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:| 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 |