Air, the invisible mixture of gases surrounding our planet, is not a liquid under normal atmospheric conditions. It is a gaseous substance defined by the high kinetic energy of its constituent molecules, which allows them to move freely and independently. The air we breathe is a complex blend of multiple gases, primarily nitrogen and oxygen. Although air exists naturally in a gaseous phase, science has developed methods to force it into a liquid state, demonstrating that its phase is not fixed.
Understanding Air as a Gaseous Mixture
Air is a mixture of gases, not a single chemical compound. Its primary components are nitrogen (about 78% of the volume) and oxygen (approximately 21%), with the remaining 1% consisting mostly of argon and carbon dioxide. In the gaseous state, these molecules are widely separated and move at high speeds, exerting pressure as they collide with surfaces.
Gases, unlike liquids or solids, do not possess a fixed shape or volume; they expand to completely fill whatever container they occupy. The molecules have sufficient kinetic energy to overcome the weak intermolecular forces, such as van der Waals forces, that would otherwise pull them closer together. This constant, rapid motion is the defining characteristic of air in its natural, gaseous form.
The Science of Phase Transition
Converting air from a gas to a liquid is a phase transition known as liquefaction, which requires manipulating temperature and pressure. To change its state, the kinetic energy of the gas molecules must be significantly reduced, allowing the weak attractive forces between them to become dominant. This is achieved by cooling the air to extremely low temperatures, a field of study called cryogenics.
The air is first compressed to a very high pressure, often hundreds of times greater than normal atmospheric pressure, forcing the molecules closer together. Since compression heats the air, the next step involves removing that heat using cooling systems. The highly compressed and cooled air is then allowed to expand rapidly. This expansion causes a drastic drop in temperature, pushing the gas past its condensation point and into a liquid state. The average boiling point of air is approximately \(-194^{\circ}\text{C}\) (or \(-317^{\circ}\text{F}\)), which is the temperature at which it condenses into a liquid at standard pressure.
Characteristics of Liquid Air
Once air is liquefied, it transforms into a pale, mobile liquid with a faint blue tint, primarily due to the presence of liquid oxygen. This substance is extremely cold, existing at temperatures around \(-195^{\circ}\text{C}\). It has a much higher density than gaseous air, roughly 870 kilograms per cubic meter. Liquid air must be stored in specialized vacuum-insulated containers, such as Dewar flasks, to prevent it from rapidly absorbing heat and reverting to a gas.
The component gases of air possess slightly varied boiling points, which is a significant characteristic of liquid air as a mixture. Nitrogen boils at about \(-196^{\circ}\text{C}\), while oxygen boils at a slightly warmer \(-183^{\circ}\text{C}\). This difference in boiling points is the basis for fractional distillation, a process used industrially to separate the liquid air into its pure gaseous components, such as oxygen, nitrogen, and argon, for commercial use.