Liquid air is air that has been liquified by compression and cooled to very low temperatures. It must be kept in a Dewar flask, as at room temperature, liquid air can absorb heat rapidly and revert to its gaseous state. It is often used for freezing other substances, and as a source of nitrogen, oxygen, argon, and other inert gases.

The most common process for distillation of liquid air is the two column Linde cycle . Air is fed at high pressure >60 psig (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 (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.