AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2020: Question 5

💡 Key knowledge

• Crude oil formed from dead marine organisms/plankton.
• They were buried in mud and compressed over millions of years.
• Fractional distillation separates hydrocarbons by different boiling points.
• Alkanes have the general formula CₙH₂ₙ₊₂ .
• Longer-chain alkanes have higher boiling points and are more viscous.

🧠 Exam technique

• For 3-mark “describe” questions, give 3 separate marking points.
• Use sequencing words: heated, evaporate, condense, collected.
• For trend questions, always state the pattern first, then explain why.

❌ Common errors

• Saying crude oil came from plants only.
• Mixing up fractional distillation with filtration.
• Writing the alkane formula as CₙH₂ₙ .
• Explaining boiling point using “strong covalent bonds break” — this is not correct for boiling.

✅ Correct answer

Crude oil formed from dead plankton / marine organisms that were buried in mud and then changed over a long period of time (or millions of years).

💡 Key knowledge

• Accept “biomass” or “marine animals/organisms” for plankton.
• “Compressed under mud” or “compressed in sedimentary rock” is accepted.
• You do not need to say “fossilised”.

🧠 Exam technique

This is a straight recall question. To score all 3 marks, include: what it came from, how it was buried, and the time scale.

❌ Common errors

• Saying “plants” instead of marine plankton/organisms.
• Leaving out the burial/compression step.
• Forgetting the long time period.

✅ Correct answer

1. Crude oil is heated.
2. The hydrocarbons evaporate / boil and become vapours.
3. The vapours rise up the column and condense at different heights.
4. Fractions have different boiling points, so they are collected at different levels.

💡 Key knowledge

• Fractional distillation works because hydrocarbons in crude oil have different boiling points.
• Higher boiling point fractions condense lower down; lower boiling point fractions go higher up before condensing.

🧠 Exam technique

• Use the process words in order: heat → evaporate/boil → condense → collect.
• Write “different boiling points” or “different levels” to secure the final mark.
• Keep it about separation by physical properties, not chemical change.

❌ Common errors

• Saying the fractions are separated by size only, without boiling point.
• Writing “melts” instead of evaporates/boils.
• Forgetting condensation in the column.

✅ Correct answer

CₙH₂ₙ₊₂

💡 Key knowledge

Alkanes are saturated hydrocarbons with only single bonds.

🧠 Exam technique

Make sure the formula is written exactly. A small symbol error can lose the mark.

❌ Common errors

• CₙH₂ₙ is wrong for alkanes.
• Missing the +2.
• Using capital letters or punctuation incorrectly.

✅ Correct answer

The boiling point increases as the number of carbon atoms increases. This is because the intermolecular forces between the molecules become stronger as the molecules get larger.

💡 Key knowledge

• Longer-chain alkanes have bigger molecules.
• Bigger molecules have stronger intermolecular forces, so more energy is needed to separate them.
• Boiling involves overcoming intermolecular forces, not breaking covalent bonds.

🧠 Exam technique

• To get 3 marks, state the trend and explain it clearly.
• Use: “as carbon number increases” + “intermolecular forces increase” + “more energy needed”.

❌ Common errors

• Talking about “stronger bonds between molecules” without saying intermolecular forces.
• Saying the molecules themselves have stronger covalent bonds.
• Reversing the trend.

✅ Correct answer

Trend: As the number of carbon atoms increases, the time increases.
Reason: The viscosity increases.

💡 Key knowledge

• More viscous liquids flow more slowly.
• Longer-chain alkanes are thicker and less runny, so they take longer to pass line X.

🧠 Exam technique

MP2 depends on getting MP1 correct. First state the pattern in the times, then explain it with viscosity.

❌ Common errors

• Saying the time decreases as chain length increases.
• Giving boiling point as the reason instead of viscosity.
• Not linking longer chains to thicker liquids / slower flow.

💡 Crude oil formation

Dead plankton → buried in mud → compressed over millions of years.

💡 Fractional distillation

Heat crude oil → vapours rise → condense at different heights because of different boiling points.

💡 Alkanes

General formula: CₙH₂ₙ₊₂

💡 Boiling point / viscosity trend

More carbon atoms → stronger intermolecular forces → higher boiling point and higher viscosity.