Most elements are in the solid state at room temperature and only enter the liquid state at elevated temperatures (Table 11.1). For this reason, data on elemental liquids is somewhat rarer than data on the properties of substances that are in their liquid state at around room temperature. Thus, we shall refer commonly to the so-called organic liquids mentioned in §8.2.3. Many properties of liquids are intermediate between those of gases and solids. Recall that the most striking property of matter in its solid state is that it displays both a well-defined volume and shape. This is in contrast with matter in its gaseous state, which expands to fill a container of any shape or volume. In line with its intermediate status, matter in its liquid state has a relatively well-defined volume, but no well-defined shape.

§9.2, §9.3, §9.4 Density, Compressibility and Thermal expansivity: In these three sections we will extend our discussion in §8.3 to allow us to understand the effects of liquid structure. We will attempt to understand the magnitude of the density change on melting, and (in more hand-waving terms) the evolution of the density under changes in both pressure and temperature.

§9.5, §9.11 Speed of sound and Thermal conductivity: Here we deal with transport of energy through liquids, and in both sections we will develop simple ‘hybrid’ models of liquids. We will consider the transport as being the sum of transport through disordered solid-like regions and small gas-like regions. We will find that even though this approach allows us to model a wide range of behaviour, it is not really sophisticated enough to accurately describe the data.

§9.6, §9.7, §9.8 Viscosity, Surface energy and Vapour pressure: In these sections we discuss properties which are especially characteristic of the liquid state. We interpret them in terms of the dynamical cell model outlined in §8.4, and then in §9.9 we consider just how applicable the cell model is to liquids. Our conclusion is that with some reservations, the cell model does allows us to understand the temperature-dependence of viscosity and vapour pressure, and the way in which all these properties vary from one liquid to another.

§9.10 Heat capacity: We will find that the heat capacity in the liquid state is, in general, greater than in the solid or gaseous states. Exploiting our understanding of heat capacities of gases and solid, we will explain this as being due to the accessibility of extra degrees of freedom in the liquid state as compared with the other two states.

§9.12, §9.13 Electrical and Optical properties: For transparent liquids, our main approach will be to treat liquids as dense gases, and we will find that this approach is surprisingly successful. However, we will also find that its shortcomings highlight the importance of understanding the time-scale on which the liquid structure changes and the importance of molecular interactions. Similarly, for liquid metals, appreciating the time-scale of electron scattering will allow us to understand how it is that the metallic state survives the melting transition.