ABSTRACT
The relationship of specific or molar volume to temperature and pressure for a pure substance in an equilibrium state can be represented by a surface in three dimensions, as shown in Fig. 3.1. The shaded surfaces marked S, L, and G in Fig. 3.1 represent, respectively, the solid, liquid, and gas regions of the diagram. The unshaded surfaces are the regions of co-existence of two phases in equilibrium, and there are three such regions: solid-gas (S-G), solid-liquid (S-L), and liquid-gas (L-G).
