Air is a Mixture, Not a Compound
One of the most important things to understand about air is that it is a mixture, not a chemical compound. This means:
- The gases in air are not chemically bonded to each other
- Their proportions can vary slightly (for example, humidity changes the amount of water vapour)
- The individual gases can be separated from each other — for example, by cooling air until it becomes liquid and then allowing different gases to boil off at different temperatures
If air were a compound, separating its components would require breaking chemical bonds. Because it is a mixture, physical methods are enough.
The Composition of Dry Air
Here is what dry air (air with no water vapour) is made of:
| Gas | Chemical formula | Approximate percentage |
|---|---|---|
| Nitrogen | N₂ | ~78% |
| Oxygen | O₂ | ~21% |
| Argon | Ar | ~0.93% |
| Carbon dioxide | CO₂ | ~0.04% |
| Other gases (Ne, He, Kr, CH₄…) | various | trace amounts |
Notice that nitrogen and oxygen together make up about 99% of all the air we breathe. The other gases, while present in tiny amounts, still play important roles.
In addition to these, real air always contains some water vapour (H₂O) — the amount varies from nearly 0% in a desert to about 4% in a tropical rainforest on a humid day.
When we say "breathing" we often think only of oxygen — but we breathe in all of these gases simultaneously. Our bodies use only the oxygen (about 21%), and we breathe out a slightly different mixture: less oxygen (~16%), more carbon dioxide (~4%), and the same amount of nitrogen and argon.
The Role of Each Gas
Nitrogen (N₂) — ~78%
Nitrogen is the most abundant gas in air but is very unreactive. It does not burn, does not support burning, and does not react easily with most substances. Its main roles are:
- Diluting oxygen — if the air were pure oxygen, fires would be far more intense and our lungs would be damaged
- Nutrient for plants — through the nitrogen cycle, nitrogen from the air is converted into forms that plants can absorb and use to build proteins
Oxygen (O₂) — ~21%
Oxygen is the gas we need to stay alive. It is used in:
- Respiration — all living cells use oxygen to release energy from glucose
- Combustion — fire and burning require oxygen; without it, flames go out
- Industry — oxygen is used in steel-making, welding, and hospitals
Carbon Dioxide (CO₂) — ~0.04%
Even though carbon dioxide makes up less than 0.1% of air, it has enormous importance:
- Photosynthesis — plants absorb CO₂ from the air and convert it into glucose using sunlight
- Greenhouse effect — CO₂ traps heat in the atmosphere, keeping Earth warm enough for life
- Industrial uses — fire extinguishers, carbonated drinks, dry ice
Argon (Ar) — ~0.93%
Argon is a noble gas — completely unreactive. It is used to fill light bulbs (preventing the tungsten filament from oxidising) and to create inert atmospheres for welding.
Water Vapour (H₂O) — variable
Water vapour is responsible for clouds, rain, snow, and humidity. It also contributes to the greenhouse effect.
Real world: What you exhale
When you breathe in, air is roughly 21% O₂ and 0.04% CO₂. When you breathe out, the mixture is about 16% O₂ and 4% CO₂, along with water vapour. Your cells have used oxygen and released carbon dioxide as a waste product of respiration. If you breathe on a cold window, the water vapour condenses and fogs the glass — that moisture is actually part of what you exhale with every breath.
The Nitrogen Cycle
Nitrogen makes up 78% of air, but most living things cannot use it directly in its gaseous form (N₂). The nitrogen cycle describes how nitrogen moves between the atmosphere, soil, plants, and animals:
- Nitrogen fixation — special bacteria in soil and in the roots of legume plants (like beans and clover) convert N₂ gas into ammonia (NH₃) or nitrates (NO₃⁻) that plants can absorb
- Absorption — plants take up these nitrogen compounds through their roots and use them to build proteins
- Consumption — animals eat plants and use plant proteins to build their own proteins
- Decomposition — when animals and plants die, decomposing bacteria break down proteins back into ammonia
- Denitrification — other bacteria convert nitrates back into N₂ gas, returning it to the atmosphere
How Lavoisier Discovered Oxygen in Air
In the 18th century, scientists did not know what air was made of. The French chemist Antoine Lavoisier (1743–1794) performed experiments showing that only part of the air supports combustion and respiration.
He heated mercury in a closed container. After some time, the mercury gained weight (forming red mercuric oxide), and the volume of air in the container decreased by about one-fifth. The remaining gas (nitrogen) did not support burning.
Lavoisier concluded that air contains (at least) two gases: one that supports burning (which he named oxygen, from Greek for "acid-maker") and one that does not. This was a revolutionary discovery that replaced the old "phlogiston" theory of burning.
Real world: Industrial uses of nitrogen and oxygen
Both nitrogen and oxygen are extracted from air industrially by cooling air to extremely low temperatures (below −196 °C) until it liquefies. The liquid is then warmed slowly — nitrogen boils off first at −196 °C, then oxygen at −183 °C, allowing them to be separated. Liquid nitrogen is used to freeze food, preserve biological samples, and cool MRI machines. Industrial oxygen is used in hospitals, steel furnaces, and rocket engines.
Air vs. Oxygen — An Important Distinction
Many people confuse air and oxygen. They are NOT the same:
| Property | Air | Oxygen |
|---|---|---|
| Composition | Mixture of N₂, O₂, Ar, CO₂, etc. | Pure gas, O₂ only |
| Colour | Colourless | Colourless |
| Supports combustion? | Yes (because it contains O₂) | Yes, more strongly than air |
| Can be breathed directly? | Yes | Yes, in medical settings |
Practice Problems
Give the approximate percentage of nitrogen, oxygen, and carbon dioxide in dry air.
Show answer
Explain why air is described as a mixture rather than a compound.