Key points (a) The extent of deviations from perfect behaviour is summarized by introducing the
compression factor. (b) The virial equation is an empirical extension of the perfect gas equation
that summarizes the behaviour of real gases over a range of conditions. (c) The isotherms of a real
gas introduce the concept of vapour pressure and critical behaviour. (d) A gas can be liquefied by
pressure alone only if its temperature is at or below its critical temperature.