Liquid air is a product made by greatly reducing the temperature of air until it turns into a fluid. Air becomes liquid at about –195 °C. The liquid is bluish and looks like water. Liquid air, like the air we breathe, consists of about 78 percent nitrogen, 21 percent oxygen, and 1 percent argon.
Scientists and engineers use liquid air in cryogenics, the study of temperatures below –150 °C and of the behavior of materials at such temperatures (see Cryogenics). Liquid air is considered a cryogenic fluid because of its low temperature. It is used to condense certain gases into liquids and liquids into solids. It is a primary source of liquid forms of nitrogen and oxygen. Scientists use liquid nitrogen in biology, chemistry, and physics research. It is also used in refrigerating and processing food. Liquid oxygen is used in compact, high-energy fuels for rocket engines that power spacecraft. It is also used to make explosives for blasting.
Properties. Liquid air affects different substances in a variety of ways. Liquid mercury becomes as hard as steel when liquid air is poured over it. A tennis ball dipped in liquid air will shatter when bounced. A lead bell, which normally makes a dull sound, will temporarily produce a clear tone when it has been exposed to liquid air.
Scientists and engineers use liquid air to study the effects of low temperatures on the characteristics of certain substances. Iron and plastics temporarily become brittle after being dipped into liquid air, but copper and brass become tougher. Exposure to liquid air also makes metals better conductors of electric current and increases the strength of certain types of magnets.
Special thermometers are needed to measure the temperature of liquid air. Regular alcohol thermometers cannot be used because alcohol freezes at temperatures higher than that of liquid air. The most accurate and widely used thermometer that measures the temperature of liquid air is the platinum resistance thermometer. It measures temperature by determining its effect on the electrical resistance of platinum. Platinum becomes a better or poorer conductor of electric current as its temperature changes. A constant-volume gas thermometer measures the effect of temperature on the pressure of a gas kept at a certain volume. Such gases as helium or neon are used to measure the temperature of liquid air because they turn into liquid at lower temperatures than air does.
Nitrogen and oxygen can be separated and used in their liquid form by distilling liquid air (see Distillation). When liquid air is heated, the nitrogen turns into a gas before the oxygen does because the boiling point of nitrogen is lower. After the nitrogen has been removed, the remaining substance consists mostly of liquid oxygen. The high oxygen content of the undistilled liquid makes it highly combustible.
Making liquid air. The process of making liquid air is based on the fact that compressed air becomes cooler when it expands. This cooling effect was described in detail in 1853 by two British physicists, James Prescott Joule and William Thomson. It later became known as the Joule-Thomson effect. In 1877, Louis-Paul Cailletet, a French physicist, liquefied air for the first time.
In 1895, the German chemist Carl von Linde invented a commercial process for liquid air production based on the Joule-Thomson effect. Linde’s method is still used today but with many improvements. Compressors raise the air pressure in a chamber to about 3,000 pounds per square inch (20,600 kilopascals). Compression heats the air. For this reason, water jackets on the compressor, plus a device called a heat exchanger, are used to lower the compressed air’s temperature before it is liquefied.
Air can be liquefied in one of two ways. In one method, called Joule-Thomson expansion, the compressed air flows through a series of throttling valves into increasingly larger chambers. The pressure and temperature of the air decrease in each chamber as the air expands. In the final chamber, some of the air has become cold enough to condense into liquid. The cold vapor from this chamber is circulated around the other chambers to help cool the air that is still going through the liquefying process.
In 1902, Georges Claude, a French engineer, developed the second method of liquefying air. It resembles Joule-Thomson expansion but is more efficient because it makes use of work done by expanding air. In the Claude method, compressed air enters a chamber and pushes a piston as it expands. As the piston moves, the volume of the chamber increases, and the chamber’s air pressure and temperature decrease. The air is sent through a series of these piston-equipped chambers, called expansion engines, until it becomes liquid.
Special containers called Dewar flasks protect liquid air from heat and evaporation. A Dewar flask is a bottle made of two layers of glass. There is space between the layers of glass to insulate the contents. The flask may be coated with silver to reflect heat. Large quantities of liquid air are stored in huge insulated tanks.