A phase boundary describes the interface two substances that can remain in contact indefinitely (that is to say, at equilibrium) without mixing, as when oil meets water or air meets stone.

The Gibbs Phase Rule relates the number of phases P which can remain in equilibrium with each other in a system of C chemical constituents given F degrees of freedom.

A degree of freedom could be the concentration of a constituent, or the temperature or pressure of the system. Chemical constituents are defined as distinct components that won't break down under the conditions of interest. For instance, water can be counted as a single constituent, unless conditions might break it down into hydrogen and oxygen, or dissociate to form hydronium and hydroxide ions.

The relation is:

P + F = C + 2

The 2 comes from counting pressure and temperature.

The states of matter – solid, liquid, and gas – are referred to as phases. A transition from one state to another is called a phase change. The freezing and melting temperature for a given material at any pressure, is the same. At this temperature the liquid and solid phases coexist in phase equilibrium. The same also happens in phase transitions between liquid and gas. The heat transfer in a phase change is given by:

where L is the heat of fusion in case of transition between liquid and solid, or heat of vaporization in case of transition between liquid and gas.