From the point of view of chemistry, how is air best described?

Introduction / Context:
Air is all around us and is essential for life, but its chemical nature is often misunderstood. In chemistry and environmental science, it is important to classify substances correctly as elements, compounds, or mixtures. This question checks your understanding of how air is categorised based on its composition.

Given Data / Assumptions:

• Air refers to the Earth atmosphere near the surface.
• It contains gases like nitrogen, oxygen, argon, carbon dioxide, and small amounts of others.
• We are describing air at normal temperature and pressure.

Concept / Approach:
An element is a substance made of only one type of atom. A compound has a fixed chemical formula and definite proportions of elements chemically bonded together. A mixture contains two or more substances combined physically, not chemically, with variable composition. Air clearly contains nitrogen, oxygen, argon, carbon dioxide, water vapour, and traces of many other gases, and their proportions can vary from place to place and time to time. Thus air is a mixture of gases rather than a pure element or compound.

Step-by-Step Solution:
Step 1: List major components of dry air: about 78 percent nitrogen, 21 percent oxygen, around 1 percent argon, and small fractions of carbon dioxide and other gases. Step 2: Note that water vapour content in air varies widely, from very dry desert air to very humid tropical air. Step 3: Because the components are not chemically bonded in a fixed ratio and can be separated by physical methods such as fractional distillation of liquefied air, air is not a compound. Step 4: Since air clearly contains several different chemical species, it cannot be a single element. Step 5: Therefore, air fits the definition of a mixture, specifically a mixture of several gases.

Verification / Alternative check:
Industrial processes for separating air into its components rely on the fact that air is a mixture. For example, oxygen plants cool and liquefy air, then separate nitrogen, oxygen, and argon by fractional distillation. This would not be possible if air were a single pure compound with fixed composition and chemical bonds between nitrogen and oxygen molecules.

Why Other Options Are Wrong:
A pure compound: A compound has a fixed chemical formula such as H2O or CO2, which air does not have. A single element: Air has multiple elements present (nitrogen, oxygen, argon, etc.), so it cannot be a single element. A liquid under normal conditions: At standard temperature and pressure, air is a gas, not a liquid.

Common Pitfalls:
One mistake is to associate the word air with oxygen alone, because we breathe it, and then to call it an element. Another is to think that because air has a fairly constant average composition, it must be a compound. Remember that mixtures can also have relatively stable average compositions while still varying locally and being separable by physical means.

Final Answer:
Chemically, air is best described as a mixture of several gases.