Andrews' experiments with carbon dioxide

The classic experiment on the behaviour of gases was devised by Andrews in 1863 and used carbon dioxide as the test gas. Using the apparatus shown in Figure 1, he plotted a series of isothermals (PV curves) to test the validity of Boyle's law over a wide range of pressures. The gases were compressed by tightening the screw and the pressure was estimated using the nitrogen assuming that under the conditions of the experiment it still obeyed Boyle's law. The capillary tubes were very strong and Andrews obtained results up to pressures of 10⁷ Pa.



The results are shown in the two graphs in Figure 2. Above about 50 ^oC Boyle's law was fairly closely obeyed. But as you can see the behaviour of the 'gas' is different above and below about 30 ^oC - in fact Andrews found that the critical temperature for carbon dioxide was 30.9 ^oC.

Above this temperature carbon dioxide could not be liquefied by pressure alone, while below this temperature an increase in pressure would finally result in liquid carbon dioxide. At the critical point the gas and liquid are in equilibrium.


These ideas form the basis for a useful definition of a vapour:

A vapour is a gas below its critical temperature

Some critical temperatures are shown in the following table.

Substance	Critical temperature (oC)	Boiling point (oC)
Helium	-268	-269
Hydrogen	-240	-253
Nitrogen	-147	-196
Air	-140	-190
Oxygen	-118	-183
Carbon dioxide	30.9	-78.2
Chlorine	146	-34
Water	374	100

To liquefy a gas by pressure alone it must first be cooled below its critical temperature.