AQA GCSE Chemistry Chemistry Paper 2 (Higher), June 2025: Question 6

✅ Correct Answers

Any one of the following:

• Calcium (or Ca²⁺ )
• Magnesium (or Mg²⁺ )
• Zinc (or Zn²⁺ )

💡 Key Knowledge

Three common metal cations produce a white precipitate with dilute sodium hydroxide solution:

• Al³⁺ + 3OH⁻ → Al(OH)₃ (s)
• Ca²⁺ + 2OH⁻ → Ca(OH)₂ (s)
• Mg²⁺ + 2OH⁻ → Mg(OH)₂ (s)

✅ Correct Answer

• Test: Add excess sodium hydroxide solution. [1 mark]
• Result: The white precipitate dissolves to form a colourless solution. [1 mark]

🧠 Exam Technique

Aluminium hydroxide is amphoteric, meaning it redissolves in excess alkali. Calcium hydroxide and magnesium hydroxide do not dissolve in excess NaOH.

Always state both the action (adding excess NaOH) and the visible change (precipitate dissolves).

❌ Common Errors

• Saying "add more" without specifying excess.
• Writing "it disappears" rather than dissolves or forms a colourless solution.
• Suggesting a flame test (aluminium has no characteristic flame colour).

✅ Correct Answers

06.3 (Flame test: orange-red flame):

Ca²⁺ [1 mark]

06.4 (Nitric acid + silver nitrate: white precipitate):

Cl⁻ [1 mark]

🧠 Critical Instruction Alert

The questions specifically asked for the symbol for the ion:

• Writing the word calcium or chloride scores 0 marks.
• Writing neutral atomic symbols ( Ca or Cl ) scores 0 marks.
• You must include the correct charges: Ca²⁺ and Cl⁻ .

💡 Halide Test Summary

• Chloride (Cl⁻): White precipitate (AgCl)
• Bromide (Br⁻): Cream precipitate (AgBr)
• Iodide (I⁻): Yellow precipitate (AgI)

Mnemonic: Milk, Cream, Butter (White, Cream, Yellow).

✅ Correct Answer

• Test: Bubble the gas through limewater (calcium hydroxide solution). [1 mark]
• Result: Limewater turns cloudy or milky. [1 mark]

❌ Common Errors

• Forgetting that Mark 2 depends entirely on Mark 1: if the test reagent (limewater) is missing or incorrect, no marks can be awarded for the observation.
• Writing "turns white" instead of cloudy, milky, or forms a white precipitate.

✅ The 4 Mark Marking Points

1. Rate Observation: The rate of reaction is lower with ethanoic acid. [1 mark]
2. Acid Strength: Ethanoic acid is a weak acid (whereas nitric acid is a strong acid). [1 mark]
3. H⁺ Concentration: Ethanoic acid is only partially ionised, so it has a lower concentration of hydrogen ions (H⁺). [1 mark]
4. Collision Frequency: There is a lower frequency of collisions between reacting particles (fewer collisions per second). [1 mark]

🧠 How to Structure a 4-Mark "Explain Rate" Answer

Always build a logical chain of cause-and-effect:

Property → Ion Concentration → Collision Theory → Rate

• State the rate difference clearly.
• Identify which acid is weak vs. strong.
• Connect weakness to partial ionisation / lower [H⁺].
• Connect lower concentration to collision frequency per unit time.

❌ Examiner Pitfalls

• "Less collisions": Never say just "less collisions". Examiners demand lower frequency of collisions or fewer collisions per second / unit time.
• Confusing strength with concentration: Both acids were 1.0 mol/dm³ (same concentration!). The difference is strictly acid strength (degree of ionisation).