AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2024: Question 7

✅ Correct Choices (Tick Two)

• Alloy [1 mark]
• Reinforcement [1 mark]

💡 Key Knowledge

Composites consist of two distinct materials:

• Reinforcement: Fibres or rods providing tensile strength (here: the high carbon steel bars).
• Matrix / Binder: Surrounds and binds the fibres together (here: the concrete).
• Steel is an alloy (a mixture of iron with carbon).

❌ Common Errors

• Ticking Matrix or Binder for the steel bars (confusing reinforcement with the surrounding material).
• Ticking Ore (iron ore is a rock containing iron, not manufactured steel bars).

🧠 Exam Technique

The question says "Tick (✓) two boxes". Always count your ticks before moving on. Giving three ticks loses marks through net marking penalties!

✅ Correct Answers

Give two distinct processes that release a greenhouse gas:

1. Burning of methane (combustion) releases carbon dioxide [1 mark]
2. Thermal decomposition of limestone releases carbon dioxide [1 mark]

💡 Key Knowledge

• Combustion of fuel: CH₄ + 2O₂ → CO₂ + 2H₂O
• Thermal decomposition: CaCO₃ → CaO + CO₂
• Both distinct chemical steps in cement manufacture generate carbon dioxide (CO₂), which is a major greenhouse gas.

❌ Common Errors

• Writing "Methane is a greenhouse gas" — the prompt asks how a gas is released in the process, not just naming a gas.
• Saying "heating the air" without naming burning methane.
• Failing to mention carbon dioxide as the gas released.

🧠 Exam Technique

Look back at the bulleted points provided in the question stem. Each bullet points you to a separate chemical reaction. Link the action directly to the greenhouse gas produced.

✅ Correct Sentence Completion

• The substance added to the limestone is: hydrochloric acid [1 mark]
• The gas produced is identified using: limewater [1 mark]

💡 Key Knowledge

Testing for carbonates (CO₃²⁻):

• Add dilute acid (e.g. HCl) → effervescence / fizzing occurs:
  CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
• Bubble the resulting gas through limewater (calcium hydroxide solution):
  Turns from clear and colourless to cloudy / milky.

❌ Common Errors

• Selecting barium chloride (which tests for sulfate ions, SO₄²⁻).
• Selecting sodium hydroxide (used to test for metal cations such as Cu²⁺, Fe²⁺, Fe³⁺).
• Writing "universal indicator" to identify the gas.

🧠 Exam Technique

Always select terms directly from the box provided. Do not alter the spelling or try to substitute your own reagents (e.g. do not write "sulfuric acid" when "hydrochloric acid" is in the box).

✅ Model Answer [2 marks]

Pre-stressed concrete is much stronger (3000 vs 600 units) [1 mark], so it can bear the heavy weight of traffic / lorries without cracking or collapsing [1 mark].

❌ Common Errors & Misconceptions

• Trap: Explaining based on density ("because it is denser"). The mark scheme states explicitly: Do NOT accept because pre-stressed concrete is more dense. High density adds dead weight; it is the strength that supports traffic.
• Quoting figures without linking them to the function (e.g. just writing "strength is 3000" without explaining that it withstands heavy loads).

✅ Correct Reasons (Any Two)

• Plain concrete slabs are cheaper (£75 vs £225 per m³) [1 mark]
• Plain concrete is lighter / less dense (2300 vs 2500 kg/m³), making slabs easier to transport or lay [1 mark]
• Garden paths do not need to carry heavy loads / vehicles (they only hold pedestrians) [1 mark]

🧠 Exam Technique

Questions asking to justify choosing a "weaker" or "simpler" material almost always come down to cost efficiency and fit for purpose:

• Don't pay for high strength if people are only walking on it!
• Lower density means less manual handling effort for gardeners.