AQA A-Level Chemistry Paper 3, June 2018: Question 20

1 mark · Easy difficulty · Multiple Choice

Identify the observation that confirms ammonia gas is released when solid ammonium chloride is warmed with solid calcium hydroxide.

Practise this question

Question

Question 20 asks: 'Which observation would confirm that ammonia gas is released when solid ammonium chloride is warmed with solid calcium hydroxide?' Four multiple choice options are given: A, Damp blue litmus paper turns red when touched onto the solid mixture; B, Damp red litmus paper turns blue when touched onto the solid mixture; C, Damp blue litmus paper turns red when held just above the solid mixture; D, Damp red litmus paper turns blue when held just above the solid mixture.
Question text

20 Which observation would confirm that ammonia gas is released when solid ammonium

chloride is warmed with solid calcium hydroxide?

[1 mark]

A Damp blue litmus paper turns red when touched onto the solid

mixture.

B Damp red litmus paper turns blue when touched onto the solid

mixture.

C Damp blue litmus paper turns red when held just above the solid

mixture.

D Damp red litmus paper turns blue when held just above the solid

mixture.

Mark scheme

Show the mark scheme Mark scheme table shows Question 20 with Marking Guidance stating the correct answer is D.

Question Marking Guidance

20 D

How to answer it

AQA A-Level Chemistry • Inorganic Analysis

Testing for Ammonia Gas and Ammonium Ions

What this question tests

This question assesses practical inorganic analysis skills (Required Practical 4 / Group 2 and ion testing): specifically, the characteristic chemical test used to confirm the evolution of ammonia gas (NH₃) from ammonium compounds, including the distinction between testing the evolved gas versus inadvertently testing the alkaline reaction mixture itself.

Question 20 Analysis

Multiple Choice Question (1 Mark)

✅ Correct Answer

D: Damp red litmus paper turns blue when held just above the solid mixture.

Mark Scheme: Award 1 mark for option D.

💡 Key Knowledge

  • Acid-Base Nature: Ammonia (NH₃) is an alkaline gas. It turns damp red litmus paper blue.
  • Chemical Reaction:
    2NH₄Cl(s) + Ca(OH)₂(s) → CaCl₂(s) + 2H₂O(l) + 2NH₃(g)
  • Why damp? The water allows the gaseous NH₃ to dissolve and form hydroxide ions:
    NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

🧠 Exam Technique & Reasoning

Break the question into two independent decisions:

  1. What colour change shows an alkaline gas?
    Ammonia is alkaline, so it turns red litmus to blue. This immediately eliminates options A and C.
  2. Gas test vs. touching the solid mixture:
    The mixture contains solid calcium hydroxide, Ca(OH)₂, which is already a base. Touching the paper onto the solid (option B) would give a false positive from unreacted Ca(OH)₂. To confirm gas release, the paper must be held above the mixture.

❌ Common Errors & Pitfalls

  • Mixing up litmus colours: Confusing acids (turn blue litmus red) with alkalis (turn red litmus blue).
  • Contaminating test paper: Touching the sides of the boiling tube or the solid reagents with the test paper. This is a classic practical error flagged by examiners.
  • Using dry litmus paper: Gaseous NH₃ itself cannot directly produce OH⁻ ions without water present; the paper must always be damp.

Topics

Inorganic Chemistry · Required Practicals · Required Practical 4: Carry out simple test-tube reactions to identify Cations and Anions

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.