Trouton's rule
![Enthalpies of melting and boiling for pure elements versus temperatures of transition, demonstrating Trouton's rule](/Images/godic/202502/17/Enthalpies_of_melting_and_boiling_for_pure_elements_versus_temperatures_of_transition.svg4522.png")
Trouton’s rule states that the entropy of vaporization is almost the same value, about 85–88 J K mol, for various kinds of liquids at their boiling points. The entropy of vaporization is defined as the ratio between the enthalpy of vaporization and the boiling temperature. It is named after Frederick Thomas Trouton.