ABSTRACT
When a pure solid species is heated through its melting point (melting temperature), the solid changes to a liquid. At the melting point, an equilibrium is established between the solid and liquid phases. Conversely, when a pure liquid is cooled through its freezing point or freezing temperature, the liquid is transformed to a solid, and at the freezing
point, equilibrium is established. Thus the melting and freezing temperatures of a pure substance are identical and in this case the terms can be used interchangeably. (However this is often not the case for a mixture, as the freezing temperature, where solid first starts to appear from a solid mixture, is often not the same as the melting temperature, where solid first starts to melt in a solid mixture (see Topic D5).)
At the melting point, the equilibrium for a pure species A is:
and as for all equilibria, the change in Gibbs free energy for the forward reaction, ∆G, is zero under all conditions (see Topic C1). For each phase, for a small change in free energy, dG (see Topic B6), dG=Vdp−SdT and therefore d∆G=∆Vdp−∆SdT, where ∆G, ∆V and ∆S are the changes in Gibbs free energy, volume and entropy during the forward reaction, so that at equilibrium, ∆G=G(1)−G(s)=0, ∆V=V(1)−V(s) and ∆S=S(1)−S(s), with ∆G, ∆V and ∆S being the change in the Gibbs free energy, volume and entropy on melting respectively. This means that:
The change in entropy on melting is always positive, as liquid species have more freedom of movement than solid species. The change in volume is also usually positive, as melting a solid produces a liquid in which the molecules move around more (have more translational energy), and as a consequence occupy more space. In this case, dp/dT is positive, and increasing the pressure increases the melting temperature. A notable exception to this is water, as solid water (ice) has an open, hydrogen-bonded structure, which occupies more volume than liquid water. This is why icebergs float, and as a consequence ∆V is negative; in this case dp/dT is negative (Fig. 1b).
