AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2018: Question 2

Recognising a greenhouse gas, naming effects of global climate change, doing a simple ratio calculation, suggesting a way to reduce carbon emissions in manufacture, and explaining how the proportions of gases in Earth’s atmosphere changed over time.

✅ Correct answer

Methane

1 mark for the correct box.

💡 Key knowledge

• Greenhouse gases trap infrared radiation in the atmosphere.
• Examples include carbon dioxide, methane and water vapour.
• Argon, nitrogen and oxygen are not greenhouse gases in this GCSE context.

🧠 Exam technique

• Tick the option that is a known greenhouse gas, not just a gas in air.
• If unsure, eliminate gases that make up most of the atmosphere but do not trap heat well.

❌ Common errors

• Choosing nitrogen or oxygen because they are common gases.
• Choosing argon because it is a gas in air, even though it is not a greenhouse gas.

✅ Correct answers

Any two of the following score 2 marks:

• rising sea levels
• melting ice
• agricultural problems
• extremes of weather
• loss of habitats

1 mark for each correct effect. You can use two from the same bullet list or two different ones.

💡 Key knowledge

These are all impacts of a warmer climate:

• Rising sea levels can happen because ice melts and sea water expands.
• Melting ice affects polar regions and glaciers.
• Agricultural problems include crop failure or reduced yields.
• Extremes of weather include droughts, storms or heatwaves.
• Loss of habitats means animals and plants may no longer survive in that area.

🧠 Exam technique

• Give short, clear effects rather than explanations unless asked.
• One effect per line helps the examiner award marks quickly.
• Make sure the effect is about global climate change, not another environmental issue.

❌ Common errors

• Writing global warming instead of an effect.
• Answering with acid rain or ozone depletion — these were ignored in the mark scheme.
• Giving vague answers like “bad weather” without a clear effect.

📐 Calculations — step by step

1. Use direct proportion:
   carbon footprint = (6.0 ÷ 4.1) × 0.0235 kg
2. Calculate the scaling factor:
   6.0 ÷ 4.1 ≈ 1.46
3. Multiply by the bottle mass:
   1.46 × 0.0235 ≈ 0.0344 kg
4. Final answer:
   0.0344 kg of carbon dioxide

The mark scheme allows 0.0344 kg for full marks. Correct rounding is accepted.

✅ Correct answer

Carbon footprint = 0.0344 kg of carbon dioxide

Equivalent acceptable forms if correct:

• 0.034 kg
• 34.4 g
• 0.344 g is not correct

🧠 Exam technique

• Keep units consistent: the answer is in kg of carbon dioxide.
• Show the proportional method, not just the final number.
• Use the calculator carefully — this is a 2-mark method + answer question.

❌ Common errors

• Using 23.5 as if it were kg instead of g.
• Forgetting to divide by 4.1 first.
• Giving an answer with no units.
• Mixing up grams and kilograms.

✅ Correct answers

• Use less plastic
• Use recycled plastic

1 mark for one correct method.

💡 Key knowledge

• Using less material means less raw plastic needs to be made.
• Recycled plastic reduces the need for new plastic production.
• The mark scheme also allowed carbon capture.

🧠 Exam technique

• Give a direct action, not a vague explanation.
• Short answers are fine here.

❌ Common errors

• Writing about “using less fuel” — this was ignored because the question is about plastic bottle manufacture.
• Answering with “recycle more” without linking it to plastic if your meaning is unclear.

💡 Key knowledge to include

• Nitrogen increased because volcanoes released nitrogen and because denitrifying bacteria produced nitrogen.
• Oxygen increased because algae and plants produced oxygen by photosynthesis.
• Carbon dioxide decreased because algae and plants used it in photosynthesis.
• Carbon dioxide also decreased because it dissolved in oceans.
• Carbon dioxide became locked up in carbonate rocks / sedimentary limestone.
• Some carbon became trapped in fossil fuels such as coal, crude oil and natural gas.

🧠 Exam technique — what top answers did

• Made clear links: cause → change in atmosphere.
• Covered all three gases: nitrogen, oxygen and carbon dioxide.
• Used scientific terms such as photosynthesis, oceans, carbonate rocks and fossil fuels.
• Wrote a connected explanation rather than three separate facts.

❌ Common errors

• Only stating that the percentages changed, without saying why.
• Talking about modern pollution rather than early atmosphere changes.
• Mixing up oxygen increase and carbon dioxide decrease.
• Not linking photosynthesis to oxygen production and carbon dioxide removal.

💡 Remember these facts

• Methane is a greenhouse gas.
• Climate change can cause rising sea levels, melting ice, extreme weather, agricultural problems and habitat loss.
• Carbon footprint calculations need proportional reasoning and correct units.
• Less plastic and recycled plastic reduce emissions.
• Photosynthesis is key to changing oxygen and carbon dioxide levels over time.

🧠 Fast exam tips

• Use bullet points for short-answer questions.
• Show working for calculations.
• For 6-mark questions, link each statement to a reason.
• Check that your answer matches the command word: tick, give, calculate, explain.