Oxygen : Liquid Air

The liquefaction of air in bulk was effected in 1895, independently, by Hampson in England and by Linde in Germany, who made use of a news principle, viz., the Joule Kelvin effect, investigated by Joule and William Thomson (later Lord Kelvin) from 1852 to 1862. When a compressed gas escaped into the free air through a plug of silk in a boxwood tube, a slight cooling effect occurred with most gases (air, oxygen nitrogen, carbon dioxide), but with hydrogen alone there was a slight heating effect.

This temperature change is quite different from that due to the external work done by a gas in adiabatic expansion. If a given mass of gas, of volume v₁

Fig: Diagram illustrating free expansion of gases

is forced under a pressure p₁ through the plug into a space under a lower pressure p₂ (say ½p₁), it occupies a larger volume v₂. The work done on the gas is p₁v₁, that done by the gas is p₂v₂. If the gas obeyed Boyle's law, p₁v₁ = p₂v₂ (v₂ = 2v₁; p₁ = 2p₂), and external work would be done; if no other effect were involved, there would be no change of temperature. Since, however, v₂ is greater than v₁ the molecules of the gas will have separated, and since a slight attraction exists between them, internal work will have been spent in separating the molecules The energy required for this internal work is taken from the heat of the gas, and a slight cooling effect results. Usually, both external and internal work are involved. Thus, in the case of air, p₂v₂ is slightly larger than p₁v₁, since the gas is slightly more compressible than an ideal gas, and p₁ is greater than p₂. A little heat is absorbed in providing this extra work, p₂v₂ - p₁v₁, but much more is absorbed to supply the internal work.

In the case of air the cooling effect in degrees C. is given by

where T₁ is the absolute temperature of the air before expansion.

If air at 0° C., and under a pressure of 100 atm., is expanded through a valve to atmospheric pressure, the fall of temperature will be

If this cool air, at -24.7°, sweeps over the surface of a copper pipe, bringing compressed air to the valve, by placing the latter inside the pipe taking away the cold expanded air

Fig: Cooling of gases by free expansion

the expanded air abstracts heat from the air coming to the valve, becoming itself warmed nearly to the atmospheric temperature. The cooled compressed air at -24.7° will, after expansion, become 30.3° colder, as the above formula shows, and this cold air at -55° sweeps over the inner tube, reducing still further the temperature of the compressed air coming down. The cooling effect accumulates, and the air issuing from the nozzle finally becomes so cold that it liquefies.

This apparatus, called a heat-interchanger, was applied by Hampson and by Linde to the liquefaction of air on a large scale. A diagrammatic representation of an air liquefaction apparatus, which is self-explanatory, is given in

Fig: Production of liquid air

The air is drawn by a pump through a purifier, and is compressed. The heat produced by compression is taken out by a cooler, and the air then passes through apparatus in which moisture is removed. The compressed air expands through a jet, and becomes cooled. The cold air sweeps over the pipe bringing the compressed air to the jet and cools it before expansion. The air finally becomes so cold that it liquefies.

Liquid air is kept in double-walled Dewar ("thermos") flasks

Fig: Vacuum vessels

the inner surfaces of which, silvered to reflect heat, have a high vacuum between them to cut down heat transmission to a minimum.

Liquid air is usually slightly turbid, because it contains particles of ice and solid carbon dioxide. If filtered through a large filter paper it forms a clear liquid, with a pale blue colour, due to liquid oxygen. If poured out into the air, it evaporates, producing thick white clouds of condensed moisture. Its temperature is about -190°, and when exposed to this extreme cold many substances undergo remarkable changes in properties. Lead becomes elastic, and rubber hard and brittle. Mercury is at once frozen to a malleable solid. Raw meat, fruits, flowers, etc., become hard, and can be reduced to powder in a mortar. A kettle containing liquid air "boils" briskly when placed on a slab of ice, and copious clouds of "steam," i.e., atmospheric moisture condensed to particles of ice by the cold of the escaping evaporated air, are emitted from the spout. The phosphorescence of calcium sulphide is at once quenched at the temperature of liquid air, but appears again on warming. Sulphur and mercuric iodide become much paler in colour on cooling in the liquid.

On standing, liquid air becomes bluer in colour; the more volatile, colourless nitrogen (b. pt. -195.7°) escapes, and pale-blue liquid oxygen (b. pt. -182.9°) is left. A cigarette soaked in liquid oxygen burns rapidly when lighted; a stick of carbon heated to redness burns brightly under the surface of liquid oxygen in a beaker, as does a piece of iron wire tipped with burning wood; and with a little care a hydrogen flame can be plunged into the liquid and continues to burn, producing an appreciable amount of ozone, recognisable by the smell.

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