Figure
5–2b shows what happens when we remove the covering
on the liquid water surface. Without the covering, some of the molecules at the surface can escape
into the overlying volume
as water vapor. The process whereby molecules
break
free of the liquid volume is known as evaporation. The opposite process is condensation, wherein
water vapor molecules
randomly collide with the water surface and bond
with adjacent molecules. At the beginning of our
hypothetical
experiment, no condensation could occur because
no
water vapor was present. As evaporation begins,
however,
water vapor starts to accumulate above the surface of the liquid.
At the
early stages of evaporation, the low water vapor
content
prevents much condensation from occurring, and
the
rate of evaporation exceeds that of condensation. This leads to an increase in the amount of water
vapor present. With
increasing water vapor content, however, the condensation rate likewise increases.
Eventually, the amount of water vapor above the surface is enough for the rates
of condensation and evaporation to become equal, as shown in Figure 5–2c. A
constant amount of water vapor now exists in the volume above the water surface
due to offsetting gains and losses by evaporation and condensation. The resulting
equilibrium state is called saturation. When this equilibrium exists in the atmosphere,
the air is said to be saturated.
The
state of saturation described here can occur whether or not air (or other
gases, for that matter) exists in the container. In other words, the water
vapor is not “held” by the air (although this erroneous statement is frequently
made). Water vapor is a gas, just like the other components of the air. Thus, it
does not need to be “held” by air any more than the oxygen, nitrogen, argon,
and other gases of the atmosphere need to be held by water vapor! When the air
is saturated, there is simple an equilibrium between evaporation and
condensation; the dry air plays no role in achieving this state. It is also
important to realize that the exchange of water vapor and liquid described here
applies as well to the change of phase between water vapor and ice. The change
of phase directly from ice to water vapor, without passing into the liquid
phase, is called sublimation. The reverse process (from water vapor to ice) is called
deposition (Figure 5–3).
