A phase diagram is a graphical representation of the various states of a substance as they relate to temperature and pressure. These diagrams can be used to predict the physical behavior of materials. Some phase diagrams are simple with three areas that represent gas, liquid and solid phases, separated by three two-phase lines. Other diagrams can be complex with multiple liquid and or solid phases, but only one gas phase.

The simplest phase diagrams are plotted as temperature versus pressure, and consist of three areas corresponding to the phases of material. In general, at high temperatures and low pressures materials usually exist as gases; at low temperatures materials usually exist as solids. It is possible for two states to exist simultaneously. The two phases are said to coexist in equilibrium and can be liquid-vapor, solid-vapor, or solid-liquid. The lines joining the three areas show where material...