I think the most straightforward way is via a phase diagram that is contoured for density (lines of constant density are called isochores). For example: http:/

I think the most straightforward way is via a phase diagram that is contoured for density (lines of constant density are called isochores). For example: http://eurjmin.geoscienceworld.org/content/15/5/773/F3.large.jpg http://eurjmin.geoscienceworld.org/content/15/5/773/F3.large... A straightforward way to distinguish between liquids and gasses is their density. Liquid water is a high density fluid and steam is low density fluid. There is line on the phase diagram that separates the liquid field from the vapour field. On that line is the only place on the diagram where liquid and vapour can coexist, i.e. where boiling can occur. It happens to be the case that the surface of the earth is in the liquid field, but within vicinity of that line, so if you heat some water up, you can watch it boil. However, at ~250°C, you need to be at a pressure of ~4 MPa to observe boiling. At those conditions the density of the liquid will be ~0.8 g/cc and the vapour ~0.2 g/cc. The liquid-vapour phase boundary (the boiling curve) terminates at the critical point (647 K and 22.064 MPa). Above the critical point H2O is supercritical. On inspection of the phase diagram we see isochores radiating out from the critical point. Above the critical point the density of water can vary smoothly as a function of P and T, and there is no boiling, condensation, etc. I don't know anything about power generation, but presumably when you can maintain temperatures higher than the T of the critical point, you don't have to worry about losing energy to phase changes.