Single Component Phase Equilibria

When there is only one degree of freedom in a single component phase diagram, it was shown above that there must be a relation between $dP$ and $dT$ for the system to remain in two phase equilibrium. Such a relation can be derived as follows:

$$\input{equations/clausius-clapeyron}$$ (26-2)

Equation 26-2 is the famous Clausius-Clapeyron equation.

Consider the behavior of the molar free energy (or $\mu$) on slices of Figure 26-1 at constant $P$ and $T$:

Figure 26-2: Considerations of the molar Gibbs free energy on slices of the single component phase diagram along lines of constant $T$ and constant $P$.

Figure: Behavior of $\ensuremath{\overline{G}} = \mu$ at constant $P$ as a function of $T$. Where the curvature of $\ensuremath{\overline{G}}$ changes sign, the system is unstable. The liquid and vapor curves must be connected to each other and this is illustrated with the "spiny-looking" curve with opposite curvature. The curve for solid is not connected to the others.

Figure: Behavior of $\ensuremath{\overline{G}} = \mu$ at constant $T$ as a function of $P$.

Figure 26-5: Example of single component phase diagram plotted with one derived intensive variable.

What would the plot look like with two extensive variables plotted?