WJEC A-Level Chemistry Unit 3, June 2025: Question 5

2 marks · Easy difficulty · Short Answer

Identify a metal halide and give reasons based on a lilac flame test and the reaction with concentrated sulfuric acid yielding rotten egg smell and coloured fumes.

Practise this question

Question

Question 5 states: 'A metal halide gives a lilac flame test and the addition of concentrated sulfuric acid causes it to release coloured fumes and a smell of rotten eggs. Identify the metal halide, giving reasons for your answer.' It carries 2 marks.
Question text

5. A metal halide gives a lilac flame test and the addition of concentrated sulfuric acid causes it

to release coloured fumes and a smell of rotten eggs.

Identify the metal halide, giving reasons for your answer. [2]

Mark scheme

Show the mark scheme Mark scheme for Question 5 lists two marking points: 'lilac flame test implies potassium ions' (1 mark), and 'smell of rotten eggs implies iodide ions' (1 mark). A guidance note states: 'award (1) for potassium iodide if no reasons given'. Total marks: 2.

5 lilac flame test potassium ions (1)

smell of rotten eggs iodide ions (1)

22 2

award (1) for potassium iodide if no reasons given

How to answer it

Identifying an Unknown Metal Halide

📌 What This Question Tests

This question assesses your qualitative analysis skills in inorganic chemistry: identifying Group 1 metal cations using flame tests, and identifying halide anions based on their reducing ability when reacted with concentrated sulfuric acid (H₂SO₄).

Question 5 • 2 Marks

Deducing Cation and Anion Identity from Observations

"A metal halide gives a lilac flame test and the addition of concentrated sulfuric acid causes it to release coloured fumes and a smell of rotten eggs. Identify the metal halide, giving reasons for your answer."

✅ Model Answer (Full 2 Marks)

Identity: Potassium iodide (or KI)

Reasons:

  • The lilac flame test confirms the presence of potassium ions (K⁺). [1 mark]
  • The smell of rotten eggs (due to H₂S gas) and purple/dark coloured fumes (I₂) confirms the presence of iodide ions (I⁻). [1 mark]

🧠 Exam Technique & Mark Breakdown

  • Mark 1: Lilac flame test → potassium ions (K⁺).
  • Mark 2: Smell of rotten eggs → iodide ions (I⁻).
  • Special note from mark scheme: Writing just potassium iodide with no reasons earns only 1 mark out of 2. Always provide the explicit link between each test and each ion!

💡 Key Knowledge: Halides + Concentrated H₂SO₄

As you descend Group 7, the halide ions become larger and lose electrons more easily — their reducing ability increases:

Halide Redox Power Key Observations with Conc. H₂SO₄
Chloride (Cl⁻) Not reducing Steamy/misty fumes only (HCl — acid-base reaction, not redox).
Bromide (Br⁻) Moderate reducer Steamy fumes (HBr), orange-brown vapour (Br₂), choking gas (SO₂). Reduces S from +6 to +4.
Iodide (I⁻) Strong reducer Purple vapour/black solid (I₂), yellow solid (S), and smell of rotten eggs (H₂S). Reduces S from +6 to -2.

Flame Tests Recap: Li⁺ = Crimson/Red, Na⁺ = Yellow/Orange, K⁺ = Lilac, Ca²⁺ = Brick-red, Ba²⁺ = Apple-green.

❌ Common Errors & Pitfalls

  • Naming elements instead of ions: Writing "potassium and iodine" instead of "potassium ions" and "iodide ions".
  • Confusing the sulfur reduction products:
    • SO₂ = sharp, choking acidic smell (formed by Br⁻ and I⁻)
    • H₂S = characteristic bad/rotten egg smell (formed only by the strongest reducer, I⁻).
  • Forgetting the compound name: Even though the reasons score the marks, the prompt asks you to identify the metal halide — always state potassium iodide clearly at the start.
Examiner Insight: This standard 2-mark question is designed to be quick. High-scoring candidates write directly and methodically: "The compound is potassium iodide. Lilac flame indicates K⁺ ions. The rotten egg smell is H₂S, which indicates that the halide reduces sulfuric acid to H₂S, confirming I⁻ ions."

Topics

Inorganic Chemistry · Practical · 1.6 The Periodic Table · AS Unit 1 practical work

Question and mark scheme from the WJEC A-Level Chemistry examination, Unit 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.