 Solution

At equilibrium between two thermodynamic phases of a pure substance, their molar Gibbs free energies are equal. Along a coexistence line,Subtracting the two differentials givesFor a nonzero volume change,The second equality uses $\Delta g=\Delta h-T\Delta s=0$ on coexistence. Total entropy and volume changes may be used instead if they refer to the same amount of material. The ratio must use one consistent transformation direction, so reversing the labels changes both signs and leaves the slope unchanged. This is the general Clapeyron equation; the ideal-vapour approximation often called Clausius-Clapeyron makes further assumptions not needed for two solids.