An important point is that, although the equation of state of a real gas may coincide
with the perfect gas law as p→0, not all its properties necessarily coincide with those
of a perfect gas in that limit. Consider, for example, the value of dZ/dp, the slope of the
graph of compression factor against pressure. For a perfect gas dZ/dp = 0 (because
Z = 1 at all pressures), but for a real gas from eqn 1.19a we obtain