Typical Phase Diagram

The figure shows a typical phase diagram of an element or a simple compound. The stability of solid, liquid and gas phases depends on the temperature and the pressure. (What is temperature?) The three phases are in equilibrium at the triple point. The gas and liquid phases are separated by a phase transition only below the temperature of the critical point. (Demonstration of critical point in liquid-liquid system, a long list of critical pressures and temperatures) It is possible to change continuously between the two phases at higher temperatures. Only the solid phase exists at the absolute zero of temperature (0 K). There are generally several phases within the solid phase corresponding different crystal symmetries (examples). (Numerical data for different elements.) For mixtures of two or more elements the phase diagrams also depend on the concentrations of the elements (examples). Many links to thermodynamics and statistical physics can be found here.

The phase diagram of helium differs essentially from the typical one.

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