AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2024: Question 6

✅ Correct answer

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

💡 Key knowledge

• Hydrocarbons contain only C and H.
• They are found in crude oil and are the main fuels and feedstocks in this topic.

🧠 Exam technique

For 1 mark, keep it simple: mention carbon and hydrogen only. That is enough for full credit.

❌ Common errors

• Saying “contains carbon and hydrogen” without saying only.
• Including oxygen or any other element.
• Describing a hydrocarbon as a mixture rather than a substance made of these atoms.

📐 Calculation / formula step

General formula for alkanes: C_nH_2n+2

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

✅ Correct answer

C₁₀H₂₂

🧠 Exam technique

Always check you have the correct alkane formula pattern: CnH2n+2. The carbon number stays the same; only the hydrogen number changes.

❌ Common errors

• Writing C10H20 or C10H18 .
• Using the alkene formula by mistake: CnH2n .
• Forgetting to substitute n = 10 correctly.

✅ Correct answer

Crude oil is heated, so some of the hydrocarbons vaporise. The vapours enter a fractionating column which has a temperature gradient — it is hotter at the bottom and cooler at the top. Different hydrocarbons have different boiling points, so they condense at different heights in the column and are collected as separate fractions.

💡 Key knowledge

• Fractional distillation separates a mixture using differences in boiling point.
• Only some hydrocarbons vaporise at the start because crude oil contains molecules with different boiling points.
• The column gets cooler going up.
• Fractions with higher boiling points condense lower down.

🧠 Exam technique

Marks are awarded for the sequence:

1. heated
2. vapourise
3. temperature gradient
4. condense at different heights / boiling points

Using Figure 4, mention that the column is hot at the bottom and cooler at the top.

❌ Common errors

• Calling it “filtering” instead of fractional distillation.
• Saying fractions “evaporate at different heights” without mentioning condensing.
• Forgetting the temperature gradient.
• Not linking separation to different boiling points.

📐 Calculation / balancing steps

1. Start with the equation: C₁₄H₃₀ → C₈H₁₈ + ___ C₃H₆
2. Carbon atoms: 14 on the left, 8 already on the right, so 6 carbon atoms remain.
3. Each propene molecule has 3 carbon atoms, so 2 propene molecules are needed.
4. Check hydrogen: 18 + 12 = 30, so it is balanced.

✅ Correct answer

C₁₄H₃₀ → C₈H₁₈ + 2C₃H₆

🧠 Exam technique

Always balance atoms on both sides. In cracking questions, the coefficient in front of the alkene is often the missing number.

❌ Common errors

• Forgetting the 2 before C3H6 .
• Balancing only carbon and not checking hydrogen.
• Changing the formula of propene instead of using the correct molecule.

✅ Correct answer

Test: Add bromine water to the sample.

Result: Bromine water changes from orange to colourless (decolourises).

💡 Key knowledge

• Alkenes contain a carbon-carbon double bond.
• Bromine water is used to identify unsaturation.
• Positive test = decolourisation.

🧠 Exam technique

For 2 marks, give both parts:

1. The test — bromine water.
2. The observation — orange to colourless.

“Turns clear” may be accepted, but colourless is the safest exam wording.

❌ Common errors

• Saying “add bromine” without mentioning bromine water.
• Writing “turns blue-black” — that is the starch test, not alkenes.
• Forgetting to state the colour change.

✅ Correct answer

Polymer

💡 Key knowledge

• Poly(propene) is made of many repeating units joined together.
• Substances like this are called polymers.
• GCSE answers may also allow plastic.

🧠 Exam technique

If you see brackets and an n in the structure, that usually means a repeating unit, so the substance is a polymer.

❌ Common errors

• Calling it a monomer — propene is the monomer, poly(propene) is the polymer.
• Giving a vague answer like “compound” or “molecule”.

💡 Key facts to remember

• Hydrocarbon = carbon and hydrogen only.
• Alkane formula = CnH2n+2.
• Fractional distillation separates crude oil by boiling point.
• Cracking makes smaller molecules, often an alkane + alkene.
• Bromine water tests for alkenes: orange → colourless.
• Poly(propene) is a polymer.

🧠 How to score well

• Use exact science words: vapourise, condense, boiling point, polymer.
• For any process, write the steps in the correct order.
• For equations, always check both sides balance.