Liquid air is air that has been cooled to very low temperatures (cryogenic temperatures), so that it has condensed into a pale blue mobile liquid. To protect it from room temperature, it must be kept in a vacuum insulated flask. Liquid air can absorb heat rapidly and revert to its gaseous state. It is often used for condensing other substances into liquid and/or solidifying them, and as an industrial source of nitrogen, oxygen, argon, and other inert gases through a process called air separation.
Liquid air has a density of approximately 870 kg/m3 (0.87 g/cm3), though the density may vary depending on the elemental composition of the air. Since dry gaseous air contains approximately 78% nitrogen, 21% oxygen, and 1% argon, the density of liquid air at standard composition is calculated by the percentage of the components and their respective liquid densities (see liquid nitrogen and liquid oxygen). Although air contains trace amounts of carbon dioxide (about 0.040%), this gas sublimates (transfers directly between gas and solid, and therefore does not exist as a liquid) at pressures less than 5.1 atmospheres.
The boiling point of liquid air is -194.35 °C, intermediate between the boiling points of liquid nitrogen and liquid oxygen. However, it can be difficult to keep at a stable temperature as the liquid boils, since the nitrogen will boil off first, leaving the mixture oxygen-rich and changing the boiling point. This may also occur in some circumstances due to the liquid air condensing oxygen out of the atmosphere.
The constituents of air were once known as "permanent gases", as they could not be liquified solely by compression at room temperature. A compression process will raise the temperature of the gas. This heat is removed by cooling to the ambient temperature in a heat exchanger, and then expanding by venting into a chamber. The expansion causes a lowering of the temperature, and by counter-flow heat exchange of the expanded air, the pressurized air entering the expander is further cooled. With sufficient compression, flow, and heat removal, eventually droplets of liquid air will form, which may then be employed directly for low temperature demonstrations.
Devices for the production of liquid air are simple enough to be fabricated by the experimenter using commonly available materials.
The most common process for the preparation of liquid air is the two-column Hampson-Linde cycle using the Joule-Thomson effect. Air is fed at high pressure (>60 psig, or 520 kPa) into the lower column, in which it is separated into pure nitrogen and oxygen-rich liquid. The rich liquid and some of the nitrogen are fed as reflux into the upper column, which operates at low pressure (<10 psig, or 170 kPa), where the final separation into pure nitrogen and oxygen occurs. A raw argon product can be removed from the middle of the upper column for further purification.
In manufacturing processes, the liquid air product is fractionated into its constituent gases in either liquid or gaseous form, as the oxygen is especially useful for fuel gas welding and cutting, and the argon is useful as an oxygen-excluding shielding gas in gas tungsten arc welding. Liquid nitrogen is useful in various low-temperature applications, being nonreactive at normal temperatures (unlike oxygen), and boiling at 77 K (-196 °C; -321 °F).
Between 1899 and 1902, the automobile Liquid Air was produced and demonstrated by a joint American/English company, with the claim that they could construct a car that would run a hundred miles on liquid air.
On 2 October 2012, the Institution of Mechanical Engineers said liquid air could be used as a means of storing energy. This was based on a technology that was developed by Peter Dearman, a garage inventor in Hertfordshire, England to power vehicles.