AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2020: Question 1

✅ Correct answer

C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O

💡 Key knowledge

• Complete combustion of a hydrocarbon makes carbon dioxide and water.
• Propane is C₃H₈ .
• Balance atoms on both sides: 3 carbons, 8 hydrogens, then oxygen last.

🧠 Exam technique

• Write the products first: 3 CO₂ and 4 H₂O .
• Then count oxygen atoms on the right: 3×2 + 4×1 = 10 oxygen atoms.
• So you need 5 O₂ on the left.

❌ Common errors

• Writing 4 O₂ or 6 O₂ instead of 5 O₂ .
• Forgetting water is a product of combustion.
• Using wrong formulae, e.g. CO instead of CO₂ .

✅ Correct answer

Test: Bubble the gas through lime water (calcium hydroxide solution).

Result: Lime water turns milky/cloudy/white, or a white precipitate forms.

💡 Key knowledge

• Lime water is the standard test for carbon dioxide.
• Accept: calcium hydroxide solution .
• The important result is a milky/cloudy white appearance.

🧠 Exam technique

• Always give both parts: how to test and what happens.
• Use the exam wording: “bubble the gas through lime water”.

❌ Common errors

• Only saying “lime water” without saying what to do with it.
• Saying “clear” or “transparent” instead of milky/cloudy.
• Confusing carbon dioxide with hydrogen or oxygen tests.

✅ Correct answer

C₃H₈ → C₃H₆ + H₂

💡 Key knowledge

• Cracking breaks a larger hydrocarbon into smaller molecules.
• Propene is an alkene with formula C₃H₆ .
• The products given are propene and hydrogen.

🧠 Exam technique

• Check atoms are conserved: 3 carbon atoms and 8 hydrogen atoms on both sides.
• C₃H₆ + H₂ gives 6 + 2 hydrogens = 8 hydrogens total.

❌ Common errors

• Writing C₃H₈ → C₃H₈ + H₂ instead of propene.
• Forgetting that propene has one fewer pair of hydrogen atoms than propane.
• Using the name only, with no formula.

✅ Correct answer

Test: Bring a burning splint to the gas.

Result: Hydrogen burns with a squeaky pop.

💡 Key knowledge

• Hydrogen is tested using a lit splint.
• The sound is often described as a squeaky pop.
• Accept “pop” or “pops” if the rest is clear.

🧠 Exam technique

• Say “lit splint” or “burning splint” — both are accepted.
• Keep the result short and exact: “burns with a pop”.

❌ Common errors

• Using a glowing splint — that is for oxygen, not hydrogen.
• Writing only “it burns” without the pop sound.
• Mixing up the tests for oxygen and hydrogen.

✅ Correct answer

Test: Add bromine water to the alkene.

Colour change: orange → colourless

💡 Key knowledge

• Alkenes are unsaturated hydrocarbons and react with bromine water.
• Bromine water starts off orange.
• If an alkene is present, it becomes colourless.

🧠 Exam technique

• Use the exact reagent name: bromine water.
• Write the colour change in order: orange to colourless.
• If you only say “decolourised”, you may not get all marks unless the colour change is clear.

❌ Common errors

• Saying bromide instead of bromine water.
• Giving the wrong colour, such as blue or green.
• Writing “clear” instead of colourless — the mark scheme specifically prefers colourless.

💡 Mark pattern

• 01.1 = 1 mark for the balanced equation.
• 01.2 = 2 marks: method + result.
• 01.3 = 1 mark for the correct formula of propene.
• 01.4 = 2 marks: test + squeaky pop result.
• 01.5 = 3 marks: reagent + starting colour + ending colour.

🧠 Top-level response tips

• Use precise scientific vocabulary.
• Give both the action and the observation for gas tests.
• Check formulae carefully in symbol equations.
• For colour change questions, state both colours clearly.

❌ Big traps across the question

• Mixing up test reagents: lime water vs bromine water.
• Mixing up gas test results: pop vs milky.
• Forgetting that the combustion equation must be balanced.

✅ Memorise these

• Propane combustion: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
• Carbon dioxide test: lime water turns milky
• Hydrogen test: lit splint gives a squeaky pop
• Alkene test: bromine water orange → colourless
• Propene formula: C₃H₆