It is possible to freeze air, but the process is far more complex than turning water into ice. Air is a mixture of different gases, each with its own specific thermal properties. To transform this gaseous blend into a solid, scientists must subject it to incredibly low, or cryogenic, temperatures. These extreme temperatures cause each component to transition sequentially from gas to liquid and then to a solid state.
The Components of Air
The primary reason freezing air is complicated is that it is a physical mixture, meaning its constituent gases are not chemically bonded. The atmosphere is composed mainly of nitrogen (about 78%) and oxygen (nearly 21%). The remaining fraction includes argon (approximately 0.93%), with trace amounts of other gases. The phase change for each component is independent, requiring a different temperature to transition from gas to liquid or solid. This difference means that air does not condense or freeze all at once like pure water.
Before the cooling process, water vapor and carbon dioxide must be removed from the air. These two substances solidify at temperatures well above the liquefaction points of nitrogen and oxygen. If not removed, they would immediately turn into a solid and clog the cryogenic machinery.
How Air Turns into a Liquid
The first major step toward freezing air is liquefaction, the transition from gas to liquid, which occurs at extremely cold temperatures. This industrial process is often achieved using cycles of compression and expansion, such as the Hampson-Linde cycle. Cooling the gas below its boiling point causes it to condense into a liquid. Nitrogen, the most abundant component, has a boiling point of approximately -196°C (-321°F), making it the first to condense. Oxygen has a slightly warmer boiling point of about -183°C (-297°F).
Because of this difference, liquid air is not a uniform liquid but a mixture of liquid nitrogen and liquid oxygen. If the liquid air warms slightly, the nitrogen will boil away first, leaving behind a liquid that is richer in oxygen.
The Solidification of Liquid Air
Once air has been condensed into its liquid form, the components must be cooled even further to achieve solidification. This process requires temperatures significantly lower than those needed for liquefaction, pushing into the deep cryogenic range. Pure liquid nitrogen reaches its solid state at approximately -210°C (-346°F), taking on a snow-like, crystalline appearance. Liquid oxygen requires an even lower temperature to freeze, solidifying at about -219°C (-362°F). The resulting solid air would be a mixture of these individual frozen components, resembling a pale blue, icy slush.
Industrial Applications of Cryogenic Gases
The complex process of liquefying and separating air is the basis for a vast global industry. The core method used to separate the components is fractional distillation, which exploits the small differences in their boiling points. By carefully warming the liquid air, each component boils off at its specific temperature, allowing for the collection of high-purity gases. These separated cryogenic gases have numerous practical uses. Liquid nitrogen is used in medicine for cryopreservation and storing biological samples; liquid oxygen is a component in rocket propellant and necessary for welding; and argon is used as an inert shielding gas in welding and specialized lighting.