In the Chapter 10 we established that changes of phase can be described with the aid of a phase diagram. We spent some time establishing the general form of a phase diagram for a ‘typical substance’ and arrived at something like Figure 11.1. The three transitions between the phases that occur as a result of temperature changes at constant pressure are shown as double-ended arrows. In this chapter we would like to examine experimental data on all aspects of this diagram: the slopes of the phase boundaries and their points of intersection. In practice the amount of data involved is amazingly large so we restrict ourselves to data on the elements and a small range of relatively simple substances (§11.2).

Figure 11.1 A generic PT phase diagram

§11.3, §11.4 The solid <==> liquid and liquid ¤ gas transitions: Although we refer to these transitions in the same manner, and they are both represented by the crossing of a single line in Figure 11.1, they are physically very different processes. We will find that we need to develop quite different microscopic models to understand each type of transition.

For the solid <==> liquid transition we test the Lindemann hypothesis that the phase transition occurs when molecular vibrations reach a certain amplitude. For the liquid <==> gas transition we develop the cell model of a liquid and its vapour pressure (§9.8) and find a connection between the microscopic parameters of the cell model and the latent heat of vaporisation.

§11.5, §11.8 The critical point and triple point: Examining the data on these points reveals that they occur at similar relative positions on the phase diagrams of quite different substances.

§11.6 Scaling: laws of corresponding states: Throughout this book, we have discovered correlations between the physical properties of different substances. This raises the hope that we might be able to use some properties of a substance to predict an unknown property, either of that substance or another one. We will find that sometimes we can do this and sometimes we can not. But what is the origin of these partial correlations?

§11.7 The solid <==> gas transitions: Compared with the phase transitions considered in §11.3 and §11.4 the solid ¤ gas transition is relatively understudied. We will find it is relatively easy to understand the process using a surprising extension of the cell model of a liquid.

§11.9 Other types of phase transitions: We mentioned in Chapter 10 that there is the concept of a phase transition could be applied more widely than to changes between solids in liquids and gas. In this section we look at what these transitions are and how they can be studied .