AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2022: Question 8

✅ Correct Answer

Complete Figure 10 by adding a C=C double bond between the central carbons, and single bonds to the attached groups:

💡 Key Knowledge

• Addition polymers: Formed when monomers with a double bond ( C=C ) open up to form single bonds ( C–C ) in a long chain.
• Reversing the process: To find the monomer, remove the extension bonds and polymer brackets, and reinstate the C=C double bond between the two carbon atoms in the repeating unit.
• Carbon must always have a total of 4 covalent bonds.

❌ Common Errors

• Leaving continuation/trailing bonds on the sides of the monomer (monomers do not have bracket bonds).
• Drawing five bonds to carbon (e.g., keeping extension bonds and adding a double bond).

🧠 Exam Technique

Always count the lines around each carbon atom before moving on. Each C atom must have exactly 4 lines (bonds) connected to it: 2 from the double bond, 1 to H, and 1 to CH₃.

✅ Correct Answers (Any one)

• Otherwise the copper produced would be impure / contaminated.
• The copper produced would be a mixture / alloy rather than pure metal.
• The polymer insulation would burn or melt during copper smelting (producing toxic fumes).
• Copper and poly(butene) require different recycling methods.

🧠 Exam Technique: Be Specific

Avoid vague statements like "because it ruins the copper". Use precise chemical vocabulary such as impure, contaminated, or state that they have different melting points / recycling processes.

✅ Correct Answer

1. Melt: Heat the scrap copper until it melts [1 mark].
2. Reform: (Re)cast, reshape, mould, or extrude the molten copper into pipes [1 mark].

💡 Key Knowledge

Metals can be recycled repeatedly without losing their properties because they are elements. The physical recycling sequence for scrap metals is always:

Collection → Sorting/Purification → Melting → Casting/Shaping

✅ Correct Reasons (Any two)

• Conserves resources: Conserves finite supplies of copper ores.
• Saves energy: Uses significantly less energy than quarrying and smelting new ore.
• Reduces waste: Produces less mining waste / sends less copper wire to landfill.
• Environmental impact: Reduces quarrying/mining impacts (e.g. habitat destruction, noise, dust) or lowers greenhouse gas (CO₂) emissions.

❌ Common Error: Mentioning Cost

Examiner Warning: The mark scheme explicitly states: "ignore references to cost".

Writing "it is cheaper" scores 0 marks. Sustainability refers to preserving natural resources and protecting the environment for future generations, not financial savings.

✅ Correct Answer

• Solution added: Sodium hydroxide (solution) or NaOH [1 mark]
• Result: Blue precipitate / blue solid [1 mark]

💡 Key Knowledge & Ionic Equation

Copper(II) ions react with hydroxide ions to form insoluble copper(II) hydroxide:

Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)

Always describe the result as a precipitate or solid, not just a "colour change" or "blue solution".

✅ Correct Answer

• Test: Add (dilute hydrochloric acid and) barium chloride solution [1 mark]
  (also accept barium nitrate / BaCl₂)
• Result: White precipitate / white solid [1 mark]

❌ Critical Trap: Acid Choice

Never acidify with sulfuric acid (H₂SO₄)!

The mark scheme notes: "do not accept add sulfuric acid". Adding sulfuric acid introduces sulfate ions into your sample, which would form a white precipitate even if no sulfate was originally present (a false positive!).

Always acidify with dilute hydrochloric acid (HCl) to remove any interfering carbonate ions.

💡 Reaction Equation

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Barium sulfate (BaSO₄) is insoluble in water, which is why it forms an unmistakable bright white precipitate.

🧠 Quick Revision Summary of Ion Tests

• Cu²⁺: NaOH → Blue precipitate
• Fe²⁺: NaOH → Green precipitate
• Fe³⁺: NaOH → Brown precipitate
• SO₄²⁻: Dilute HCl + BaCl₂ → White precipitate