liquid equals the external (most often atmospheric) pressure is called the boiling point. In general,
then, the temperature at which a liquid boils is dependent on the pressure surrounding it. We know
water to boil at 100°C because it is at this temperature that its vapor pressure (or the pressure
exerted by the gas phase H 2 O molecules) is equal to one atmosphere. At places of high elevation, the
surrounding pressure is lower than 1 atm and so water boils at a lower temperature. By controlling
the ambient pressure, then, we can change the temperature at which water boils. This is the
principle behind the pressure cooker: By maintaining a high pressure, water can reach a
temperature higher than 100°C before it vaporizes, thus making it more effective at heating things.
LIQUID-SOLID EQUILIBRIUM
The liquid and solid phases can also coexist in equilibrium (such as in the ice-water mixture
discussed above). Even though the atoms or molecules of a solid are confined to definite locations,
each atom or molecule can undergo motions about some equilibrium position. These motions
(vibrations) increase when energy (most commonly in the form of heat) is supplied. If atoms or
molecules in the solid phase absorb enough energy in this fashion, the solid’s three-dimensional
structure breaks down and the liquid phase begins. The transition from solid to liquid is called
fusion or melting. The reverse process, from liquid to solid, is called solidification, crystallization,
or freezing. The temperature at which these processes occur is called the melting point or freezing
point, depending on the direction of the transition. Whereas pure crystals have distinct, very sharp
melting points, amorphous solids, such as glass, tend to melt over a larger range of temperatures,
due to their less-ordered molecular distribution.
GAS-SOLID EQUILIBRIUM
A third type of phase equilibrium is that between a gas and a solid. When a solid goes directly into
the gas phase, the process is called sublimation. Dry ice (solid CO 2 ) sublimes under atmospheric
pressure; the absence of the liquid phase makes it a convenient refrigerant. The reverse transition,
from the gaseous to the solid phase, is called deposition.