By plotting a gas’ temperature and pressure on a p-T diagram and seeing what region of the graph it’s in, we can find what phase the material is in:
In each region, only one phase (solid, liquid or gas) exists.
Along the lines, the material is in phase equilibrium, meaning that two phases exist: line 1 is the sublimation line, where solid and gas both exist, line 2 is the fusion line, where liquid and solid both exist, and line 3 is the vaporisation line, where liquid and gas both exist.
At point a, all three phases exist. This is called the triple point.
Point b is the critical point. Beyond this there is no distinct change between liquid and gas.
Gibb’s phase rule is often used with this graph:
This gives the number of degrees of freedom, F, using the number of components in the mixture, C, and the number of phases, P (given by the location on the graph). The degrees of freedom are the things that can be changed with the material remaining in the same phase, eg temperature and pressure.