AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2021: Question 2

✅ Correct answer

plankton

Accept: ancient biomass, or microscopic plants/animals.

💡 Key knowledge

• Crude oil formed from the remains of tiny organisms over millions of years.
• These are usually described as plankton in GCSE answers.
• “Ancient biomass” is also accepted because it means old biological material.

🧠 Exam technique

For a 1-mark recall question, give the exact keyword. If you write a longer answer, make sure it still includes plankton or ancient biomass.

❌ Common errors

• Writing just “dead animals” is usually too vague.
• “Dinosaurs” is not the expected answer.
• Do not say crude oil comes from rocks themselves.

✅ Correct answer

propane

Accept: C₃H₈

💡 Key knowledge

• Alkane names depend on the number of carbon atoms.
• 1 = methane, 2 = ethane, 3 = propane, 4 = butane.

🧠 Exam technique

Learn the first few alkane names in order. This is a quick recall mark.

❌ Common errors

• Saying “propanol” or “propene” — these are different compounds.
• Giving the formula instead of the name, unless the question allows it.

✅ What a full-mark answer could include

Methane has 1 carbon atom and 4 hydrogen atoms, while hexane has 6 carbon atoms and 14 hydrogen atoms.

Both are hydrocarbons and both contain C–H bonds. Hexane also contains C–C bonds, but methane does not.

Hexane molecules are larger than methane molecules, so the forces between hexane molecules are stronger. Therefore hexane has a higher melting point and higher boiling point than methane.

Methane is a gas at room temperature, while hexane is a liquid.

💡 Key knowledge

• Methane = CH₄
• Hexane = C₆H₁₄
• Both are alkanes, so they are saturated hydrocarbons.
• Longer-chain alkanes have stronger intermolecular forces, so they usually have higher boiling and melting points.
• From the table: methane b.p. = −162 °C, hexane b.p. = 69 °C.
• From the table: methane m.p. = −183 °C, hexane m.p. = −95 °C.

🧠 Exam technique

Top answers do more than list facts. They explain the link:

• larger molecule → stronger intermolecular forces → higher mp/bp
• Use comparative words: higher than, larger than, stronger than.
• To reach Level 2, make the differences clear and, where possible, mention the size of the difference using the data.

❌ Common errors

• Saying hexane has a lower boiling point because it is bigger — that is the opposite of the correct trend.
• Forgetting to compare both structure and properties.
• Confusing molecular size with number of atoms without linking it to forces.
• Saying “they both have C–H bonds” and stopping there — this is not enough for 6 marks.

✅ Correct answer

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

💡 Key knowledge

Cracking breaks a long-chain alkane into a smaller alkane and an alkene.

Here, the missing product is C₈H₁₈ (octane).

🧠 Exam technique

Check atoms balance on both sides:

• Carbon: 10 = 8 + 2
• Hydrogen: 22 = 18 + 4

Always ensure the equation is balanced.

❌ Common errors

• Writing the wrong alkane formula.
• Forgetting that alkenes contain two fewer hydrogens than the matching alkane.
• Not balancing carbon atoms.

📐 Calculations / balancing steps

1. Start with the given reactant: C₁₀H₂₂.
2. The product includes C₂H₄, so count what is left for the other product.
3. Carbon left: 10 − 2 = 8.
4. Hydrogen left: 22 − 4 = 18.
5. So the missing product is C₈H₁₈.

✅ Correct answer

Test: Add bromine water.

Result: It turns from orange/brown to colourless.

💡 Key knowledge

• Alkenes are unsaturated hydrocarbons.
• Bromine water is used to test for a carbon–carbon double bond.
• If an alkene is present, the bromine water is decolourised.

🧠 Exam technique

For 2 marks, you need both parts:

• the test = bromine water
• the result = colour change to colourless

If you only write “it changes colour”, you may lose the mark.

❌ Common errors

• Writing chlorine water or limewater instead of bromine water.
• Not stating the final colour is colourless.
• Writing “clear” instead of colourless — “colourless” is the safer exam word.

💡 Must-know facts

• Crude oil formed from plankton / ancient biomass.
• 3-carbon alkane = propane.
• Methane = CH₄; hexane = C₆H₁₄.
• Longer hydrocarbons usually have higher boiling points.
• Cracking can make alkanes + alkenes.
• Alkenes turn bromine water from orange/brown to colourless.

🧠 Best exam habits

• Use the exact scientific terms.
• Compare data clearly: “higher than”, “lower than”.
• For equations, check atoms are balanced.
• For tests, always give both the reagent and the colour change.