The van der Waals equation sheds some light on the principle. First, we express
eqn 1.21b in terms of the reduced variables, which gives
pr pc= −
Then we express the critical constants in terms of a and b by using eqn 1.22:
= −
which can be reorganized into
pr= − (1.25)
This equation has the same form as the original, but the coefficients a and b, which
differ from gas to gas, have disappeared. It follows that, if the isotherms are plotted in
terms of the reduced variables (as we did in fact in Fig. 1.20 without drawing attention
to the fact), then the same curves are obtained whatever the gas. This is precisely the
content of the principle of corresponding states, so the van der Waals equation is
compatible with it.
Looking for too much significance in this apparent triumph is mistaken, because
other equations of state also accommodate the principle (Table 1.7). In fact, all we
need are two parameters playing the roles of a and b, for then the equation can always
be manipulated into reduced form. The observation that real gases obey the principle
approximately amounts to saying that the effects of the attractive and repulsive interactions
can each be approximated in terms of a single parameter. The importance of
the principle is then not so much its theoretical interpretation but the way in which it
enables the properties of a range of gases to be coordinated on to a single diagram (for
example, Fig. 1.21 instead of Fig. 1.14).