Reversible phase transitions occur at constant temperature and pressure. The reversible heat is the enthalpy change for the transition, and the entropy change is the enthalpy change divided by the thermodynamic temperature. For fusion (i.e., melting) of a solid to a liquid at the melting point T m , the entropy of fusion is: Δ Δ S f u s = Δ Δ H f u s T m . Similarly, for vaporization of a liquid to a gas at the boiling point T b , the entropy of vaporization is: Δ Δ S v a p = Δ Δ H v a p T b