A classification of phase transitions in terms of their thermodynamic properties put forward by the Dutch physicist Paul Ehrenfest (1880 -1933).

A first-order phase transition is a phase transition in which the first derivative of the chemical potential is discontinuous. In a first-order phase transition there is a nonzero change in the value of the enthalpy, entropy, and volume at the transition temperature. Melting and boiling are examples of first-order phase transitions.

In a second-order phase transition there is no jump in the value of the enthalpy, entropy, and volume at the transition temperature.