The behaviour of real gases

In our consideration of gases so far we have assumed that the intermolecular forces are zero and therefore that they follow the kinetic theory of gases exactly. However this is not the case with actual gases.

A gas that follows the gas laws precisely is known as an ideal gas and one which does not is called a real gas.

In 1847 Regnault constructed PV curves up to 400 atmospheres and found that Boyle's law was not obeyed at these high pressures.

Amagat went a stage further in 1892, working with nitrogen to pressures of some 3000 atmospheres (3x10⁸ Pa) down a coal mine.


The idea that actual gases did not always obey the ideal gas equation was first tested by Cagniard de Ia Tour in 1822, using the apparatus shown in Figure 1.

A liquid such as water or ether was trapped in a tube and the end of the tube placed in a bath whose temperature could be controlled. The temperature was then varied and the behaviour of the liquid observed. The space above the liquid is obviously filled with vapour and it was noticed that at a particular temperature no difference could be seen between the liquid and vapour states - this was called the critical temperature. This phenomenon was not predicted by Boyle's law, which says nothing about the liquefaction of gases.