Air’s classification in chemistry requires understanding how matter is categorized at the molecular level. The distinction between a pure substance and a mixture is based on composition, chemical bonding, and the ability to separate the components. Applying these scientific rules allows us to definitively classify the air we breathe.
Defining Pure Substances in Chemistry
A pure substance is a form of matter with a uniform and definite composition throughout. This constant composition gives the substance distinct, fixed properties, such as a precise melting or boiling point. Pure substances cannot be separated into simpler components by physical means like filtration or evaporation.
The two subcategories of pure substances are elements and compounds. Elements, such as oxygen or gold, consist of only one type of atom and represent the simplest chemical forms of matter. Compounds, like water (H2O) or carbon dioxide (CO2), are formed when two or more different elements are chemically bonded together in a fixed ratio.
Understanding Mixtures
A mixture is the physical combination of two or more substances, where each substance retains its own chemical identity and properties. Unlike compounds, the components are not chemically bonded and can be combined in varying proportions. Since the components are only physically combined, they can typically be separated using physical processes like distillation or filtration.
Mixtures are broadly separated into two types: heterogeneous and homogeneous. A heterogeneous mixture has a non-uniform composition, where components are visibly distinct, such as sand mixed with water. In contrast, a homogeneous mixture exhibits a uniform appearance and composition throughout, making the components indistinguishable, like salt dissolved in water.
The Variable Composition of Air
Air is primarily composed of nitrogen (approximately 78% of the volume) and oxygen (about 21%). The remaining 1% consists of argon (about 0.93%), carbon dioxide (about 0.04%), and various trace gases.
A significant portion of air’s composition is highly variable depending on location, altitude, and climate. Water vapor, for example, can range from nearly 0% in arid regions to as much as 5% in hot, humid areas. Pollutants like smog, dust, and pollen are also present in inconsistent amounts. This inability to assign a fixed, unchanging chemical formula means air fails the fundamental test for being a pure substance.
Why Air Is Classified as a Homogeneous Mixture
Air is classified as a mixture because its components are only physically intermingled and not chemically bonded. The constituent gases, such as nitrogen and oxygen, maintain their original chemical properties within the mix. For instance, oxygen remains available for respiration, and nitrogen remains relatively inert.
Air is specifically a homogeneous mixture because the gases are uniformly distributed, making the air appear transparent and uniform. While the total proportions of components like water vapor vary regionally, any sample taken from a single location will exhibit a consistent distribution. The gases can also be separated by physical means, specifically through fractional distillation of liquefied air, which exploits the different boiling points of the components.