because of strong ionic or covalent bonds in network solids. (Examples: NaCl
and diamond, respectively.)
When heated at certain pressures, some solids vaporize directly without
passing through the liquid phase. This is called sublimation. Solids like solid
carbon dioxide and solid iodine exhibit this property because of unusually high
vapor pressure.
The temperature at which atomic or molecular vibrations of a solid become
so great that the particles break free from fixed positions and begin to slide freely
over each other in a liquid state is called the melting point. The amount of energy
required at the melting point temperature to cause the change of phase to occur is
called the heat of fusion. The amount of this energy depends on the nature of the
solid and the type of bonds present.
PHASE DIAGRAMS
The simplest way to discuss a phase diagram is by an example, such as Figure 25.
A phase diagram ties together the effects of temperature and also pressure on
the phase changes of a substance. In Figure 25 the line BD is essentially the
vapor-pressure curve for the liquid phase. Notice that at a pressure of 760
millimeters of mercury (1 atmosphere) the water will boil (change to the vapor
phase) at 100°C (point F). However, if the pressure is raised, the boiling point
temperature increases; and, if the pressure is less than 760 millimeters, the
boiling point decreases along the BD curve down to point B.
TIP
Know the significance of each gray area and boundary line.