Phase Equilibria in One-Component Systems

Abstract

On melting a substance heat is absorbed in order to transform the crystal into the melt, in which the interatomic interaction is weaker. In addition, the degree of order is lower than in the solid body. The melting process is therefore connected with an absorption of heat (heat of melting or enthalpy of fusion) as well as with a volume change. This implies that the melting equilibria are determined by an interaction of variables of state, that is, pressure and temperature. The direction of the effect of an external pressure is given by Le Chatelier’s principle of the smallest force (1884). According to this principle, an equilibrium is always shifted into the direction leading towards a reduction of a coercion imposed from without (of the given change of the state variables). If, on melting, the volume of a substance is increasing, a pressure rise will result in an increase of the melting temperature, as on solidification of the melt a solid body is formed which is able to counterbalance the external force.