SAT Subject Test Chemistry




Liquids, Solids, and Phase Changes


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.

Figure 25. Partial Phase Diagram for Water (distorted somewhat to distinguish the triple point from the freezing point)


Know the significance of each gray area and boundary line.

At 0°C the freezing point of water is found along the line BC at point E for pressure at 1 atmosphere or 760 millimeters. Again, this point is affected by pressure along the line BC so that, if pressure is decreased, the freezing point is slightly higher up to point B or 0.01°C.

Point B represents the point at which the solid, liquid, and vapor phases may all exist at equilibrium. This point is known as the triple point. It is the only temperature and pressure at which three phases of a pure substance can exist in equilibrium with one another in a system containing only the pure substance.


The triple point is the only temperature and pressure at which all three phases of a substance can exist.