liquid air, ordinary air that has been liquefied by compression and cooling to extremely low temperatures (see liquefaction). Its commercial preparation involves purification by washing to remove soluble impurities and by passage over calcium oxide (lime) to remove the carbon dioxide; compression, under a pressure of 200 atmospheres, or about 3,000 lb per sq in.; cooling, by passage through pipes immersed in cold water; treatment with sodium hydroxide to remove excess water; and rapid expansion, the expanding air passing back over the pipe from which it has just escaped absorbing so much heat that the air remaining in the pipe becomes liquid. Freshly liquefied air consists of 78.1% nitrogen, 21.0% oxygen, 0.9% argon, and very small amounts of rare gases and hydrogen in solution. Its boiling point is approximately -195°C. Because of fractional evaporation, its oxygen concentration and its boiling point increase with time. It must be kept in a specially designed container, the Dewar flask, because at ordinary temperatures it absorbs heat rapidly and reverts to the gaseous state. Liquid air is used commercially for freezing other substances and especially as an intermediate step in the production of nitrogen, oxygen, and argon and the other inert gases. As the temperature of liquid air rises, the nitrogen evaporates first at -195.8°C, the argon next at -185.7°C, and the oxygen last at -183°C. See low-temperature physics.