I was reading Chandler's Introduction to Modern Statistical Mechanics and noticed a strange feature in one of the figures. The phase diagram in the image has two triple points; however, according to the Gibbs phase rule, a one-component system with three coexisting phases should have zero degrees of freedom.
The last paragraph in the image talks about single-component systems, so I assume that is what is shown in the figure. I have two questions about this:
- Is a discrete variable (only two points in $(p,T)$ space) considered a true degree of freedom?
- If yes to the first question, how are two triple points possible (as in Chandler's Figure 2.4)?