Phase Equilibria and Phase Changes

The different phases of matter interchange upon the absorption or release of energy, and more than
one of them may exist in equilibrium under certain conditions. For example, at 1 atm and 0°C, an ice
cube floating in water is a system in which the liquid and the solid phases coexist in equilibrium. On
the microscopic level, however, the two phases are constantly interconverting while in this
seemingly static state. Some of the ice may absorb heat and melt, but an equal amount of water will
release heat and freeze. That is, individual H 2 O molecules are going between the solid and liquid
phases constantly, but in a way such that the relative amounts of ice and water remain constant.
This condition in which two opposing processes occur such that the net change in the outcome is
zero is known as dynamic equilibrium.

GAS-LIQUID EQUILIBRIUM

The temperature of a liquid is related to the average kinetic energy of the liquid molecules;
however, the kinetic energy of the individual molecules will vary (just as there is a distribution of
molecular speeds in a gas). A few molecules near the surface of the liquid may have enough energy
to leave the liquid phase and escape into the gaseous phase. This process is known as evaporation
(or vaporization). Each time the liquid loses a high-energy particle, the average kinetic energy of
the remaining molecules decreases, which means that the temperature of the liquid decreases.
Evaporation is thus a cooling process. Given enough kinetic energy, the liquid will completely
evaporate.

If a cover is placed on a beaker of liquid, the escaping molecules are trapped above the solution.
These molecules exert a countering pressure, which forces some of the gas back into the liquid
phase; this process is called condensation. Atmospheric pressure acts on a liquid in a similar fashion
as a solid lid. As evaporation and condensation proceed, an equilibrium is reached in which the rates
of the two processes become equal; that is, the liquid and the vapor are in dynamic equilibrium. The
pressure that the gas exerts when the two phases are at equilibrium is called the vapor pressure.
Vapor pressure increases as temperature increases, because more molecules will have sufficient
kinetic energy to escape into the gas phase. The temperature at which the vapor pressure of the