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This lesson brings together all the key concepts from the Chemistry of the Atmosphere unit to help you prepare for the AQA GCSE Chemistry exam (specification 5.9). You will review the essential knowledge, practise applying it to exam-style questions, and learn strategies for maximising your marks. This topic typically appears in Paper 2 and can include multiple-choice, short-answer, and extended-response (6-mark) questions.
Before the exam, make sure you can confidently answer questions on all of these topics:
| Topic | Key Points You Must Know |
|---|---|
| Early atmosphere | Formed by volcanic outgassing; rich in CO₂ and water vapour; no oxygen; similar to Mars/Venus today |
| How oxygen increased | Photosynthesis by cyanobacteria, algae, then plants; Great Oxidation Event; formation of ozone layer |
| How CO₂ decreased | Dissolved in oceans; photosynthesis; formation of sedimentary rocks (limestone); formation of fossil fuels |
| Current atmosphere | N₂ 78%, O₂ 21%, Ar 0.93%, CO₂ 0.04%, other noble gases in trace amounts |
| Greenhouse gases | CO₂, CH₄, H₂O vapour; sources of each; the greenhouse effect mechanism |
| Climate change | Evidence (temperature records, ice cores, sea level rise, glacier retreat); consequences; scientific consensus |
| Carbon footprint | Definition (total CO₂ and other greenhouse gases); how to reduce at individual/business/government level |
| Pollutants | CO₂, CO, SO₂, NOₓ, particulates; sources, formation, and effects of each |
| Complete combustion | Hydrocarbon + O₂ → CO₂ + H₂O; balanced equations |
| Incomplete combustion | Hydrocarbon + limited O₂ → CO + H₂O or C + H₂O; balanced equations |
| Acid rain | Caused by SO₂ (→ H₂SO₄) and NOₓ (→ HNO₃); effects on buildings, lakes, vegetation |
| Global dimming | Caused by particulates reflecting/scattering sunlight; opposing effect to greenhouse warming |
| Reducing emissions | Catalytic converters, FGD, CCS, renewable energy, electric vehicles, international agreements |
Exam Tip: Use this checklist as a revision tool. Cover the "Key Points" column and test yourself on each topic. If you cannot recall the key points from memory, go back and revise that topic before the exam.
Understanding command words is essential for writing answers that match what the examiner expects:
| Command Word | What It Means | How to Answer |
|---|---|---|
| State | Give a brief, factual answer | No explanation needed — just the fact |
| Describe | Say what happens or what something is like | Give details but no explanation of WHY |
| Explain | Say what happens AND why | Include reasons, mechanisms, or scientific principles |
| Compare | Identify similarities and/or differences | Use comparative language (e.g. "whereas," "both," "unlike") |
| Evaluate | Weigh up evidence or arguments and reach a conclusion | Give advantages AND disadvantages, then give your judgement |
| Suggest | Apply your knowledge to an unfamiliar situation | Use scientific principles even if the exact answer is not in the specification |
| Calculate | Work out a numerical answer | Show all working and include units |
Exam Tip: Always read the command word carefully. If a question says "explain," you MUST give reasons, not just describe what happens. If it says "evaluate," you MUST discuss both sides and give a conclusion. Misreading the command word is one of the most common reasons students lose marks.
These test recall of key facts. For the atmosphere topic, common questions include:
Strategy: Read all options carefully before selecting your answer. Eliminate obviously wrong options first.
Examples:
Strategy: Be precise and specific. Each mark requires a distinct point.
You may need to:
Strategy: Show all working, use correct units, and check your answer makes sense.
These questions require you to write a structured, detailed answer. Common 6-mark questions on this topic include:
flowchart TD
A["Read the question carefully"] --> B["Identify the command word"]
B --> C["Plan your answer<br/>(3–4 key points)"]
C --> D["Write in a logical order"]
D --> E["Use scientific terminology"]
E --> F["Include specific examples<br/>and equations where relevant"]
F --> G["Check: Have you answered<br/>what was actually asked?"]
style A fill:#2980b9,color:#fff
style C fill:#27ae60,color:#fff
style G fill:#e74c3c,color:#fff
Exam Tip: For 6-mark questions, spend 1–2 minutes planning before you start writing. Jot down 3–4 key points. Then write each point as a separate paragraph with scientific detail. A structured, logical answer will always score better than a rambling one.
Question: "Describe how the composition of the Earth's atmosphere has changed over time. Include the processes that caused these changes." (6 marks)
Model Answer:
The Earth's early atmosphere was formed by volcanic outgassing about 4.6 billion years ago. It was rich in carbon dioxide and water vapour, with small amounts of nitrogen, methane, and ammonia. There was no oxygen. This was similar to the atmospheres of Mars and Venus today.
As the Earth cooled, water vapour condensed to form the oceans. Carbon dioxide dissolved in the oceans, reducing its concentration in the atmosphere.
About 2.7 billion years ago, cyanobacteria (and later algae and plants) evolved and began carrying out photosynthesis: CO₂ + H₂O → glucose + O₂. This process removed carbon dioxide from the atmosphere and produced oxygen.
Carbon dioxide was also locked away in sedimentary rocks (such as limestone, formed from the shells of marine organisms) and in fossil fuels (coal, oil, and gas formed from buried organisms).
Over billions of years, oxygen levels rose to approximately 21%, carbon dioxide fell to approximately 0.04%, and nitrogen accumulated to approximately 78% because it is unreactive and was not removed. The atmosphere has been approximately at this composition for about 200 million years.
Why this scores 6 marks: It covers all three phases, names specific gases and percentages, identifies key processes (volcanic outgassing, condensation, photosynthesis, formation of sedimentary rocks), includes the photosynthesis equation, and is written in a logical chronological order.
| Reaction | Equation |
|---|---|
| Complete combustion of methane | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Incomplete combustion (CO) | 2CH₄ + 3O₂ → 2CO + 4H₂O |
| Incomplete combustion (C) | CH₄ + O₂ → C + 2H₂O |
| Sulfur burning | S + O₂ → SO₂ |
| Formation of sulfuric acid | SO₂ + H₂O → H₂SO₃ |
| Nitrogen oxide formation | N₂ + O₂ → 2NO |
| Catalytic converter reaction | 2CO + 2NO → 2CO₂ + N₂ |
| Acid rain on limestone | CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂ |
| Flue gas desulfurisation | CaCO₃ + SO₂ → CaSO₃ + CO₂ |
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