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.