Phase Changes

When a body changes phase (e.g., melts, freezes, evaporates, changes crystal structure), its enthalpy increases or decreases depending on whether heat is absorbed or given off at constant pressure. When the body changes phase at the equilibrium temperature, the entropy of the universe should not change therefore $G \equiv H - TS$ does not change. Therefore the Gibbs free energy per mole of two phases in equilibrium are equal. Such equalities are very useful.