ABSTRACT
The thermodynamic treatment of simple phase transitions is straightforward and is discussed in A2.1.6 and therefore need not be repeated here. In a one-component two-phase system, the phase rule yields one degree of freedom, so the transition between the two phases can be maintained along a pressure-temperature line. Figure A2.5.1 shows a typical p, T diagram with lines for fusion (solid-liquid), sublimation (solid-gas), and vaporization (liquid-gas) meeting at a triple point (solid-liquid-gas). Each of these lines can, at least in principle, be extended as a metastable line (shown as a dashed line) beyond the triple point. (Supercooling of gases below the condensation point, supercooling of liquids below the freezing point and superheating of liquids above the boiling point are well known; superheating of solids above the melting point is more problematic.) The vaporization line (i.e. the vapour pressure curve) ends at a critical point, with a unique pressure, temperature, and density, features that will be discussed in detail in subsequent sections. Because this line ends it is possible for a system to go around it and move continuously from gas to liquid without a phase transition; above the critical temperature the phase should probably just be called a ‘fluid’.
