AQA GCSE Chemistry 4.9 Chemistry of the atmosphere: a complete overview

What topic 4.9 actually demands

Chemistry of the atmosphere connects chemistry to the environment and current affairs. Topic 4.9 rewards clear recall of the atmosphere's composition and history, precise explanation of the greenhouse effect and pollutants, and balanced discussion of climate change evidence. It links to organic chemistry (combustion) and using resources.

This guide walks through all three dot points of the topic in specification order, then sets out the exam patterns AQA repeats. Each dot point has a matching page with practice questions; this overview ties them together.

The evolution of the atmosphere

Today's atmosphere is about $80\%$ nitrogen and $20\%$ oxygen, with small amounts of carbon dioxide, water vapour and noble gases. The early atmosphere came from volcanic activity and was rich in carbon dioxide with little oxygen; water vapour condensed to form the oceans. Oxygen increased through photosynthesis by algae and plants, while carbon dioxide decreased as it dissolved in the oceans, was used in photosynthesis, and was locked into sedimentary rocks and fossil fuels.

Greenhouse gases and climate change

Greenhouse gases (carbon dioxide, methane, water vapour) absorb heat radiated from the Earth and re-emit some back, keeping the planet warm. Human activities increase carbon dioxide (burning fossil fuels, deforestation) and methane (cattle, landfill), enhancing the effect and driving global climate change, with effects such as rising sea levels and extreme weather. The carbon footprint is the total greenhouse gases over a product's life cycle.

Atmospheric pollutants

Burning fuels releases pollutants. Incomplete combustion gives carbon monoxide (toxic, reduces oxygen transport) and soot (breathing problems, global dimming). Sulfur dioxide comes from sulfur impurities, and oxides of nitrogen form from nitrogen and oxygen at high temperatures; both cause acid rain and respiratory problems.

How topic 4.9 is examined

A typical AQA profile for this topic:

• Recall. The atmosphere's composition and the steps in its evolution.
• Explanation. The greenhouse effect, how oxygen rose and carbon dioxide fell, and how each pollutant forms and harms.
• Evaluation. Discussing climate change evidence, bias and the carbon footprint.
• Combustion. Linking complete and incomplete combustion to the pollutants produced.

Check your knowledge

A mix of recall and explanation questions covering topic 4.9. Attempt them under timed conditions, then check against the solutions.

1. State the approximate composition of today's atmosphere. (2 marks)
2. Describe what the early atmosphere was thought to be like. (2 marks)
3. Explain how the amount of oxygen in the atmosphere increased. (2 marks)
4. Explain the greenhouse effect. (2 marks)
5. Name two human activities that increase greenhouse gases. (2 marks)
6. Explain why carbon monoxide is dangerous. (2 marks)
7. State which two pollutants cause acid rain and how each forms. (4 marks)
8. Define the carbon footprint. (1 mark)