Air a Mixture

As has already been mentioned, the apparent constancy of composition of the atmosphere led many early chemists to believe that it was a definite compound of oxygen and nitrogen and not a mechanical mixture of these two gases. When later and more accurate analyses of air were made, however, small but decided variations in the relative proportions of its oxygen and nitrogen were found - a discovery that disposed entirely of the suggestion that air is a compound.

Several other lines of argument point to the same conclusion however. For example, when air is shaken with water each constituent dissolves to an extent dependent upon its own solubility and partial pressure. Since oxygen is approximately twice as soluble in water as nitrogen, the dissolved air is slightly richer in oxygen. If air were a compound, on the other hand, the gases would dissolve in the water in the same proportions as they exist in the free air.

When oxygen and nitrogen gases are intermixed in the relative proportions necessary to form air, no heat is set free, yet the resulting mixture possesses the properties of pure air, and its constituents admit of separation again by means of diffusion. It seems highly improbable, therefore, that a compound can have been formed.

When air is liquefied and subsequently boiled, a vapour rich in nitrogen gas is the first to escape, leaving a liquid proportionately richer in oxygen. If the air were a compound, however, the escaping gas would have the same composition as the liquid.

Finally, the properties of air, whether in the gaseous or liquid phase, are intermediate between those of oxygen and nitrogen respectively, thereby suggesting a mixture, for, as a general rule, compounds do not resemble their components either physically or chemically.