Phase transition

Summary

A phase transition is a change of a substance from one phase to another, such as melting, boiling or the onset of magnetism. In thermodynamics it shows up as a point where the equilibrium free energy is not analytic in the control parameters, such as temperature and pressure. Transitions are sorted by the Ehrenfest classification or, today, into first-order and continuous ones. They are mapped in a phase diagram and studied near critical points by Landau theory and the theory of critical phenomena.