AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2023: Question 3

💡 Big ideas across this question

• Crude oil is a mixture, not a formulation.
• Fractions are separated because they have different boiling points.
• Cracking makes smaller, more useful molecules.
• Hydrogen is tested with a lit splint and gives a squeaky pop.
• Alkenes follow the general formula CₙH₂ₙ .

🧠 Examiner insight

• Many students lose marks by giving vague answers like “crude oil is useful” or “it is separated by heating” without the key detail.
• For 4-mark process questions, include a sequence: heat → vapourise → temperature change in column → condense at different heights → because of different boiling points.
• For gas tests, both the test and the result are needed for full marks.

✅ Correct answers

• Crude oil is not a useful product.
• It is not mixed in carefully measured quantities.

💡 Key knowledge

A formulation is a useful mixture designed for a particular purpose, with components in carefully measured amounts.

🧠 Exam technique

Do not just say “crude oil is a mixture” because the question already tells you that. You need to explain why it is not a formulation.

❌ Common errors

• Saying only “it is a mixture” — that does not answer the question fully.
• Talking about crude oil being separated into fractions — wrong focus.
• Missing the idea of “carefully measured quantities”.

✅ Correct answer

A full-mark answer could be:

Crude oil is heated to vaporise the hydrocarbons. The fractionating column is hottest at the bottom and cooler at the top. The hydrocarbons condense at different heights in the column because they have different boiling points.

💡 Key knowledge

• Fractional distillation separates substances in a mixture by boiling point.
• Large hydrocarbons usually have higher boiling points.
• Small hydrocarbons condense higher up the column.

🧠 Exam technique

For a 4-mark “describe how” question, aim for 4 linked points in the correct order. Use process language such as heated, vaporise, temperature decreases, condense, different boiling points.

If you only write “it is heated and separated by boiling point”, that is too short for 4 marks.

❌ Common errors

• Saying the column gets hotter towards the top — this is wrong.
• Saying fractions evaporate at different heights — they condense at different heights after vaporising.
• Forgetting to mention different boiling points.

✅ Correct answers

Alkanes are cracked to make smaller hydrocarbon molecules. These are more useful, for example as fuels or as starting materials for making other chemicals.

💡 Key knowledge

Cracking breaks long-chain hydrocarbons into shorter alkanes and alkenes.

🧠 Exam technique

This answer needs both parts:

1. What cracking does: makes smaller molecules.
2. Why that matters: they are more useful / in higher demand.

❌ Common errors

• Saying “to make energy” — not the mark scheme answer here.
• Saying only “to make alkenes” without mentioning usefulness may miss a mark.
• Talking about fractional distillation instead of cracking.

✅ Correct answer

Test: Put a lit splint into the gas.

Result: It burns with a squeaky pop.

💡 Key knowledge

Hydrogen is identified using a lit splint. Oxygen uses a glowing splint, so do not mix them up.

🧠 Exam technique

When the question gives separate lines for Test and Result, make sure you fill both clearly. Examiners award one mark for each.

❌ Common errors

• Writing glowing splint — this is not accepted.
• Writing “it relights” — that is the test for oxygen.
• Only saying “splint” without saying lit.

✅ Correct answer

C₂H₄

💡 Key knowledge

Alkenes have the general formula CₙH₂ₙ .

Check each option:

• C₂H₂ → not CₙH₂ₙ
• C₂H₄ → matches CₙH₂ₙ ✅
• C₂H₆ → alkane pattern, not alkene
• C₃H₈ → alkane pattern, not alkene

🧠 Exam technique

Substitute the carbon number into the formula. If n = 2, then H must be 4. That gives C₂H₄ .

❌ Common errors

• Choosing C₂H₆ because it looks familiar — that is an alkane.
• Not checking the hydrogen number carefully.

✅ 03.1 model answer

Crude oil is not a useful product and it is not mixed in carefully measured quantities.

✅ 03.2 model answer

Crude oil is heated to vaporise the hydrocarbons. The temperature decreases from the bottom to the top of the fractionating column. The hydrocarbons condense at different heights because they have different boiling points.

✅ 03.3 model answer

Alkanes are cracked to make smaller hydrocarbon molecules that are more useful, such as fuels or starting materials for other chemicals.

✅ 03.4 model answer

Test the gas with a lit splint. Hydrogen burns with a squeaky pop.

✅ 03.5 model answer

C₂H₄

🧠 How top answers scored well

• Used exact science vocabulary: vaporise, condense, boiling points.
• Answered with enough detail for the number of marks available.
• Separated test and result clearly in the gas test question.
• Applied the general formula correctly rather than guessing.

❌ Where students lost marks

• Confusing a formulation with just any mixture.
• Missing out the temperature gradient in the fractionating column.
• Using the oxygen test instead of the hydrogen test.
• Giving one reason instead of two in 03.1 and 03.3.