The two types of material equilibrium are phase equilibrium and reaction equilibrium. Phase equilibrium refers to the transport of matter between phases of the system without conversion of one species to another. That is, the same chemical species may be present in several different forms in a system. These phases may inter-convert from one to another, but the overall amount of any one particular phase is constant in time if the system is in equilibrium.

The crossing of any two-phase curve in a phase diagram is called a transition. Special names are given some particular transitions. Melting is the process of a solid going to a liquid, and freezing is a liquid transitioning to a solid. Vaporization is when a liquid transitions to a gaseous or vapor phase. Condensation is a vapor transitioning to a liquid. Sublimation is when a...