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