Consider the coexistence curve between the liquid and gaseous phases of a pure substance. This is indicated as the vaporization curve between the triple A and critical point C in Figure 3.4.
If we neglect the volume changes of the liquid phase, we have
where the last equality assumes the vapor to be an ideal gas.
The Clausius-Clapeyron equation thus simplifies to
where is the molar latent heat of vaporization. Treating as a constant, we get, along the vaporization curve,
where is the pressure for .
Find the molar heat capacity along the vaporization curve.
On the vaporization curve, we have
where the last equality is valid for an ideal gas, we have
which means for , we have so that the vapor gives off heat as T is raised.