PVT Surface

The (pvT) surface shown opposite is for a material that contracts on freezing. The surface is described by the Equation of State of the material and is the locus of all equilibrium states of the material.

The diagram shows regions associated with solid, liquid and gas phases. Regions in which two phases co-exist are normal to
the (p,T) plane as in such regions isotherms and isobars are identical. As a phase change takes place, material goes from one
phase to the other and the volume of the
system changes. The Gibbs function is the
same for the two phases in equilibrium.