AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2021: Question 7

✅ Correct Answer

Test: Flame test (or dip a nichrome wire loop into the solution and place into a blue Bunsen flame) [1 mark]

Result: Lilac flame [1 mark]

💡 Key Knowledge

Look at the formula: KAl(SO₄)₂ . The Group 1 metal is potassium ( K⁺ ).

• Lithium (Li⁺) = Crimson
• Sodium (Na⁺) = Yellow
• Potassium (K⁺) = Lilac
• Calcium (Ca²⁺) = Orange-red
• Copper (Cu²⁺) = Green

❌ Common Errors

• Writing "purple" or "violet" instead of the standard GCSE specification term lilac.
• Suggesting chemical precipitate tests (like NaOH) which do not form a precipitate with potassium ions.

✅ Correct Answer

Flame emission spectroscopy [1 mark]

💡 Key Knowledge

Flame emission spectroscopy is an instrumental method used to:

• Identify metal ions by matching their unique line spectra (qualitative).
• Determine concentration by measuring the intensity/absorbance of the light emitted (quantitative).

🧠 Exam Technique

When an exam question mentions an instrumental method that finds both identity and concentration of a metal ion, the required answer in AQA GCSE Chemistry is always flame emission spectroscopy.

✅ Correct Answer

Result: White precipitate [1 mark]

💡 Key Knowledge

The "other metal ion" in KAl(SO₄)₂ is aluminium ( Al³⁺ ).

When sodium hydroxide solution ( NaOH ) is added dropwise, aluminium forms insoluble aluminium hydroxide:

Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s)

This forms as a white precipitate.

🧠 Exam Technique

The question says "until a change is seen" (meaning added dropwise, not yet in excess). Therefore, the correct observation is simply the appearance of the white solid.

✅ Correct Answer

Additional step: Add excess sodium hydroxide solution (or add more sodium hydroxide solution) [1 mark]

Result: Precipitate dissolves / redissolves (forming a colourless solution) [1 mark]

💡 Key Knowledge

Three metal ions form white precipitates with NaOH: Al³⁺, Ca²⁺, and Mg²⁺.

• Al³⁺: White precipitate forms, which dissolves in excess NaOH.
• Ca²⁺: White precipitate forms, does not dissolve in excess.
• Mg²⁺: White precipitate forms, does not dissolve in excess.

❌ Common Errors

• Saying "the solution turns clear" instead of stating that the precipitate dissolves. A coloured solution can be clear; "dissolves" describes the phase change correctly.
• Forgetting the word "excess" and just saying "add sodium hydroxide again".

✅ Correct Answer

Test:

1. Add (dilute) hydrochloric acid [1 mark]

2. Add barium chloride solution [1 mark]

Result: White precipitate [1 mark]

💡 Key Knowledge

The chemical reaction produces insoluble barium sulfate:

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

The dilute hydrochloric acid is added first to remove any carbonate ions ( CO₃²⁻ ), which would otherwise produce a false positive white precipitate of barium carbonate.

❌ Common Errors & Pitfalls

• Wrong acid: Adding sulfuric acid ( H₂SO₄ ) introduces sulfate ions into the sample and ruins the test!
• Missing the result mark: The mark for "white precipitate" depends on mentioning barium chloride (or barium nitrate). You cannot get the result mark if you use the wrong reagent.