Earth and atmosphere · GCSE Chemistry
The atmosphere
How Earth’s atmosphere changed, the greenhouse effect, climate change, carbon footprint and pollutant gases from combustion.
The early atmosphere was mostly carbon dioxide. Photosynthesis locked carbon into biomass, sediments and fossil fuels, and released oxygen. What we burn now is that carbon returning.
The important bits
What you need to know
- 1
For 200 million years the atmosphere has been about four-fifths nitrogen, one-fifth oxygen, plus small amounts of carbon dioxide, water vapour and noble gases.
- 2
The early atmosphere was dominated by carbon dioxide from volcanic activity, with little or no oxygen, and some methane, ammonia and water vapour. Oceans formed as the Earth cooled and water vapour condensed.
- 3
Carbon dioxide decreased as it dissolved in oceans, formed sedimentary carbonates, and was locked into fossil fuels. Algae and plants photosynthesised, producing oxygen and reducing carbon dioxide further.
- 4
Greenhouse gases (carbon dioxide, methane, water vapour) absorb long-wave radiation from the Earth’s surface and reduce the energy radiated to space, keeping the surface warmer than it would otherwise be.
- 5
Human activities increase carbon dioxide (burning fossil fuels, deforestation) and methane (cattle, rice fields, landfill). This enhanced greenhouse effect is linked to global climate change.
- 6
A carbon footprint is the total greenhouse gas emissions caused by a person, event or product. It can be reduced, but not always to zero, because of energy demand and incomplete international action.
- 7
Complete combustion of a hydrocarbon produces carbon dioxide and water. Incomplete combustion produces carbon monoxide (toxic) and carbon (soot). Sulfur impurities make sulfur dioxide; high temperatures make oxides of nitrogen. Both contribute to acid rain. Particulates cause global dimming and respiratory harm.
Go deeper
The atmosphere we have is a biological product
Volcanoes filled the early air with carbon dioxide. As the planet cooled, water condensed into oceans and some carbon dioxide dissolved, forming carbonates in sediments. Then photosynthetic organisms appeared. They used carbon dioxide and water, stored carbon in biomass, and released oxygen. Over geological time that oxygen reached the modern 21% and carbon dioxide fell to a trace. Coal, oil and limestone are stores of that ancient carbon. Burning them is not a new process in chemistry — it is combustion — but it returns carbon to the atmosphere much faster than weathering and photosynthesis can remove it. That rate mismatch is the climate problem in one sentence.
Go deeper
Pollutants are specific molecules with specific harms
Do not write “fumes”. Carbon monoxide binds to haemoglobin and reduces oxygen transport. Soot (carbon particles) causes respiratory problems and reflects sunlight, contributing to global dimming. Sulfur dioxide and nitrogen oxides dissolve to form acid rain, which damages limestone, lakes and trees. Nitrogen oxides also form at high engine temperatures from nitrogen and oxygen in the air, not from sulfur in the fuel. Catalytic converters, low-sulfur fuels, wet scrubbers in power stations and burning less fuel are linked solutions. Complete combustion in a good supply of oxygen is cleaner than a yellow, sooty flame, but it still produces carbon dioxide, so it is not climate-neutral.
See the idea in action
Methane burns completely: CH₄ + 2O₂ → CO₂ + 2H₂O. In a limited oxygen supply, carbon monoxide or carbon can form instead of some of the carbon dioxide. A gas fire with a blocked flue is dangerous because colourless, odourless CO can build up. Separately, sulfur in some fuels burns to SO₂, which oxidises and dissolves to form acid rain. Reducing the carbon footprint of the same fire means burning less methane or replacing it with a lower-emission energy source — the combustion chemistry itself still produces CO₂ when complete.
Exam technique
Turn knowledge into marks
Describe the early atmosphere, then give two processes that reduced CO₂ and one that increased O₂. For climate questions, name the greenhouse gas, the human activity, and the mechanism (absorbs outgoing long-wave radiation). For pollutants, name the gas and a specific effect.
Common mistakes
Do not give these marks away
- 01
Saying the greenhouse effect is only a human invention — the natural effect keeps Earth warm enough for life; the enhancement is the problem.
- 02
Mixing up carbon monoxide toxicity with carbon dioxide’s greenhouse effect.
- 03
Writing that oxygen in the early atmosphere came from volcanoes, rather than from photosynthesis.
Which process is mainly responsible for the rise of oxygen in Earth’s atmosphere?
AVolcanic activity
BPhotosynthesis by algae and plants
CComplete combustion
DAcid rain
Show the answer
Photosynthesis by algae and plants. Photosynthesis uses carbon dioxide and water and releases oxygen. Over geological time this produced the oxygen-rich atmosphere we have now.
Quick questions
If this is the bit you searched
How did the amount of carbon dioxide in the atmosphere decrease?
It dissolved in oceans, formed sedimentary rocks such as limestone, was locked into fossil fuels, and was used in photosynthesis.
What is the difference between complete and incomplete combustion?
Complete combustion in plenty of oxygen gives carbon dioxide and water. Incomplete combustion gives carbon monoxide and/or carbon (soot) as well, and less energy.