Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

11.2 Phase Transitions

Melting is the change of a solid to the liquid state (melting is also referred to as

fusion). For example,
H ₂O_(s) H₂O_(l) (melting, fusion)

ice, snow liquid water

Freezing is the change of a liquid to the solid state. The freezing of liquid water to

ice is a common example

H ₂O_(l) H₂O_(s) (freezing)

liquid water ice

The casting of a metal object involves the melting of the metal, then its freezing in a mold.

Vaporization is the change of a solid or a liquid to the vapor. For example,

H ₂O_(l) H₂O_(g) (vaporization)

liquid water water vapor

The change of a solid directly to the vapor is specifically referred to as sublimation. Although the vapor pressure of many solids is quite low, some (usually molecular solids) have appreciable vapor pressure. Ice, for instance, has a vapor pressure of 4.7 mmHg at 0_C. For this reason, a pile of snow slowly disappears in winter even though the temperature is too low for it to melt. The snow is being changed directly to water vapor.

H ₂O_(s) H₂O_(g) (sublimation)

ice, snow water vapor

Sublimation can be used to purify solids that readily vaporize. Figure 11.3 shows

a simple way to purify iodine by sublimation. Impure iodine is heated in a beaker so that it vaporizes, leaving nonvolatile impurities behind. The vapor crystallizes on the bottom surface of a dish containing ice that rests on top of the beaker. Freeze-drying of foods is a commercial application of sublimation. Brewed coffee, for example, is frozen and placed in a vacuum to remove water vapor. The ice continues to sublime until it is all gone, leaving freeze-dried coffee. Most freeze-dried foods are easily reconstituted by adding water.Condensation is the change of a gas to either the liquid or the solid state (the change of vapor to solid is sometimes called deposition). Dew is liquid water formed by condensation of water vapor from the atmosphere. Frost is solid water formed by direct condensation of water vapor from the atmosphere without first forming liquid water. Snow is formed by a similar process in the upper atmosphere.

H ₂O_(g) H₂O_(l) (condensation)

water vapor dew

H ₂O_(g) H₂O_(s) (condensation, deposition)

water vapor frost, snow