AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2024: Question 3

💡 Key knowledge

• Crude oil formed from plankton (dead microscopic marine organisms).
• The plankton died and were buried in mud/sediments.
• Over time they were compressed.
• This happened over millions of years.

✅ Correct answer

Crude oil formed from dead plankton that were buried by mud/sediments. They were then compressed and changed over millions of years.

🧠 Exam technique

• This is a 4-mark recall question: each correct stage can earn 1 mark.
• Use the full sequence: dead plankton → buried → compressed → millions of years.
• Short bullet points are fine if the idea is clear.

❌ Common errors

• Saying crude oil came from plants or dinosaurs.
• Missing the idea of burial or compression.
• Forgetting the long timescale — millions of years.

💡 Key knowledge

• Some fuels contain sulfur.
• When the fuel burns, sulfur reacts with oxygen.
• This produces sulfur dioxide.

✅ Correct answer

Transport fuels can contain sulfur. When these fuels are burnt, the sulfur reacts with oxygen to produce sulfur dioxide.

🧠 Exam technique

• You need both parts: fuel contains sulfur and reaction with oxygen.
• You can mention a named fuel, e.g. petrol or diesel.

❌ Common errors

• Writing that sulfur dioxide comes from carbon dioxide or nitrogen.
• Forgetting the word oxygen.
• Saying only “from exhaust fumes” without explaining the chemistry.

💡 Key knowledge

• Sulfur dioxide causes acid rain.
• It can also cause respiratory problems in humans, such as asthma.

✅ Correct answer

1. Acid rain
2. Respiratory problems in humans, for example asthma

🧠 Exam technique

• Any two separate valid problems score the marks.
• If you write “harmful” or “bad for the environment,” that is usually too vague.
• Give a specific effect to be safe.

❌ Common errors

• Repeating the same idea twice in different words.
• Saying “global warming” — this is not the expected answer here.
• Not linking respiratory problems to humans.

💡 Key knowledge

• Carbon monoxide is made when fuels are burnt.
• This happens in insufficient / limited oxygen.
• This is called incomplete combustion.

✅ Correct answer

Carbon monoxide is produced when fuels are burnt in insufficient oxygen, so the combustion is incomplete.

🧠 Exam technique

• To get full marks, include both the fuel burning and the oxygen idea.
• “Incomplete combustion” is excellent scientific wording and matches the mark scheme.

❌ Common errors

• Writing only “cars produce carbon monoxide” with no explanation.
• Saying it is produced by too much oxygen — this is the opposite of the correct idea.
• Mixing up carbon monoxide with carbon dioxide.

💡 Key knowledge

• Carbon monoxide reacts with nitrogen dioxide in a catalytic converter.
• The products are nitrogen and carbon dioxide.
• The equation must be balanced with correct numbers in front of each formula.

✅ Correct answer

4 CO + 2 NO₂ → N₂ + 4 CO₂

🧠 Exam technique

• Write the formulas first, then balance using coefficients only.
• Do not change the small numbers in the formulas.
• If you cannot fully balance it, a partially correct unbalanced equation can still earn 1 mark.

❌ Common errors

• Writing NO instead of NO₂ .
• Forgetting that nitrogen is N₂ .
• Changing formula subscripts instead of adding coefficients.
• Not balancing oxygen atoms correctly.

✅ Top-level features

• Used the exact scientific sequence for crude oil formation.
• Named the pollutant and the process: incomplete combustion.
• Gave specific effects like acid rain and asthma.
• Wrote the balanced equation with all correct formulas and coefficients.

🧠 Final exam tip

For these questions, the safest strategy is: use the mark scheme wording, keep answers short and precise, and check equations atom by atom.