If we raise the temperature of a solid it will, commonly, become liquid, and then on further heating it will evaporate to become a gas. In this sequence of states (solid <--> liquid <--> gas) the liquid state is intermediate between the solid and gaseous states. This intermediate position is a reflection of the fact that the arrangement of atoms or molecules in a liquid is, in general, intermediate between the crystalline order of the solid state and the random molecular motions of the gaseous state. In what follows we will discuss liquids as being structurally intermediate between solids and gases.

Figure 8.1 Illustration of the motion of atoms in a liquid. Notice the small separation between the atoms, and the random orientation of the vibrations of the molecules. The atoms themselves are shown as a central darkly-shaded region, where the electron charge density is high, and a peripheral lightly-shaded region. The electric field in this peripheral region significantly affects the motion, and disturbs the electronic charge density, of neighbouring atoms.

In general, we will find that most properties of liquids can be understood by explanations that begin in one of two ways. Either we will begin by saying that liquids are similar to solids, but more disordered and slightly less dense, or we will begin by saying that liquids are similar to gases, but more ordered and much more dense. Both approaches are useful for understanding the behaviour of liquids.

When we discussed the properties of gases we were able to arrive at the theory of an ‘ideal gas’ which was, for many purposes, a good approximation to the properties of real gases. However, there was no single model of an ‘ideal solid’ that could explain the diverse properties of solids. Liquids fall into an intermediate category and we will discuss their properties in terms of two simplified models. One model describes the structure of a liquid (§8.3): we will use this model to understand properties such as the density. The other model describes the dynamics of the liquid molecules (§8.4): we will use this model to understand properties such as the viscosity. We will find that we are able to understand many of the properties of real liquids in terms of these models. However, the models are so simplified that we will not really be able to ‘believe’ them in the way that we ‘believe’ the model of an ideal gas. The models capture just one or two key features of liquid behaviour and ignore many properties of the molecules that make up the liquid. The predictions of the models tend to be rather qualitative, allowing to us to examine trends among groups of substances, or variations with temperature, rather than predicting that the viscosity of, say, water at temperature T will be X.