During the process of melting, the solid and liquid phases of a pure substance are in equilibrium with each other. The amount of heat required to convert one unit amount of substance from the solid phase to the liquid phase — leaving the temperature of the system unaltered — is known as the latent heat of fusion. It is also equal to the enthalpy difference between the solid and liquid phases, ΔHSL.
As a consequence, the latent heat of fusion depends upon the crystal form of the solid phase which should be strictly specified. Direct measurements of moderate accuracy of the latent heat of fusion is relatively straightforward with modern differential scanning calorimeters over a moderate range of conditions of temperature and pressure. In principle, the Clausius-Clapeyron Equation is available,
where the subscript m denotes along the melting line. This implies that information on the variation of the melting temperature with pressure is available; but this is seldom the case so it is not a practical route to evaluate ΔHSL. Compilations of experimental results of DHSL are available for a number of materials [Weast (1982)], but there is no reliable means of their estimation [Reid et al.(1977)].
Weast, R. C., Ed. (1982) Handbook of Chemistry and Physics, CRC Press. Boca Raton, FL.
Reid, R. C., Prausnitz, J. M., and Sherwood, T. K. (1977) The Properties of Gases and Liquids. McGraw-Hill, New York.