Heat of Transition

(or heat of transformation), the quantity of heat that must be absorbed or given up by a substance when it undergoes an equilibrium constant-pressure and constant-temperature transition from one phase to another. Boiling, melting, crystallization, and changes from one polymorph to another are examples of first-order transitions. In second-order transitions, the heat of transition is zero.

An equilibrium phase transition at a given temperature occurs at a constant temperature known as the transition point, or transition temperature. The heat of transition is equal to the product of the transition point and the difference in the entropy of the two phases between which the transition occurs. Heats of transition may be determined per unit mass, for example, per kg, or per mole.