The Foundations of Chemistry

EVAPORATION

Evaporation,or vaporization,is the process by which molecules on the surface of a liquid
break away and go into the gas phase (Figure 13-10). Kinetic energies of molecules in
liquids depend on temperature in the same way as they do in gases. The distribution of
kinetic energies among liquid molecules at two different temperatures is shown in Figure
13-11. To break away, the molecules must possess at least some minimum kinetic energy.
Figure 13-11 shows that at a higher temperature, a greater fraction of molecules possess
at least that minimum energy. The rate of evaporation increases as temperature increases.
Only the higher-energy molecules can escape from the liquid phase. The average molec-
ular kinetic energy of the molecules remaining in the liquid state is thereby lowered,
resulting in a lower temperature in the liquid. The liquid would then be cooler than its
surroundings, so it absorbs heat from its surroundings. The cooling of your body by evap-
oration of perspiration is a familiar example of the cooling of the surroundings by
evaporation of a liquid. This is called “cooling by evaporation.”

13-6

Figure 13-11 Distribution of

kinetic energies of molecules in a

liquid at different temperatures. At

the lower temperature, a smaller

fraction of the molecules have the

energy required to escape from the

liquid, so evaporation is slower and

the equilibrium vapor pressure

(Section 13-7) is lower.

13-6 Evaporation 495

Breeze

Molecules

of the air

(a) (b) (c)

Figure 13-10 (a) Liquid continuously evaporates from an open vessel. (b) Equilibrium
between liquid and vapor is established in a closed container in which molecules return to
the liquid at the same rate as they leave it. (c) A bottle in which liquid–vapor equilibrium
has been established. Note that droplets have condensed.

Minimum kinetic energy required

for escape of molecules from

surface of liquid

Kinetic energy

Fraction of molecules atgiven kinetic energy

Higher temperature

Lower temperature

The dew on this spider web was

formed by condensation of water

vapor from the air.