Air is the gaseous envelope surrounding Earth, and it is classified as a mixture. The air we breathe is not a single chemical substance but rather a physical combination of various gases, each retaining its own distinct properties. Understanding why air is categorized this way requires looking at the foundational concepts that distinguish mixtures from other forms of matter.
Defining Mixtures and Pure Substances
A pure substance is any form of matter that has a constant chemical composition and uniform properties throughout the sample. Pure substances can be either elements, like gold, or compounds, which are two or more elements chemically bonded in a fixed ratio, such as water (H2O) or salt (NaCl). A defining characteristic of a pure substance is that it cannot be separated into simpler components by physical means.
A mixture, conversely, is a physical blend of two or more pure substances, where each component retains its individual chemical identity. The composition of a mixture can vary, meaning the ratio of its components is not fixed. For example, salt water can be made with varying amounts of salt, and the salt can be physically separated from the water through evaporation. Mixtures are further categorized as either heterogeneous, where the components are visibly distinct, or homogeneous, where the composition is uniform and visually indistinguishable throughout.
The Chemical Composition of Air
Air is composed of multiple gases that are physically mixed but not chemically bonded together. The two most abundant components of dry air are nitrogen (N2), which makes up approximately 78% by volume, and oxygen (O2), which accounts for about 21%. Argon (Ar) is the next most plentiful gas at nearly 0.93%.
A small fraction of air consists of other gases, including carbon dioxide (CO2), typically around 0.04%, and trace amounts of neon, helium, and methane. In addition to these gases with relatively stable percentages, air also contains a highly variable component: water vapor (H2O). The concentration of water vapor can fluctuate greatly, ranging from almost 0% in arid regions to as much as 4% in humid, tropical areas.
Physical Evidence that Air is a Mixture
The variable composition of air is the first major piece of evidence confirming its status as a mixture. A pure substance must have a fixed composition, but the percentage of water vapor, pollutants, and dust in the air constantly changes based on location and weather. Since air appears uniform and its components are not visually distinguishable, it is specifically classified as a homogeneous mixture.
The individual gases in the atmosphere also retain their distinct properties, which would not happen if they were chemically bonded into a compound. For instance, the oxygen in the air still actively supports combustion, while the nitrogen remains largely inert, or unreactive, under normal conditions. If air were a compound, its properties would be entirely different from those of its constituent elements, similar to how table salt is nothing like its components, sodium and chlorine.
Furthermore, the components of air can be separated using physical processes, which is a hallmark of a mixture. Industrial processes commonly use fractional distillation of liquid air to obtain pure nitrogen and oxygen. This process involves cooling and compressing air until it liquefies, then gradually warming it so that each gas boils off and is collected at its specific boiling point.