Phase Transition

Materials may exist in the Solid, Liquid, and Gas phase. The (p,v,T) conditions determine which phase or phases are in thermodynamic equilibrium. For certain ranges of (p.v,T) more than one phase can be present in equilibrium. Under these conditions, the addition or removal of heat energy will cause the mass of material in one phase to increase and in the other to decrease. This process is known as a Phase Transition and the amount of heat energy required to transform unit mass from one phase to the other is known as the Heat of Transformation.

The solid to liquid transition is a melting process and the heat required is the heat of melting. The liquid to solid transition is the reverse process and the heat liberated is the heat of freezing. The solid to gas transition is a sublimation process and the heat required is the heat of sublimation. The liquid to gas transition is a vaporization process and the heat required is the heat of vaporization (heat of boiling). Both the gas to solid and the gas to liquid processes are condensation processes and have an associated heat of condensation.