A rule stating that if the density of the vapour phase above a liquid is constant, the entropy of vaporization of a mole of liquid is constant. This law does not hold if molecular association takes place in the liquid or if the liquid is subject to quantum-mechanical effects, e.g. as in superfluidity. The Hildebrand rule is named after the US chemist Joel Henry Hildebrand (1881–1983), who stated it in 1915.