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 (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 fixed positions of atoms in a solid and the random molecular motion that characterises a gas.
Consequently, even though liquids are obviously uniquely distinct from solids or gases, we will discuss them as being intermediate between solids and gases. 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. But for solids there was no single model of an ‘ideal solid’. Liquids again 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.