TABLE OF CONTENTS Consider a one-component system in equilibrium in field-free space so that the only external force acting on it is pressure and its only thermodynamic variables are temperature, pressure, and volume. Over the temperature and pressure ranges of interest, the system can exist in gas, liquid, and solid phases, but assume that there is only one solid phase. For each phase, the pressure, temperature, and volume are connected by an equation of state. The first step in understanding the possible equilibria among these phases is to determine the conditions of temperature and pressure under which the phase can coexist. Quite a lot can be learned by simple applications of thermodynamics.
From the phase rule, if the solid, liquid, and gas phases exist simultaneously, then there are no degrees of freedom. That is, there is only one pair of values of temperature and pressure at which the three phases can coexist. In a pressure-temperature diagram, this pair of values is represented by a point called the triple point.
If two phases coexist, then there is one degree of freedom, which means that either the pressure or the temperature, but not both, can be freely chosen. This corresponds to a curve in the pressure temperature diagram for each pair of phases solid-liquid, solid-vapor, and liquid-vapor. For brevity, let us call these S-L, S-V, and L-V curves. The three curves obviously meet at the triple point since each pair of curves (S-L and S-V, S-V and L-V, S-L...
