Air Is a Mixture of Gases

Introduction

Every breath you take draws in about half a litre of air, yet air is not one substance. It is a blend of several gases — mainly nitrogen and oxygen, with smaller amounts of argon, carbon dioxide, water vapour and traces of others. Chemists describe air as a mixture , not a compound. On this page you will see what is in air and the evidence that its gases are simply mixed rather than chemically joined together.

Core explanation

What is in dry air? Because the amount of water vapour changes from day to day, scientists quote the composition of dry air . By volume, it is approximately:

Gas Formula Percentage by volume --- --- --- Nitrogen N₂ 78% Oxygen O₂ 21% Argon Ar 0.9% Carbon dioxide CO₂ about 0.04% Neon, helium, krypton, xenon and others — traces

A handy summary is "about four-fifths nitrogen and one-fifth oxygen".

Evidence that air is a mixture. A compound has a fixed composition, properties different from its elements, and can only be separated by a chemical reaction. Air behaves differently on every count:

1. Its composition varies. Air in a crowded room has slightly more carbon dioxide and water vapour than air on a mountain top. A compound such as water always has exactly the same proportions of hydrogen and oxygen. 2. No formula can be written for it. The gases are not present in a simple whole-number ratio. 3. Each gas keeps its own properties. The oxygen in air still supports burning and respiration; the nitrogen is still unreactive; the carbon dioxide still turns limewater milky. 4. It can be separated by physical means. When air is cooled until it liquefies, its gases can be separated by fractional distillation because each has its own boiling point. 5. Mixing the gases gives no energy change. Blending nitrogen and oxygen in the right proportions produces a gas that behaves like air, with no heat given out and no new substance formed.

Particles in air. In the particle model, air is made of separate molecules and atoms — N₂ molecules, O₂ molecules, Ar atoms, CO₂ molecules — moving rapidly and randomly with large spaces between them. They collide with one another but do not bond together at room conditions.

Step-by-step reasoning

To decide whether a substance such as air is a mixture or a compound:

1. Ask whether its composition is fixed. Air's is not. 2. Ask whether its components keep their own properties. In air they do. 3. Ask whether it can be separated physically. Air can, by liquefying and distilling it. 4. Three "mixture" answers mean air is a mixture.

Visual explanation

Picture a box of air drawn as particles: about four in every five are pairs of blue spheres (N₂), about one in five are pairs of red spheres (O₂), and very occasionally there is a single grey sphere (Ar) or a black-and-red trio (CO₂). All are separate and moving; none are joined to each other.

Real-world analogy

Air is like a bag of mixed nuts rather than a baked cake. In the bag, each nut is still a peanut or an almond and you could pick them apart by hand. In a cake, the flour, eggs and sugar have changed into something new that cannot simply be sorted back out.

Real-world example

Divers sometimes breathe "nitrox", which is air with extra oxygen blended in. Suppliers make it simply by mixing gases in different proportions. This is only possible because air is a mixture: you can change its composition without making a new substance.

Why?

Why do nitrogen and oxygen not react to form a compound in the air around us? The nitrogen molecule is held together by a very strong triple bond, so a great deal of energy is needed to start a reaction. At normal temperatures the collisions between molecules are far too gentle, so the gases stay mixed but unreacted.

Common misconception

"Air is just oxygen." Many people think we breathe pure oxygen, but oxygen is only about one-fifth of air. Most of each breath is nitrogen, which goes in and comes out again almost unchanged.

Worked example

Question: A classroom has a volume of 200 m³. Estimate the volumes of nitrogen and oxygen in the air in the room.

Reasoning: Nitrogen is about 78% of air: 0.78 × 200 = 156 m³. Oxygen is about 21%: 0.21 × 200 = 42 m³.

Answer: About 156 m³ of nitrogen and 42 m³ of oxygen.

Quick check

1. Give one piece of evidence that air is a mixture and not a compound. Answer: Its composition varies from place to place (or each gas keeps its own properties, or it can be separated physically).

Exam focus

Learn the approximate percentages: nitrogen 78%, oxygen 21%, argon about 0.9%, carbon dioxide about 0.04%. When asked why air is a mixture, give at least two separate reasons, such as variable composition and physical separation.

Advanced insight

The proportions of nitrogen, oxygen and argon are almost identical all over the lower atmosphere because winds and convection stir it thoroughly. Carbon dioxide and water vapour vary more, because they are constantly being added and removed by living things, the oceans and the weather.

Summary

Air is a mixture of gases: about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide, plus variable water vapour and traces of other gases. It is a mixture because its composition varies, it has no formula, each gas keeps its own properties and the gases can be separated by physical methods.

Practice questions

1. What is the approximate percentage of nitrogen in dry air? Answer: About 78% by volume. 2. Explain why no chemical formula can be written for air. Answer: Air is a mixture whose gases are not combined in a fixed whole-number ratio, and its proportions vary slightly. 3. Why do scientists quote the composition of dry air rather than ordinary air? Answer: The amount of water vapour changes a lot with weather and place, so removing it gives values that stay nearly constant. 4. A balloon holds 5.0 litres of air. Roughly what volume of oxygen does it contain? Answer: About 21% of 5.0 litres, which is about 1.05 litres, or roughly 1 litre.