Phase Equilibrium

We begin this chapter by following the efforts of Maxwell to show that the attractive forces between molecules were responsible for vapor condensation, at temperatures below the critical temperature found by Andrews for \(\text {CO}_2\) . In his 1875 paper ‘On the dynamical evidence of the molecular constitution of bodies’, Maxwell recognizes the impact of the 1873 Doctoral Thesis of Johannes Diderik van der Waals for clearing the role of the interaction of real molecules on phase transition and does not omit to mention the important work of Willard Gibbs who, in Maxwell’s words, “ has given us a remarkably simple and thoroughly satisfactory method of representing the relations of the different states of matter by means of a model”. Van der Waals equation of state leads to a two-parameter law of corresponding states in the sense that only the critical temperature and pressure are needed for describing the liquid-vapor states of a substance. Most substances require a third parameter for their description. The acentric factor, introduced by Pitzer in 1955, gave rise to a sequence of empirically derived equations of state of substantial practice importance in deriving liquid-vapor phase diagrams. The chapter ends with the presentation of a method for calculating the thermodynamic functions of liquid-vapor systems.