Liquid air

When air, cooled below the critical temperatures of its constituents, is subjected to compression, liquefaction may occur, but the constituents will not separate out in strict proportionality because oxygen is more easily condensed than nitrogen. For a similar reason liquid air, when kept, loses nitrogen more rapidly than oxygen, so that the boiling-point gradually approaches that of the latter element and the gas evolved at first extinguishes a lighted match whereas the last portions increase its combustion.

Curves showing composition of liquid and gaseous air in equilibrium at various temperatures under atmospheric pressure (Baly, 1900).

On account of the uncertainty introduced by this behaviour of liquid air, it is important to bear in mind that statements of the physical properties are of relatively little value unless accompanied by figures giving the composition of the liquid air examined. Freshly prepared liquid air usually has a boiling-point near -193° C. at 760 mm., a temperature as low as -220° C. being attainable by rapid evaporation under a pressure of a few millimetres of mercury; under constant pressure the boiling-point gradually rises, approaching that of liquid oxygen. The compositions of the gaseous mixture in equilibrium with liquid air of varying oxygen content are given numerically in the accompanying table and shown diagrammatically in Fig.

Liquid oxygen and nitrogen mix without appreciable change in volume; hence the approximate composition of the liquid mixture can be calculated from its density at any particular temperature. Measurements which have been made lead to the expression

d = 0.86 + 0.00289x

for the value at the boiling-point under ordinary pressure, when x represents the percentage of oxygen and d the density required.

Measurement has also been made of the heat of evaporation, the refractive index, and the absorption spectrum of liquid air, these properties again being almost entirely the resultant of the properties of the constituents.

The critical constants of liquid air have been estimated as having the following values: temperature -140° C., pressure 39 atmospheres, and volume 0.00257.

A distinction is sometimes made between the point at which the two co-existing phases, namely, vapour and liquid, become identical - the plait point - and the point representing the limiting condition for the separation into two phases - the critical point of contact. The difference between the two is very small, as is evident from the following estimation:

	Temperature, °C.	Pressure, Atm.	Density of Liquid.
Plait point	-140.73	37.25	0.35
Critical point of contact	-140.63	37.17	0.31

The critical density of air, as calculated from the critical densities of oxygen and nitrogen by the simple rule of mixtures, is 0-34. This lies within the range given above.

The specific heat of liquid air is about half that of water, whilst the latent heat of evaporation is approximately 50 calories.

The viscosity of liquid air of boiling-point 79.6° absolute is given as 0.001678.

Liquid air is of relatively small value as a source of energy; its conversion into gas at the ordinary temperature is capable of yielding only as much energy as the combustion of one-tenth its weight of petrol, and the heat absorbed from the surroundings during the change is only sufficient to melt times its weight of ice.

Percentages of Oxygen in equilibrium in gaseous and liquid phases of Air at various temperatures under atmospheric pressure

Temperature. °C.	Oxygen in Liquid. Per cent.	Oxygen in Vapour. Per cent.
-182	100	100
-184	91.98	79.45
-186	82.95	60.53
-188	72.27	44.25
-190	59.55	29.93
-192	43.38	17.66
-194	21.60	6.80
-195	8.10	2.10
-195.46	0.00	0.00

Although it evaporates rapidly in ordinary vessels, liquid air can be preserved for a considerable time with relatively little loss in glass vessels, the hollow, silvered walls of which enclose an evacuated space; these are manufactured in various shapes and sizes.

Liquid air is now a common commercial product and can be applied to various useful purposes. An obvious application is to the production of low temperatures for experimentation work, and a valuable extension is to the production of high vacua by filling with carbon dioxide the apparatus which has already been attached by hermetically sealing to a bulb of coconut charcoal; when this bulb is immersed in liquid air, the carbon dioxide is absorbed so rapidly and completely that the pressure may be almost immediately reduced to a fraction of a millionth of an atmosphere.

Mention has already been made of the employment of liquid air as a blasting agent. It also finds application as a convenient source of pure nitrogen and oxygen.

By the rapid evaporation of liquid air under reduced pressure sufficient further cooling can be effected to cause the air to solidify, when it yields a colourless, transparent mass. The fact that nitrogen has a higher melting-point (namely -210° C.) than oxygen ( -219° C.) tends to the predominance of the former element in the solid mixture.