Phase transition
A change from one phase to another — melting, boiling, sublimation — that occurs at a definite temperature and pressure and absorbs or releases latent heat.
Definition
A phase transition is the transformation of a substance from one phase to another: melting (solid to liquid), freezing, vaporisation (liquid to gas), condensation, sublimation (solid to gas), and deposition. It occurs along a coexistence curve in the pressure–temperature plane, at a sharply defined temperature for a given pressure.
First-order transitions, such as ordinary melting and boiling, absorb or release latent heat at constant temperature and involve a discontinuous change in density and other properties. Continuous (second-order) transitions, such as the loss of ferromagnetism at the Curie point, have no latent heat but show diverging response functions — the subject of critical phenomena.
Transitions generally require nucleation: melting and freezing begin at sites such as impurities or surfaces. In their absence a substance can be driven past the transition into a metastable state — supercooled liquid or superheated liquid — until a disturbance triggers the change.
History
Latent heat at transitions was discovered by Joseph Black (1760s); the modern theory of continuous transitions and universality developed through the twentieth century, culminating in the renormalization group.