Air is the natural atmosphere of the earth--a nonflammable, colorless, odorless gas that consists of a mixture of gaseous elements (with water vapor, a small amount of carbon dioxide, and traces of many other constituents). Synthesized air is produced by combining pure oxygen and nitrogen and contains between 19.5 and 23.5 percent oxygen, with the balance nitrogen and with a major portion of the other components eliminated. Dry air is non-corrosive. Liquefied air is transparent with a bluish cast and has a milky color when it contains carbon dioxide.
Because air is a mixture, not a compound, it can be separated into its components. The most common method is the liquefaction of air by reducing its temperature to approximately -320 degrees F (-195.6 degrees C), then fractionally distilling to remove each of the constituents as fractions.
Air can be compressed at the point of use for most practical applications. To meet needs for air of special purity or specified composition (as in certain medical, scientific, industrial, fire protection, undersea, and aerospace uses), it is purified or compounded synthetically and shipped in cylinders as a non-liquefied gas at high pressures.