In this diagram, each region—solid, liquid, and gas—represents the phase that
will exist for substance X at a given set of pressures and temperatures. For
example, at a pressure of 0.75 atm and 110°C (point A), substance X is a gas.
The normal freezing point (at 1 atm) for substance X is 85°C, and the normal
boiling point (at 1 atm) is 110°C. Any point that lies on a line on the phase
diagram represents a temperature and pressure at which the substance can exist
in both phases. For instance, substance X can be a solid or a liquid at 1 atm and
85°C. Point T is a special combination of pressure and temperature called the
triple point. At this particular pressure and temperature, the substance can exist
as a solid, liquid, or a gas. For substance X, the triple point is at 0.75 atm and
80°C.
In general, when a substance is at relatively low pressure and high temperature,
it exists as a gas. When it is at relatively high pressure and low temperature, it is
a solid. The liquid phase dominates at moderate pressures and temperatures.
Keeping these relationships in mind can help you to predict how a change in
pressure or temperature will affect the phase of a substance. For instance, if
substance X, at 0.75 atm and 110°C (point A) is put under increasing pressure
but its temperature is maintained, what phase change will eventually occur?
Look at the phase diagram for substance X. Follow the dotted line up from point
A (in the direction of increasing pressure). You’ll see that beyond 1 atm (at 110°
C), substance X will become a liquid. So an increase in pressure at constant
temperature will cause substance X to condense.
Vapor Pressure
Even if a solid is well below its melting point, a small number of its molecules
will always have enough kinetic energy to enter the liquid phase. So when you
have a block of ice stored in a freezer, a little bit of it is always melting to form
liquid. The molecules of that liquid immediately lose kinetic energy and form a
solid again, but right away, a few other molecules gain enough kinetic energy to
become liquid. They, too, become solid again after a few seconds. But a few
other molecules take their place, becoming liquid, and then solid again. In other
words, every solid is always melting—on the molecular level—and the
molecules that melt are always refreezing.
The same goes for liquids. Even if it’s well below the boiling point, a few
molecules of a particular liquid always have enough kinetic energy to escape