Entropy of Saturated Liquid-Vapour Mixtures, and Trouton's Rule

Abstract

IT is well known that the entropy of a saturated vapour usually diminishes continually as the saturation temperature and pressure increase up to the critical values. On the other hand, the entropy of the saturated liquid increases continually. At any equilibrium below the critical, the vapour entropy S_v is greater than the critical entropy S_c, while the liquid entropy S_l is less. Thus it is possible to define a mixture of saturated liquid and vapour, say, of dryness q, such that the entropy of the mixture will be equal to the entropy at the critical point. The defining equation will be (1 - q)S_l + q S_v = S_c.......(1)