AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2018: Question 1

✅ Correct answer

condenses

💡 Key knowledge

• In a fractional distillation column, the vapour cools as it rises.
• Different fractions condense at different heights because they have different boiling points.
• Higher-boiling fractions condense lower down where it is hotter.

🧠 Exam technique

• The question says crude oil vapour passes up the column, so the process is condenses.
• Do not overthink it: GCSE questions often want one exact word.

❌ Common errors

• melts is wrong — this is not a solid changing to liquid.
• freezes is wrong — vapour does not freeze in this context.
• dissolves is wrong — no solvent is involved.

✅ Correct answer

The fractions have different boiling points.

💡 Key knowledge

• Fractions are mixtures of hydrocarbons with different chain lengths.
• Longer hydrocarbons usually have higher boiling points.
• They condense at different temperatures, so they collect at different levels.

🧠 Exam technique

• If asked why they separate, link it to boiling point differences.
• The best answer is short and direct.

❌ Common errors

• flammability is not the separation property.
• melting points are not used in fractional distillation.
• viscosity is a property of the liquid, but it does not cause the separation in the column.

✅ Correct answer

propane

💡 Key knowledge

• Alkanes are saturated hydrocarbons with only single bonds.
• The first three are methane, ethane, propane.
• Propane has 3 carbon atoms: C₃H₈.

🧠 Exam technique

• Count the carbon atoms in the model carefully.
• If the molecule has only single bonds and 3 carbons, the name is propane.

❌ Common errors

• propene is wrong because propene is an alkene with a double bond.
• Students often count the white atoms; only carbon atoms matter for the name.

✅ Correct answer

CₙH₂n+2

💡 Key knowledge

• Alkanes are saturated hydrocarbons.
• They fit the formula CₙH₂n+2.
• Example: if n = 3, then H = 8, so propane is C₃H₈.

🧠 Exam technique

• Use the exact subscript form: CₙH₂n+2.
• Remember the +2 is important — this is where many students lose the mark.

❌ Common errors

• CₙH₂n is the formula for an alkene, not an alkane.
• CₙH₂n−2 is not the alkane formula.

✅ Correct answer

CH₄ + 2 O₂ → CO₂ + 2 H₂O

📐 Calculations / balancing steps

1. Start with the unbalanced word equation: methane + oxygen → carbon dioxide + water.
2. Write the formula equation: CH₄ + O₂ → CO₂ + H₂O.
3. Count atoms:
   • Left: C = 1, H = 4, O = 2
   • Right: C = 1, H = 2, O = 3
4. Put 2 in front of H₂O to make hydrogen balanced: CH₄ + O₂ → CO₂ + 2 H₂O
5. Now oxygen on the right is 4 atoms, so put 2 in front of O₂: CH₄ + 2 O₂ → CO₂ + 2 H₂O

🧠 Exam technique

• Balance carbon, then hydrogen, then oxygen.
• Only change the numbers in front of substances, not the formula itself.
• Check your final atom counts before moving on.

❌ Common errors

• Writing CH₄ + O₂ → CO₂ + H₂O is not balanced.
• Putting a 2 inside water as H₂O₂ is wrong — that changes the substance.
• Forgetting the coefficient 2 before O₂ is a very common mistake.

✅ Correct answer

bromine water

💡 Key knowledge

• Alkenes react with bromine water and decolourise it.
• This test shows the presence of a carbon-carbon double bond.

🧠 Exam technique

• Learn the standard pair: alkenes + bromine water.
• In exam questions, the answer is usually the reagent, not the observation.

❌ Common errors

• Anhydrous copper sulfate tests for water.
• Damp litmus paper tests acidity/alkalinity.
• Limewater tests carbon dioxide.

✅ Correct answer

To assess the environmental impact of the stages in the life of a product.

💡 Key knowledge

• An LCA looks at a product from raw materials to disposal.
• It helps compare different methods fairly.
• It considers impacts on the environment across the whole life of the product.

🧠 Exam technique

• Use the phrase environmental impact if you can.
• Simple, direct answers score best for 1-mark recall questions.

❌ Common errors

• Do not just say “to save energy” — that is too narrow.
• The mark scheme accepts references to harm or damage to the environment, but not vague comments with no clear link.

✅ What a top answer includes

• Burning and using energy to generate electricity produces 25 kg of carbon dioxide, compared with 15 kg for landfill.
• So landfill produces 10 kg less carbon dioxide than burning.
• Burning produces 0.050 kg of solid residue, compared with 0.070 kg for landfill.
• So landfill produces 0.02 kg more solid residue than burning.
• Burning produces 0.20 kg of sulfur dioxide, compared with 0.30 kg for landfill.
• So landfill produces 50% more sulfur dioxide than burning.

📐 Step-by-step comparison

1. Carbon dioxide: 25 kg vs 15 kg → difference = 10 kg → landfill produces less carbon dioxide.
2. Solid residue: 0.050 kg vs 0.070 kg → difference = 0.020 kg → landfill produces more solid residue.
3. Sulfur dioxide: 0.20 kg vs 0.30 kg → difference = 0.10 kg → landfill produces more sulfur dioxide.
4. Make sure every comparison says which method has more/less.

🧠 Exam technique

• For 4-mark compare questions, do not just list numbers — always say what the numbers show.
• A strong answer often uses the pattern: “A produces more/less than B by…”
• The mark scheme rewards clear, scientific comparisons with the magnitude of the difference.

❌ Common errors and traps

• Just saying “they are different” is not enough for high marks.
• Forgetting to include units loses precision.
• Mixing up which method produces more carbon dioxide or sulfur dioxide.
• Writing “50% more” for solid residue would be wrong — that percentage applies to sulfur dioxide.

💡 Core facts to remember

• Fractional distillation works because fractions have different boiling points.
• Alkanes have the general formula CₙH₂n+2.
• Propane is C₃H₈.
• Methane burns as: CH₄ + 2 O₂ → CO₂ + 2 H₂O.
• Alkenes are tested using bromine water.
• LCA is used to assess environmental impact.

🧠 Last-minute exam advice

• 1-mark questions: give the exact keyword.
• Compare questions: use numbers and say what they mean.
• Balancing equations: check atom counts on both sides.
• If a word is in the mark scheme, learn it exactly.