3. Changes of state of a pure body by isobaric heating

Consider a single-phase system consisting of a quantity n of a pure solid body. The system is heated at constant pressure by supplying heat from an external source. The temperature is assumed to be uniform at all times (this is one of the constraints of the representation model). The variation in temperature T as a function of time can thus be noted. The heating curve shows the changes in state of the pure body (figure 1 ).

During melting (step AB of the curve), all the energy received in the form of heat is used to effect the change of state. Let Q _p be the amount of heat required to melt n moles of solid.

According to the first principle...