A collection of molecules of H₂O are capable of smashing the steel hull of an ocean liner if their temperature is 272 K. If their temperature is increased by less than 1 % to 274 K they pose no danger at all. When ice melts some of its properties change dramatically even though the temperature changes by only a tiny amount. The phenomenon of ‘sudden change’ of properties is the most general, and the most striking, characteristic of phase changes.

In the preceding chapters we have examined the properties of gases, solids and liquids, and seen how it is possible to understand their behaviour in terms of the atoms and molecules of which they are made. The division into just three phases is natural, since in our experience solids, liquids and gases behave strikingly differently. However, the division raises several questions that we will address in this chapter.

§10.2 Free energy: It is frankly impossible to understand phase transitions in anything more than a qualitative sense without understanding the role of free energy. In this section we will introduce the concept and use it to estimate the melting and boiling temperatures of potassium metal. The key result (which is demonstrated explicitly in Appendix 3) is that the equilibrium phase of a substance is the phase with the lowest Gibbs free energy.

§10.3, §10.4 Phase transitions and Enthalpy change on transformation: In these sections we look at the key result from §10.2 and extend our analysis to enable us to estimate not only the temperatures at which transitions take place, but also the amount of heat energy required to make them happen.

§10.5 The ‘order’ of a phase transition: The transition temperature of a phase transition can mark one of two things depending on its order. It can mark the temperature below which one phase has completely changed into another. This is the ‘normal’ or first-order type of phase change. For example, on cooling below its melting temperature, a substance becomes completely solid. Alternatively, for so called non-first order or continuous transitions, the transition temperature marks the temperature below which one phase begins to change into another.

§10.6 Nucleation: supercooling and superheating: In order to change from one phase (e.g. liquid) to another (e.g. solid) a substance has to somehow ‘know’ that a solid has a lower free energy than a liquid. But how can a liquid ‘know’ anything about a solid?

§10.7 Phase diagrams: The transition temperatures between phases change with pressure in an interesting and complicated manner. Sometimes we wish to summarise a great deal of information about the general form of these changes and this can be achieved with a phase diagram. Here we learn what phase diagrams are, how they are constructed, and the significance of some of their special features.