On Transitions in Condensed Systems

Abstract

In this paper there is presented a method of studying transitions in condensed phases based on the thermodynamic equation (δE/δV)_T = T(δS/δV)_T, which holds for low pressures. (δE/δV)_T and T(δS/δV)_T are plotted as functions of V, the latter for various temperatures. Intersections of these curves give the points of equilibrium and the transition points. It is shown that very general and qualitative considerations, taking into account only facts that we definitely know to be true of these curves, with one very reasonable assumption, are sufficient to indicate that as the temperature of a system is increased from absolute zero there must be at least one change of phase, which is taken to be fusion. Changes of phase due to setting in of molecular rotation in the solid are also considered. Curves of N₂ and O₂ are constructed from available experimental data. Finally, an equation of Eyring and Hirschfelder, which would give the trend of the function V(δS/δV)_T with V for liquids, is tested by a new method, and found not to accord very well with the experimental data, especially for low boiling substances.