AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2024: Question 1

✅ Correct answer

A hydrocarbon is a substance made only of carbon and hydrogen atoms.

💡 Key knowledge

• “Atoms only” is the important idea.
• If oxygen, nitrogen, sulfur or any other element is present, it is not a hydrocarbon.
• Hydrocarbons are the main compounds in crude oil.

🧠 Exam technique

For 1 mark, keep it short and exact: carbon and hydrogen only . Extra detail is fine, but not needed.

❌ Common errors

• Saying “contains carbon and hydrogen” without making it clear that only these atoms are present.
• Mixing up hydrocarbons with carbohydrates.

✅ Correct answer

C₁₀H₂₂

📐 Calculation

Use the general formula for alkanes: CₙH₂ₙ₊₂

1. Substitute n = 10 .
2. Hydrogen = 2 × 10 + 2 = 22 .
3. So the formula is C₁₀H₂₂ .

🧠 Exam technique

Always substitute the number of carbon atoms into the formula carefully. This is a quick 1-mark recall question, so write the final molecular formula clearly.

❌ Common errors

• Writing C10H20 by forgetting the +2 .
• Using the wrong formula for alkenes instead of alkanes.
• Using full-size numbers without subscripts.

✅ Correct answer

Crude oil is heated so some hydrocarbons vaporise. The column has a temperature gradient, so it is hotter at the bottom and cooler at the top. The vapours rise and condense at different heights because different hydrocarbons have different boiling points. The fractions are collected at the trays/levels where they condense.

💡 Key knowledge

• Heating turns part of the crude oil into vapour.
• A temperature gradient means temperatures decrease up the column.
• Fractions with higher boiling points condense lower down.
• Fractions with lower boiling points rise higher before condensing.

🧠 Exam technique

For 4 marks, give the process in order. The examiner is looking for these points:

1. Crude oil is heated.
2. Some hydrocarbons vaporise.
3. The column is cooler higher up / has a temperature gradient.
4. Fractions condense at different heights according to boiling point.

Mentioning “different heights” and “boiling points” usually secures the final mark.

❌ Common errors

• Saying the fractions are “filtered” out — this is not filtration.
• Forgetting to mention heating and vaporisation.
• Describing “melting” instead of condensing.
• Not linking the separation to different boiling points.

✅ Correct answer

C₁₄H₃₀ → C₈H₁₈ + 2C₃H₆

📐 Calculation

1. Count atoms on the left: C = 14 , H = 30 .
2. On the right, one C₈H₁₈ gives 8 carbon and 18 hydrogen.
3. That leaves 14 - 8 = 6 carbon atoms.
4. Each propene molecule is C₃H₆ , so 6 ÷ 3 = 2 molecules of propene are needed.
5. Check hydrogen: 18 + (2 × 6) = 30 , so the equation is balanced.

🧠 Exam technique

This is a balancing question, so the total number of each atom must match on both sides. Write the coefficient before the whole molecule: 2C₃H₆ , not C₃H₆₂ .

❌ Common errors

• Forgetting to check both carbon and hydrogen.
• Putting the 2 in the wrong place.
• Writing an unbalanced equation such as C₁₄H₃₀ → C₈H₁₈ + C₃H₆ .

✅ Correct answer

💡 Key knowledge

• Alkenes react with bromine water.
• The colour change is the key result: orange → colourless.
• If the question only asks for “test”, bromine water alone is fine. If it asks for result, state the colour change.

🧠 Exam technique

You need both parts for 2 marks: the test and the result. A strong answer is short and direct:
Add bromine water; it turns from orange to colourless.

❌ Common errors

• Saying “it goes clear” is less precise than “colourless”.
• Using iodine solution or limewater instead of bromine water.
• Not giving a result at all.

✅ Correct answer

Polymer

💡 Key knowledge

• Poly(propene) is made by joining many propene molecules together.
• Anything with “poly-” in the name is usually a polymer.
• A polymer is a large molecule made from many repeating units.

🧠 Exam technique

This is a simple recall mark. Write the one-word answer clearly: polymer . “Plastic” can sometimes be accepted, but polymer is the safest answer.

❌ Common errors

• Saying “alkene” because it is made from propene.
• Writing “monomer” — propene is the monomer, poly(propene) is the polymer.

💡 Top-level response features

• Uses exact GCSE keywords: hydrocarbon, vaporise, temperature gradient, condense, boiling point, decolourises, polymer.
• Gives answers in the same order as the process described in the question.
• Uses clear scientific language rather than vague phrases like “separated by heat”.

❌ Where students lose marks

• Missing the word only in the hydrocarbon definition.
• Forgetting the temperature gradient in the fractionating column.
• Not balancing the cracking equation properly.
• Giving the test for alkenes but not the colour change.