TWO SPECIAL CASES:

1. In solid to liquid transition, the specific volume and specific heat are constant in each phase and the value of in each phase is negligible; consequently,

(33.21)

Where h_sf is the latent heat of Fusion

2. If the transition is from liquid to ideal gas, then

Also, so that

(33.22)

This equation is also applicable in the case of sublimation with subscript f (for liquid) replaced by subscript s (for solid).

Phase Equilibrium

If two phases are in a state of equilibrium, the temperature and pressure of both the phases must be identical. The chemical potential of component must be identical in both the phases. That is

for all

(33.23)

Where and refer to two different phases.

The criterion of equilibrium at constant temperature and pressure is given by