Review of structural differences between solids, liquids, and gases

6 REVIEW OF STRUCTURAL DIFFERENCES BETWEEN SOLIDS, LIQUIDS, AND GASES 

We have described the structure of a gas simply i n terms o f the chaotic motion o f molecules (thermal motion), which are separated from one another by distances that are very large compared with their own diameter. The influence of intermolecular forces and finite molecular size is very small and vanishes in the limit of zero pressure. 

Since in a liquid the molecules are separated by a distance of the same magnitude as the molecular diameter, the volume occupied by a liquid is about the same as the volume of the molecules themselves. At these close distances the effect of the intermolecular forces is very large, with the result that each molecule has a low potential energy compared with its energy in the gas. The difference in potential energy between gas and liquid is the energy that must be supplied to vaporize the liquid. The motion of the molecules in the liquid is still chaotic, but since the liquid occupies a much smaller volume, there is less randomness in the space distribution of the molecules. The liquid has a very low compressibility simply because there is very little empty space left between the molecules. The liquid is capable of flow under stress because the molecule does have freedom to move anywhere within the volume it must, however, push other molecules aside to do so, and as a consequence the resistance to flow is greater than for the gas. 

The distribution of energies in solids and liquids is essentially the same as in the gas and, so long as the temperature is sufficiently high, is described by the Maxwell-Boltzmann 

2 A typical liquid with a = 10 K and k = 10 atm is heated by 10 K. Estimate the external pressure required to keep the density of the liquid constant. 

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