Real gases show deviations from the perfect gas law because molecules interact with
one another. A point to keep in mind is that repulsive forces between molecules assist
expansion and attractive forces assist compression.
Repulsive forces are significant only when molecules are almost in contact: they are
short-range interactions, even on a scale measured in molecular diameters (Fig. 1.13).
Because they are short-range interactions, repulsions can be expected to be important
only when the average separation of the molecules is small. This is the case at high
pressure, when many molecules occupy a small volume. On the other hand, attractive
intermolecular forces have a relatively long range and are effective over several
molecular diameters. They are important when the molecules are fairly close together
but not necessarily touching (at the intermediate separations in Fig. 1.13). Attractive
forces are ineffective when the molecules are far apart (well to the right in Fig. 1.13).
Intermolecular forces are also important when the temperature is so low that
the molecules travel with such low mean speeds that they can be captured by one
another