have indicated. Not only does the water go into the air, but
also, from time to time, one of the oxygen or nitrogen molecules will come in and
“get lost” in the mass of water molecules, and work its way into the water. Thus
the air dissolves in the water; oxygen and nitrogen molecules will work their way
into the water and the water will contain air. If we suddenly take the air away
from the vessel, then the air molecules will leave more rapidly than they come ïn,
and in doïng so will make bubbles. 'Phis is very bad for divers, as you may know.
Now we go on to another process. In Fig. I-6 we see, from an atomie point of
view, a solid dissolving in water. lf we put a crystal of salt in the water, what
will happen? 5alt is a solid, a crystal, an organized arrangement oŸ “salt atoms.”
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S ) SS C seo \ °,
có )›ề Ả J@” x©
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SALT DISSOLVING IN WATER
® CHLORINE C SODIUM
Figure 1-6
Jigure 1-7 is an ilHustration of the three-dimensiona]l structure oŸ common salt,
sodium chloride. Strictly speaking, the crystal is not made of atoms, but oŸ what
we call jons. An ion is an atom which either has a few extra electrons or has lost
a few electrons. In a salt crystal we find chlorine ions (chlorine atoms with an
extra electron) and sodium ions (sodium atoms with one electron missing). The
1ons all stick together by electrical attraction in the solid salt, but when we put
them in the water we fñnd, because of the attractions of the negative oxygen and
positive hydrogen for the ions, that some of the ions jiggle loose. In Eig. 1-6 we
see a chlorine ion getting loose, and other atoms foating in the water in the form
of lons. This picture was made with some care. Notice, for example, that the
hydrogen ends of the water molecules are more likely to be near the chlorine ion,
. _ˆ 8
Rode S52 LEEL.
Sylvine K ClI | 6.28 È
Ag | Cl | 5.54
H55
Pb | Se | 6.14 đd mi
Pb | Te | 6.34 ù Ò ©
Nearest neighbor
distance d = a/2
Figure 1-7
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