AQA GCSE Chemistry (8462) · Paper 2

🌫️ Chemistry of the Atmosphere

Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.

Evolution of the atmosphere

Today, the atmosphere is made up of roughly four-fifths (about 80%) nitrogen and roughly one-fifth (about 20%) oxygen, plus small amounts of carbon dioxide (about 0.04%), water vapour and noble gases.

During the first billion years or so, intense volcanic activity built up an atmosphere that was mostly carbon dioxide, with water vapour and nitrogen, probably similar to Mars and Venus today. The evidence from this period is limited, since it happened around 4.6 billion years ago, so every theory about it remains provisional.

The water vapour then condensed into oceans, and carbon dioxide began to leave the air in several ways: it dissolved in the seawater, formed carbonate rocks such as limestone, and became locked into fossil fuels (coal from plants, oil and gas from plankton). Once algae evolved, around 2.7 billion years ago, photosynthesis also began removing carbon dioxide while releasing oxygen. Enough oxygen eventually built up in the atmosphere for animals to evolve.

🎯 Examiner tip: There are four ways carbon dioxide left the early atmosphere: dissolving in the oceans, becoming locked in carbonate rock, becoming locked in fossil fuels, and being used in photosynthesis. Listing all four is usually needed for full marks.

Greenhouse effect & climate

Greenhouse gases, including water vapour, carbon dioxide and methane, let the Sun's short-wavelength radiation through to warm the Earth, then absorb the long-wavelength infrared radiation the warm Earth radiates back out, trapping the energy. Without any greenhouse gases the Earth would freeze; the problem is the extra amount that human activity is adding.

The main human causes are burning fossil fuels and deforestation, which release carbon dioxide, and cattle farming, rice paddies and landfill sites, which release methane. Based on peer-reviewed evidence, most scientists agree that this is driving global warming and climate change.

Sea levels rise as ice melts and seawater expands, causing flooding and coastal erosion.
Storms, droughts and heatwaves become more frequent and more severe.
Rainfall patterns change, putting pressure on food production.
Species are forced to migrate, and some face extinction.

A carbon footprint is the total amount of greenhouse gases emitted over the full life cycle of a product, service or event. It can be reduced using renewable energy, greater efficiency, less flying and less meat consumption, carbon capture, carbon taxes and carbon offsets. The main barriers are cost, the need for lifestyle change, and the need for every country to act together.

Pollutants

Burning fuels does not only produce carbon dioxide; incomplete or dirty combustion also releases several other pollutants:

Carbon monoxide (CO)Carbon monoxide comes from incomplete combustion, where there is too little oxygen. It is colourless, odourless and toxic, because it binds to red blood cells so that blood can no longer carry oxygen, and it cannot be detected without an alarm.

Sulfur dioxide (SO₂)Sulfur dioxide comes from sulfur impurities in fuels. It causes breathing problems, and it dissolves in rain to form acid rain, which damages trees, lakes and buildings.

Nitrogen oxides (NOₓ)Nitrogen oxides form when the heat of an engine forces the nitrogen and oxygen in the air to react. They cause respiratory problems and contribute to acid rain.

ParticulatesParticulates are specks of soot and unburned carbon. They cause deep-lung damage and cardiovascular harm, and in the atmosphere they contribute to global dimming by reflecting sunlight.

🎯 Examiner tip: Match each pollutant to its source: carbon monoxide comes from too little oxygen, sulfur dioxide comes from sulfur in the fuel, nitrogen oxides come from nitrogen in the air reacting in engine heat, and particulates are unburned carbon. Giving both the cause and the effect together earns the marks.

Turn these notes into memory. 12 flashcards cover this topic. Short daily sessions, spaced repetition up to your exam date, quizzes with friends. Free during the beta.Practise Chemistry of the Atmosphere in the app ↗

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