Heat of Transformation

When a material changes phase heat is either liberated or must be supplied. The amount of heat associated with the transformation of unit mass of the material is the Heat of Transformation, l12 = Q/m. In general, there will also be a volume change associated with this process and so work is done against the ambient pressure: w = p(v2 - v1). From the First Law, the change in the specific internal energy of the system is given by:
du = (u2 - u1) = l12 - p(v2 - v1).

The phase transition process occurs under conditions of constant temperature and pressure and hence the enthalpy change is: dh = du + pdv. Substituting into the expression for the heat of transformation gives: l12 = (h2 - h1)p = (Tds)p. The entropy change associated with the phase transition is therefore: ds = (l12 / T)p .
For Water at One Atmosphere:

Heat of Melting (fusion) = 333 kJ/kg; Heat of vaporization (condensation) = 2260 kJ/kg