The atmosphere is a thin layer of gases held around Earth by gravity. Its composition changed dramatically over 4.5 billion years — from toxic volcanic gases to the oxygen-rich air supporting complex life today. What is in the air, where those gases come from, and how human activity alters them are core KS3 chemistry themes.
What is the composition of the present-day atmosphere?
The modern atmosphere is a mixture of gases with a remarkably stable composition near Earth's surface:
| Gas | Approximate percentage by volume |
|---|---|
| Nitrogen (N₂) | 78% |
| Oxygen (O₂) | 21% |
| Argon (Ar) | ~0.9% |
| Carbon dioxide (CO₂) | ~0.04% |
| Water vapour (H₂O) | Variable (0–4%) |
A useful approximation: four-fifths nitrogen, one-fifth oxygen, with tiny amounts of other gases. The amount of water vapour varies with weather conditions; the 78% + 21% + other gases sum to 100% after excluding variable water vapour.
Predict first: What percentage of the air is NOT nitrogen or oxygen? About 1% — mostly the noble gas argon, with a tiny fraction carbon dioxide.
How did the early atmosphere differ from today's?
Scientists believe Earth formed about 4.5 billion years ago. The early atmosphere was very different from today's:
~4.5 billion years ago (early Earth):
- Produced mainly by volcanic outgassing (gases released from within the Earth as the surface cooled)
- Consisted chiefly of: water vapour, carbon dioxide, nitrogen, and sulfur dioxide — similar to the gas emissions from modern volcanoes
- Very little or no oxygen
- No ozone layer to shield the surface from ultraviolet radiation
Key changes over billions of years:
- Water condensed — as the Earth cooled, water vapour condensed and fell as rain, forming the oceans. CO₂ dissolved in the oceans and was incorporated into sedimentary rocks (e.g. limestone) and the shells of marine organisms, removing it from the atmosphere.
- Oxygen appeared — about 2.4 billion years ago, photosynthesising cyanobacteria (and later plants) began releasing oxygen as a waste product. Oxygen built up slowly in the atmosphere.
- Ozone layer formed — oxygen high in the atmosphere reacted to form ozone (O₃), which absorbs harmful UV radiation and allowed life to colonise the land.
- Nitrogen increased — nitrogen was released by volcanic activity and produced by denitrifying bacteria. It is chemically unreactive, so it accumulated.
What are the main air pollutants and where do they come from?
Burning fossil fuels (coal, oil, and natural gas) releases several harmful pollutants:
| Pollutant | Source | Effect |
|---|---|---|
| Carbon dioxide (CO₂) | Complete combustion of carbon-containing fuels | Greenhouse gas — contributes to climate change |
| Carbon monoxide (CO) | Incomplete combustion (insufficient oxygen) | Toxic — binds to haemoglobin in red blood cells, preventing oxygen transport |
| Sulfur dioxide (SO₂) | Combustion of sulfur-containing fuels (coal, some diesel) | Causes acid rain; irritates respiratory system |
| Nitrogen oxides (NOₓ) | High-temperature combustion in engines (N₂ reacts with O₂) | Causes acid rain; contributes to smog; irritates lungs |
| Particulates | Incomplete combustion; diesel engines | Respiratory damage; blackening of buildings |
What is incomplete combustion and why is carbon monoxide dangerous?
Complete combustion occurs when there is enough oxygen for a fuel to burn fully:
fuel + oxygen (excess) → carbon dioxide + water
Incomplete combustion occurs when there is insufficient oxygen:
fuel + oxygen (limited) → carbon monoxide (+ carbon particles + water)
Carbon monoxide (CO) is a colourless, odourless, poisonous gas. It binds to haemoglobin in red blood cells about 200 times more strongly than oxygen does, forming carboxyhaemoglobin — a stable compound that prevents the blood from carrying oxygen. This causes headaches, loss of consciousness, and at high concentrations, death. It is particularly dangerous indoors — faulty gas boilers and poorly ventilated spaces are hazards.
What is acid rain?
Acid rain forms when sulfur dioxide (SO₂) or nitrogen oxides (NOₓ) dissolve in rainwater in the atmosphere:
SO₂ + H₂O → H₂SO₃ (sulfurous acid)
NO₂ + H₂O → HNO₃ (nitric acid)
Normal rain has a pH of about 5.6 (slightly acidic due to dissolved CO₂). Acid rain has a pH of 4–5 or lower.
Effects of acid rain:
- Acidifies lakes and rivers, killing fish and aquatic life
- Damages tree bark and leaches nutrients from soils, harming forests
- Corrodes limestone and marble buildings and statues (calcium carbonate reacts with the acid)
- Damages iron and steel structures
Measures to reduce acid rain include fitting catalytic converters to vehicles (to convert NOₓ to N₂), using low-sulfur fuels, and fitting flue-gas desulfurisation (scrubbers) to power station chimneys.
What is the difference between carbon dioxide and carbon monoxide as pollutants?
Both are products of burning carbon-containing fuels, but their effects are very different:
- CO₂ is not directly toxic in small quantities but is a greenhouse gas that contributes to climate change by trapping heat in the atmosphere. CO₂ levels in the atmosphere have risen from about 280 ppm before industrialisation to over 420 ppm today.
- CO is directly toxic even in small quantities because it prevents oxygen transport in the blood. It is produced only when there is insufficient oxygen for complete combustion.
Frequently asked questions
What percentage of the air is oxygen?
Approximately 21% of the air (by volume) is oxygen. This can be demonstrated by the classic rusting iron experiment: iron wool is placed in a sealed tube with water at the bottom; as the iron rusts, it consumes the oxygen in the air above it. After a few days, the water level rises to fill approximately one-fifth of the tube, confirming that about 21% of air was oxygen.
What is the difference between complete and incomplete combustion?
Complete combustion occurs when a fuel burns in a plentiful supply of oxygen, producing carbon dioxide and water. Incomplete combustion occurs when oxygen is limited, producing carbon monoxide (CO) and/or soot (carbon particles). Complete combustion releases more energy and is safer. Incomplete combustion is a concern in enclosed spaces with poor ventilation — gas appliances, car engines in garages — because of the risk of toxic carbon monoxide build-up.
What causes acid rain and how does it damage the environment?
Acid rain is caused by sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) released mainly by burning fossil fuels. These gases dissolve in atmospheric water vapour to form sulfuric and nitric acids, which fall as rain with a pH below 5. Acid rain acidifies lakes (killing aquatic life), damages forests (by leaching soil nutrients and directly harming plant tissue), and corrodes limestone buildings and metal structures. Reducing acid rain requires using cleaner fuels, catalytic converters, and power station scrubbers.
How has human activity changed the composition of the atmosphere?
Since the Industrial Revolution, burning fossil fuels has significantly increased atmospheric CO₂ (from ~280 ppm to over 420 ppm today), contributing to the enhanced greenhouse effect and climate change. Combustion also releases SO₂, NOₓ, and particulates, causing acid rain and air quality problems. Deforestation reduces the number of trees that absorb CO₂ through photosynthesis, accelerating the rise. The release of chlorofluorocarbons (CFCs) from refrigerants and aerosols in the 20th century caused significant thinning of the ozone layer, allowing more UV radiation to reach Earth's surface — though CFC emissions have now been largely banned under the Montreal Protocol.
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