PHASE CHANGES
We refer to the condition of being a solid, liquid, or gas as being in a particular
state or phase. Whether a substance is in one phase or another depends on
temperature and pressure. H 2 O, for instance, turns from solid to liquid or from
liquid to solid at 0°C and 1 atm. It turns from liquid to gas or gas to liquid at
100°C and 1 atm.
When a substance turns from solid to liquid, we say it melts. When it moves in
the reverse direction—from liquid to solid—we say it freezes. So when we think
of H 2 O at 1 atm, we say 0°C is the melting point or freezing point: the
temperature at which it melts or freezes. When a substance turns from liquid to
gas, it vaporizes; when it goes from gas to liquid, it condenses. When we think
of H 2 O at 1 atm, we say 100°C is the boiling point, which means, generally, the
temperature at which it vaporizes or condenses.
You should also be aware of sublimation. This is the process in which a solid
turns directly into a gas. Dry ice (solid carbon dioxide) does this when it is
exposed to room temperature. The opposite of sublimation is deposition, which
is the name of the phase change that occurs when a gas turns directly into a
solid. A common example of deposition is water vapor in the atmosphere turning
directly into solid ice; this is how frost forms on the ground after a cold night.