Phase diagrams

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.

Back to Contents: Physics: Thermodynamics

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