172 ❯ STEP 4. Review the Knowledge You Need to Score High
+++++++++++++++++++−−−−−−−−−−− −−−−−−−−Na+ Cl−Figure 12.3 Sodium chloride crystal lattice.ENRICHMENTPhase Diagrams
The equilibrium that exists between a liquid and its vapor is just one of several that can
exist between states of matter. A phase diagram is a graph representing the relation-
ship of a substance’s states of matter to temperature and pressure. The diagram allows
us to predict which state of matter a substance will assume at a certain combination of
temperature and pressure. Figure 12.4, on the next page, shows a general form of the
phase diagram.
Note that the diagram has three general areas corresponding to the three states
of matter––solid, liquid, and gas. The line from A to C represents the solid’s change
in vapor pressure with changing temperature, for the sublimation equilibrium. The
A-to-D line represents the variation in the melting point with varying pressure. The
A-to-B line represents the variation of a liquid’s vapor pressure with varying pressure.
The B point shown on this phase diagram is called the critical point of the substance,
the point beyond which the gas and liquid phases are indistinguishable from each other.
At or beyond this critical point, no matter how much pressure is applied, the gas cannot
be condensed into a liquid. Point A is the substance’s triple point, the combination