Air is an ever-present part of daily life. While it appears to be a uniform substance, air is not a pure element or compound. Instead, it represents a combination of various gases, each maintaining its distinct characteristics. Understanding the specific components of air and how they interact reveals why this invisible blanket is best described as a mixture.
The Building Blocks of Air
Dry air primarily consists of three main gases. Nitrogen (N₂), the most abundant gas, accounts for approximately 78% of the atmosphere by volume. Oxygen (O₂), the second most common, comprises about 21% of the atmosphere and is essential for many life processes. The inert gas argon (Ar) is the third most abundant, making up about 0.93% of dry air.
Air also contains smaller amounts of other gases, including carbon dioxide (CO₂), which typically forms around 0.04% of dry air, though its concentration has been increasing due to human activities. Trace gases like neon (Ne), helium (He), methane (CH₄), krypton (Kr), and hydrogen (H₂) are present in even smaller quantities. Water vapor (H₂O) is also a significant component of air, though its concentration varies greatly depending on factors like temperature and humidity, ranging from nearly zero to about 4%. These various gases are individual elements or compounds.
Defining a Mixture
In chemistry, a mixture is a substance formed when two or more different chemical substances are physically combined without undergoing a chemical reaction. No new chemical bonds are created. A fundamental characteristic of mixtures is that their individual components retain their original chemical properties. For example, if sugar and sand are mixed, the sugar remains sweet and the sand retains its gritty texture.
Components within a mixture can be present in variable proportions. This contrasts with chemical compounds, which have a fixed formula. Another distinguishing feature of mixtures is that their components can typically be separated by physical means. These separation methods do not involve chemical changes and often rely on differences in physical properties, such as boiling points, particle size, or solubility.
Why Air Fits the Bill
Air aligns with the definition of a mixture. The gases within air, such as nitrogen and oxygen, maintain their distinct chemical identities. Oxygen still supports combustion, while nitrogen remains unreactive, demonstrating that their inherent properties are preserved. There are no chemical bonds forming between the nitrogen and oxygen molecules, or any other gases, within the atmosphere. They simply exist together in a shared space.
The composition of air is also variable. The amount of water vapor in the air fluctuates based on geographical location, temperature, and weather conditions. Carbon dioxide levels can vary in different environments, such as higher concentrations in urban or industrial areas. These changing proportions illustrate that air does not have a fixed chemical formula like a compound.
Furthermore, the components of air can be separated using physical processes. For example, air can be cooled and compressed into a liquid, and then its constituent gases like nitrogen, oxygen, and argon can be separated through fractional distillation, which exploits their different boiling points. This physical separation confirms that air is indeed a mixture, not a compound.