Composition of the Atmosphere and Air as a Mixture
Air is a homogeneous mixture of gases that makes up the atmosphere, consisting of about 78% nitrogen, 21% oxygen, 0.9% argon and small amounts of carbon dioxide, water vapor and other gases. In brief: in 100 liters of dry air there are about 78 L of nitrogen and 21 L of oxygen; because air is a mixture, its components can be separated by cooling it until it becomes liquid and then distilling it.
Air is so familiar that it is easy to forget it is matter. This article lists the composition of dry air, explains why air counts as a mixture and not a compound, describes the layers of the atmosphere, shows how oxygen and nitrogen are obtained industrially, works through a volume calculation and summarizes the history of the discovery of the components.
Why is air a mixture?
The gases in air are not chemically bonded to each other. Each gas keeps its own properties, and the proportions can vary from place to place: air near a forest has more oxygen than air in a crowded room, and humid air contains more water vapor than dry air. A compound would have a fixed composition. In addition, the components can be separated by physical methods, as the section on liquefaction shows.
Air is a homogeneous mixture, because the gases mix evenly and cannot be seen separately.
What is air made of?
| Gas | Formula | Percent by volume |
|---|---|---|
| Nitrogen | N2 | 78.08 |
| Oxygen | O2 | 20.95 |
| Argon | Ar | 0.93 |
| Carbon dioxide | CO2 | about 0.04 |
| Neon, helium, methane, krypton, hydrogen and others | various | less than 0.01 in total |
Real air also contains variable amounts of water vapor, from nearly 0% in a desert to as much as about 4% in very warm, humid air. The numbers for dry air add up to about 100%. The concentration of carbon dioxide has been rising because of the burning of fossil fuels.
What are the layers of the atmosphere?
The atmosphere is divided into layers according to how the temperature changes with height. Most of the mass of the atmosphere lies in the lowest layer.
| Layer | Approximate height above the surface | Notable feature |
|---|---|---|
| Troposphere | 0 to about 12 km | Weather occurs here; temperature falls with height |
| Stratosphere | about 12 to 50 km | Contains the ozone layer |
| Mesosphere | about 50 to 85 km | Meteors burn up |
| Thermosphere | about 85 to 600 km | Auroras, very low density |
| Exosphere | above about 600 km | Gradually merges with space |
The height of the troposphere varies, from about 8 km near the poles to about 18 km above the equator.
How are the gases in air separated?
Industry separates air by fractional distillation of liquid air. The process has four main steps.
- Filter the air to remove dust, and remove water vapor and carbon dioxide, which would freeze and block the equipment.
- Compress the air and cool it to about −200 °C, so it becomes a liquid.
- Let the liquid air warm slowly in a fractionating column. Nitrogen boils first at −196 °C, then argon at −186 °C and oxygen at −183 °C.
- Collect each gas as it boils off at the top of the column, or collect liquid oxygen at the bottom.
The products are used widely: nitrogen for making fertilizer and for freezing food, oxygen for hospitals and steelmaking, and argon for welding and light bulbs.
Worked example: gas volumes in a room
A classroom measures 8.0 m × 6.0 m × 3.0 m. What volume of oxygen and nitrogen does it contain?
- Volume of the room. 8.0 × 6.0 × 3.0 = 144 m3.
- Oxygen. 20.95% of 144 = 0.2095 × 144 = 30.2 m3.
- Nitrogen. 0.7808 × 144 = 112.4 m3.
- Argon. 0.0093 × 144 = 1.3 m3.
- Check. 30.2 + 112.4 + 1.3 = 143.9 m3. The small difference of 0.1 m3 is the carbon dioxide and other trace gases (about 0.06 m3 for 0.04%), together with rounding.
Why does each component matter?
- Nitrogen: relatively unreactive, it dilutes oxygen, which slows combustion. Bacteria convert nitrogen into forms that plants can use.
- Oxygen: needed for respiration and burning.
- Carbon dioxide: needed by plants for photosynthesis, and a greenhouse gas that warms the planet.
- Water vapor: forms clouds and rain, and is also a greenhouse gas.
- Noble gases: inert, used in lighting and welding.
How was the composition of air discovered?
Air was considered an element until the 1700s. Daniel Rutherford isolated nitrogen in 1772. Joseph Priestley and Carl Wilhelm Scheele each prepared oxygen in the early 1770s, and Antoine Lavoisier showed that oxygen is the part of air that supports burning and gave it its name. Argon was discovered in 1894 by Lord Rayleigh and William Ramsay, and Ramsay later identified neon, krypton and xenon in liquid air.
Which mistakes are common?
Mistake 1: Saying that air is mostly oxygen
Air is about 78% nitrogen and only about 21% oxygen.
Mistake 2: Treating air as a compound
Air has no fixed formula and its components keep their own properties.
Mistake 3: Thinking that exhaled air contains no oxygen
Exhaled air contains roughly 16% oxygen and about 4% carbon dioxide, so it still contains plenty of oxygen.
Mistake 4: Believing that pure oxygen is safe in large concentrations
Pure oxygen makes fires burn much faster and can be harmful when breathed for long periods at high pressure.