AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2019: Question 7

💡 Key knowledge

• Crude oil is separated by fractional distillation.
• Combustion products depend on the amount of oxygen.
• Greenhouse gases absorb and re-emit infrared radiation.

🧠 Exam technique

• Give the process in the correct order.
• Use key terms exactly: vapourises, condenses, oxygen, infrared.
• For equations, balance atoms on both sides.

❌ Common errors

• Saying cracking instead of fractional distillation.
• Mixing up complete and incomplete combustion.
• Forgetting that greenhouse gases trap long-wave infrared, not short-wave radiation.

✅ Correct answer

Crude oil is heated/vaporised so that it turns into a vapour. The vapour enters a fractionating column which has a temperature gradient: it is hotter at the bottom and cooler at the top. As the vapour rises, it condenses at different heights. Different fractions condense at different levels because they have different boiling points.

💡 Key knowledge

• Fractional distillation separates hydrocarbons by boiling point.
• Smaller molecules have lower boiling points and rise higher.
• Larger molecules condense lower down the column.

🧠 How the 4 marks are awarded

• 1 mark for heating/vaporising the crude oil.
• 1 mark for mentioning the temperature gradient in the column.
• 1 mark for vapour condensing into fractions.
• 1 mark for different boiling points / different levels.

❌ Common errors

• Saying “fractional distillation” is enough on its own — it is not.
• Using the word cracking, which is a different process.
• Saying the fractions “separate by density” instead of boiling point.
• Writing “fracking” or “tracking” — the examiners ignore incorrect references, but you lose marks for missing the right science.

✅ Correct answer

Different amounts of oxygen are available.

💡 Key knowledge

• Complete combustion happens when oxygen is plentiful.
• Incomplete combustion happens when oxygen is limited.
• Incomplete combustion can produce carbon monoxide or carbon (soot).

🧠 Exam technique

For a 1-mark question, keep it short and direct. The mark scheme accepts:

• different amounts of oxygen available
• complete and incomplete combustion

❌ Common errors

• Describing the products only, without explaining why they differ.
• Writing that the fuel changes instead of the oxygen supply.

✅ Correct answer

2 C₄H₁₀ + 9 O₂ → 8 CO + 10 H₂O

📐 Step-by-step balancing

1. Start with 1 butane molecule: C₄H₁₀.
2. To avoid half numbers in the final answer, use 2 C₄H₁₀.
3. This gives 8 carbon atoms and 20 hydrogen atoms.
4. So the products must be 8 CO and 10 H₂O.
5. Count oxygen atoms on the right: 8 + 10 = 18 oxygen atoms.
6. So you need 9 O₂ on the left.

🧠 How to get the mark

• You need the correct balanced equation.
• Examiners allow correct multiples or halves, but the atoms must balance.
• Always check C, then H, then O.

❌ Common errors

• Writing 4 CO instead of 8 CO.
• Forgetting to balance oxygen after balancing carbon and hydrogen.
• Using the wrong product, such as CO₂ instead of CO.

✅ Correct answer

Short-wavelength radiation from the Sun, including UV/short-wave radiation, enters the atmosphere. It is absorbed by the Earth’s surface and then re-emitted as longer-wavelength infrared radiation. Greenhouse gases such as carbon dioxide and methane absorb this infrared radiation and re-emit it. This traps energy in the atmosphere, so temperature increases.

💡 Key knowledge

• Incoming energy: short wavelength radiation.
• Outgoing energy from Earth: long wavelength infrared radiation.
• Greenhouse gases absorb and re-emit infrared, reducing energy escaping to space.

🧠 What top answers include

• Mentions radiation enters the atmosphere.
• Says it is absorbed and re-emitted.
• Clearly states the re-emitted radiation is infrared / longer wavelength.
• Explains that greenhouse gases trap radiation and warm the atmosphere.

❌ Common errors

• Saying greenhouse gases trap short-wave radiation.
• Forgetting to mention re-emission.
• Not naming infrared as the longer-wavelength radiation.
• Writing only “carbon dioxide causes global warming” without explaining the process.

💡 One-line recall

Fractional distillation separates crude oil by boiling point; oxygen supply affects combustion products; and greenhouse gases absorb and re-emit infrared radiation.

🧠 Final exam reminder

For GCSE Science, short structured answers often score best: use the correct term, then explain it in the right order. If balancing an equation, check every atom and keep coefficients whole numbers where possible.