Negative deviations from Rauolt's Law yield phase diagrams similar to that below. These have a maximum boiling azeotrope.
Remember open and closed systems can behave differently. If we were to conduct a distillation (the system would be open). The first would initially have vapor composition closer to the azeotrope than the original liquid. As the distillation proceeds, the liquid and vapor composition move more toward the pure component. In contrast, for the negative deviations, the vapor composition is initially more like the pure component, and later both the liquid and vapor move toward the composition of the azeotrope. The blue arrows indicate the direction of composition drift.
The critical temperature, Tc, is the temperature above which the two liquids are miscible in all proportions. When the Tc gets higher, the two regions approach each other and the following diagram results.
On the above diagram, A (liquid heavy in A), B (solution of A and B), and C (liquid heavy in B) can all be in equilibrium at one temperature (within a certain overall composition range). In other words, a 3 phase region separates the two two-phase regions.
Sometimes compounds are formed in the mixture, such as MgZn(2). This has a fixed stoichiometric ratio. A typical phase diagram might be:
Note that this is almost like two separate diagrams with the compound, C in between. Thus on each side the pure A or B is in equilibrium with the compound. There are two different eutectics in this diagram.
Think about what would happen to the liquid in each of the different regions of the phase diagram as it was cooled. What would be in equilibrium with what?
Sorry, but haven't yet had the ambition to draw in solid solutions or phase diagrams with incongruent melting points.