Air is fundamental to life on Earth. A common question is its chemical classification: is it a compound or a mixture? Air is not a chemical compound; it is a mixture. This distinction helps understand how its gases behave and interact.
Understanding Chemical Classifications
To understand why air is a mixture, first define chemical classifications. A chemical compound forms when two or more elements chemically bond in a fixed ratio. The resulting compound has properties distinct from its elements. For example, water (H₂O) is a compound of hydrogen and oxygen, but its characteristics differ from either gas.
In contrast, a mixture involves two or more substances physically combined, not chemically bonded. Its components retain individual properties, and their proportions can vary. Mixtures are either homogeneous or heterogeneous. A homogeneous mixture has a uniform composition, with components evenly distributed and indistinguishable, like air. A heterogeneous mixture has a non-uniform composition where different components are visually identified, such as oil and water.
The Composition and Nature of Air
Air is composed of gases, primarily nitrogen (N₂) at 78% and oxygen (O₂) at 21% of its volume. Other gases include argon (Ar) at 0.93% and carbon dioxide (CO₂) at 0.04%, along with trace amounts of neon, helium, methane, krypton, and hydrogen. Water vapor is also present, with concentration varying from nearly 0% to 4% depending on location and conditions.
These gases are not chemically bonded; they coexist. Each gas retains its unique chemical properties. For example, oxygen supports combustion and is necessary for respiration. Nitrogen, despite its abundance, is relatively unreactive due to a strong triple bond, which dilutes oxygen’s reactivity.
The variable proportions of air’s components support its classification as a mixture. Water vapor, for instance, changes with humidity and altitude. Carbon dioxide concentration can also vary due to factors like local pollution. This lack of a fixed ratio defines mixtures.
Another evidence for air being a mixture is that its components can be separated physically. Industrial processes use fractional distillation of liquid air to separate nitrogen and oxygen. This method cools air to liquefy it, then gradually warms it. Since each gas has a different boiling point (e.g., nitrogen at -196 °C, oxygen at -183 °C), they vaporize at different temperatures for individual collection. This physical separation confirms air is a homogeneous mixture of gases.