Rationale

A (p,v,T) surface for a pure substance represents the locus of thermodynamic equilibrium states for the material described by its equation of state. The surface displays zones in which individual phases (solid, liquid, gas) are stable, and regions in which two phases are in equilibrium. When this surface of state is projected onto the (p,T) plane the two-phase regions become lines separating the single phase regions. The phase transition occurs at constant temperature if the pressure is constant. Along the lines separating the phases the value of the Gibbs function is the same for each phase. The slope of the line is described by the Clausius-Clapeyron equation. Materials undergoing a phase transition at constant pressure experience an entropy change and absorb or release heat, the heat of transformation. Phase changes are important in temperature measurement where they serve to define reference temperatures. They may also be used to provide thermal energy at constant temperature in heat storage devices, and in combustion devices condensing the water produced in the combustion reaction provides additional heat, the heat of transformation from vapor to liquid water.