Clausius–Clapeyron equation and saturation vapour pressure: simple theory reconciled with practice

While the Clausius–Clapeyron equation is very important as it determines the saturation vapour pressure, in practice it is replaced by empirical, typically Magnus-type, equations which are more accurate. It is shown that the reduced accuracy reflects an inconsistent assumption that the latent heat of vaporization is constant. Not only is this assumption unnecessary and excessive, but it is also contradictory to entropy maximization. There is an additional erroneous assumption for the derivation of the Clausius–Clapeyron equation, related to the equality of chemical potentials of the two phases, which does not affect the final result but puts into question the logical coherence of the equation's derivation. Removing these assumptions and using a pure entropy maximization framework we obtain a simple closed solution which is both theoretically consistent and accurate. Our discussion and derivation are relevant to students and specialists in statistical thermophysics and in geophysical sciences, and our results are ready for practical application in physics as well as in such disciplines as hydrology, meteorology and climatology.