A phase is defined as a physically distinct region. Under certain conditions, every substance may exist in one or more phases. It may be a solid, a liquid, or a gas (or vapor). Also, some substances have allotropic solid forms. For instance, carbon may exist as either graphite or diamond in its solid phase.

It has long been known that most substances can change from one phase to another. Aristotle (384-322 B.C.) noted that water could be changed into "air" (gas) and could come out of "air" as well.

The phase change from liquid to gas is called vaporization or boiling (depending on the circumstances; see below), and the opposite change, from gas to liquid, condensation; from solid to liquid, melting, and the opposite, from liquid to solid, freezing; and from solid to gas, sublimation.

Energy must be added or removed from substances during phase...