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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
does not change. Therefore the Gibbs free energy per mole of
two phases in equilibrium are equal. Such equalities are very useful.
W. Craig Carter