13.2. Liquids http://www.ck12.org
In order for a molecule to escape into the gas state, it must have enough kinetic energy to overcome the intermolecular
attractive forces in the liquid. Recall that a given liquid sample will have molecules with a wide range of kinetic
energies. Liquid molecules with a kinetic energy that is above a certain threshold are able to escape the surface and
become vapor. Because only the highest energy molecules are leaving the liquid state, the collection of molecules
that remain in the liquid now have a lower average kinetic energy. Thus, as evaporation occurs, the temperature of the
remaining liquid decreases. You have observed the effects of evaporative cooling. On a hot day, the water molecules
in your perspiration absorb body heat and evaporate from the surface of your skin. The evaporating process leaves
the remaining perspiration cooler, which, in turn, absorbs more heat from your body.
A given liquid will evaporate more quickly when it is heated. This is because the heating process results in a greater
fraction of the liquid’s molecules having the necessary kinetic energy to escape the surface of the liquid. The figure
below (Figure13.9) shows the kinetic energy distribution of liquid molecules at two temperatures. The number
of molecules that have the required kinetic energy to evaporate are shown in the shaded area under the curve at the
right. The higher temperature liquid (T 2 ) has more molecules that are capable of escaping into the vapor phase than
the lower temperature liquid (T 1 ).
FIGURE 13.9
Kinetic energy distribution curves for a
liquid at two temperatures, T 1 and T 2. The
shaded area represents the molecules
with enough kinetic energy to escape the
liquid and become vapor.Vapor Pressure
When a partially filled container of liquid is sealed with a stopper, some liquid molecules at the surface evaporate
into the vapor phase. However, the vapor molecules cannot escape from the container. Over time, some of the
molecules lose energy through collisions with other molecules or with the walls of the container. At this point, the
less energetic vapor molecules are trapped by the attractive forces of the molecules in the liquid, and they begin to
condense back into the liquid form. Eventually, the system reaches a point where the rate of evaporation is equal to
the rate of condensation (Figure13.10). This is called a dynamic equilibrium between the liquid and vapor phases.