The Kinetic Molecular Theory
Applied to the Condensed States

As we saw in Section 9.3, the kinetic molecular theory of gases has five postulates: (1) the volume occupied by a gas is largely empty space; by comparison, the real volume of the gas molecules is negligible; (2) there are no attractive forces between the molecules of a gas; (3) the gaseous molecules are in constant, random, rapid, straight-line motion; (4) the molecules collide frequently with one another and with the walls of the container, but no energy is lost by these collisions; and (5) the average kinetic energy of the molecules is proportional to the temperature of the sample.

These postulates do not hold completely for liquids and solids. We have already seen that, when the average kinetic energy of a gas sample decreases, the sample eventually reaches a temperature at which this kinetic energy is no longer great enough to overcome the attractive forces between the molecules, and they start to interact with one another. These forces become more effective as the temperature lowers, and the sample becomes first a liquid and then, at a lower temperature, a solid. In these states, the sample has the properties described in Section 9.1. For condensed states, the kinetic molecular theory must be modified. The properties of molecules in these states can be described as follows:

1. Instead of negligible volume, the molecules of a liquid or a solid occupy most of the volume of the sample. This fact is apparent from the lack of compressibility of matter in these two states.

2. There are appreciable attractive forces between the molecules; these forces keep the liquid or the solid together.

3. Although not moving as rapidly as in a gas, the molecules are still in constant, rapid motion.

4. In a liquid this movement can be seen by gently placing a drop of ink (or dye) on the surface of a large amount of water. Without any stirring, the colored ink will be rapidly dispersed throughout the entire container as the colored molecules are buffeted about by the constantly moving water molecules (Section 10.4), we will discuss movement of particles in a solid.

5. In a liquid the molecules collide without loss of kinetic energy; in a solid the particles are held so rigidly in place that they do not collide.

6. Just as in gases, each molecule has some kinetic energy and the average of these energies is proportional to the temperature of the sample. There is a distribution of kinetic energies in the sample similar to that in a gas sample (see Section 9.2).