We can draw conclusions from the virial equations of state only by inserting specific
values of the coefficients. It is often useful to have a broader, if less precise, view of all
gases. Therefore, we introduce the approximate equation of state suggested by J.D.
van der Waals in 1873. This equation is an excellent example of an expression that can
be obtained by thinking scientifically about a mathematically complicated but physically
simple problem; that is, it is a good example of ‘model building’.