The heat of vaporization is the heat that is absorbed to transform a substance from its liquid state to its vapor, that is, to boil or evaporate the liquid substance completely. In common usage, the heat of vaporization is used in place of the more precise term the enthalpy of vaporization, which has the symbol ()H_vap. The enthalpy of vaporization is the heat of vaporization for vaporizing 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 vaporization for a substance is only valid for conversion of the pure liquid to the pure gaseous state of the substance.

Enthalpies of vaporization can be measured directly for a large number of substances whose natural physical...