The heat of fusion is the heat that is absorbed to transform a substance from its solid state to its liquid state at constant, that is, to melt the solid substance. In common usage, the heat of fusion is used in place of the more precise term the enthalpy of fusion, which has the symbol ()H_fus. The enthalpy of fusion is the heat of fusion for melting of one mole of the substance under three specific conditions: (1) the pressure remains constant, (2) the only possible work that occurs is expansion against the atmosphere (so-called P()V work) and (3) the temperature remains constant during the process. A heat of fusion for a substance is only valid for conversion of the pure solid to the pure liquid state of the substance.

Unlike many other types of enthalpy changes, enthalpies of fusion for a large number of...