Phase diagram surfaces

Question

When looking at the 3D phase diagram of water I felt unsure whether the only physical points were the ones on the surface of the plot.

If it is unphysical to move outside the surface of the phase diagram ("on shell"?) the question might be moot, but I guess there are conditions for when the phase diagram applies/does not apply. I am not sure when thermodynamics applies and does not apply; is perhaps heating ice by whispering at it too fast a process, or using a frying pan too fast a process, or microwaving too fast a process, or exploding with a nuke too fast a process, is it the energy flow per unit time over some volume that decides whether the process is too fast, etc.

So the question is when is a phase diagram valid and what kinds of processes follow the phase diagram and does the process necessarily need to lie embedded as a curve in the surface of the phase diagram?

Answer 1

I am not sure to understand your question.

A phase diagram typically describes the equilibrium state, in this case of water, depending on different thermodynamic parameters. So a phase exists for every point of the graph and typically a shell, as you describe, represents a phase transition, an interface between two phases.

You can connect two points of the phase space by changing the thermodynamic parameters and letting relax. You could describe non-equilibrium process, but should be very careful about what you define as your parameter.

Answer 2

The 3D phase diagram know also as thermodynamic phase diagram is associated to a binary system. 2 axis are composition of each component respectively. Then the third axis is for temperature. Therefore, phased are not limited to a state phase as liquid, solid or gas.

The diagrams show phase diagrams in the context of solutions. In all diary system. Some authors call these diagrams a crystallization diagram. Here is a good presentation on the subject here