The air surrounding the Earth is a complex combination of various gases, suggesting it is not a single pure substance. From a scientific perspective, the atmosphere is a mixture, specifically a solution of gases dissolved within each other. Understanding this classification requires differentiating between the two primary ways substances combine: forming a compound or creating a mixture.
Classifying Matter: Mixtures and Compounds
Matter is classified based on its composition and whether its components are chemically bonded. A compound is formed when two or more different elements chemically bond together in a fixed, definite proportion. Water (\(\text{H}_2\text{O}\)) is a compound because hydrogen and oxygen atoms are chemically linked in a precise 2:1 ratio, resulting in a substance with new properties distinct from the original gases.
In contrast, a mixture involves two or more substances that are physically combined, not chemically bonded. The components of a mixture retain their individual chemical properties, and their proportions can vary widely. Mixtures can often be separated into their original components using physical methods, such as evaporating water to recover dissolved salt.
The Primary Components of Dry Air
The classification of air as a mixture is supported by its consistent composition in its dry form, which is a blend of several gases. By volume, the vast majority of dry air consists of two main gases: nitrogen (\(\text{N}_2\)), which makes up approximately 78%, and oxygen (\(\text{O}_2\)), accounting for about 21%. Even “pure” dry air is composed of multiple distinct substances.
The remaining 1% of the volume is primarily composed of the noble gas argon (\(\text{Ar}\)), at about 0.93%, along with trace amounts of other gases. Carbon dioxide (\(\text{CO}_2\)), while only about 0.04% of the volume, is a trace gas involved in atmospheric processes. These gases are intermingled and do not chemically react with one another under normal atmospheric conditions.
Why Air Qualifies as a Homogeneous Mixture
Air is classified as a homogeneous mixture because its components are uniformly distributed throughout, making it appear as a single substance. The individual gas molecules are thoroughly intermixed, meaning any sample of air taken from a particular location will have the same composition as another sample. This uniform distribution is why a homogeneous mixture is often referred to as a solution, with nitrogen acting as the solvent gas.
A defining characteristic confirming air is a mixture, not a compound, is the lack of chemical bonds between the gases. Oxygen molecules remain \(\text{O}_2\), retaining their ability to support respiration, while nitrogen molecules remain chemically inert. Furthermore, the components do not combine in a fixed mass ratio, which is a requirement for a compound.
The physical separability of air’s components provides further proof of its mixture status. Industrial processes use fractional distillation to cool air until it liquefies. The liquid air is then separated into its constituent gases, such as pure nitrogen and oxygen, by slowly raising the temperature and collecting the gases as they boil off at different points. Since compounds require chemical reactions to separate their elements, this physical separation confirms air is a mixture.
Real-World Air: The Role of Variable Components
While “pure” air is often discussed in an idealized, dry context, real-world atmospheric air always contains variable components, which solidifies its classification as a mixture. The most significant variable component is water vapor, or humidity, which can range from nearly 0% to as much as 4% by volume in hot, tropical air masses. This constantly changing proportion, driven by weather and location, is a definitive trait of a mixture.
Real-world air also contains various solid and liquid particulates, known as aerosols, including dust, pollen, sea salt, and pollutants. The concentration of these particulates changes constantly based on natural events like wind and volcanic activity, and human activity like industrial emissions. The presence of these variable components, which are physically suspended in the gas solution, confirms that air’s composition is not fixed, classifying it as a dynamic, complex mixture.