AQA A-Level Chemistry Paper 3, June 2022: Question 18

1 mark · Easy difficulty · Multiple Choice

State the reason for adding dilute nitric acid prior to silver nitrate when testing for aqueous chloride ions.

Practise this question

Question

Question 18 asks: 'A test for chloride ions in aqueous solution involves adding dilute nitric acid followed by aqueous silver nitrate. What is the reason for adding the nitric acid?' followed by four multiple choice options: A 'To convert AgNO3 into [Ag(NO3)2]-', B 'To decrease the solubility of silver chloride', C 'To increase the pH of the solution', and D 'To prevent the precipitation of other silver compounds'.
Question text

18 A test for chloride ions in aqueous solution involves adding dilute nitric acid followed

by aqueous silver nitrate.

What is the reason for adding the nitric acid?

[1 mark]

A To convert AgNO into [Ag(NO ) ]−

33 2

B To decrease the solubility of silver chloride

C To increase the pH of the solution

D To prevent the precipitation of other silver compounds

Mark scheme

Show the mark scheme Mark scheme table row for question 18 indicates the correct answer is D (AO1), worth 1 mark, with the description 'To prevent the precipitation of other silver compounds'.

18 D (AO1) 1 To prevent the precipitation of other silver compounds

How to answer it

Halide Ion Identification: Role of Nitric Acid

📌 What This Question Tests

This question assesses foundational knowledge of qualitative inorganic analysis (AQA Inorganic Chemistry Section 3.2.3: Group 7 / Halogens). Specifically, it tests your understanding of the reagent sequence used to identify aqueous halide ions and the chemical reason why acidifying the solution with dilute nitric acid is an essential preliminary step.

Question 18 Breakdown

Multiple Choice: Purpose of Dilute Nitric Acid in Halide Testing

✅ Correct Option: D

To prevent the precipitation of other silver compounds

Mark Scheme: Option D [1 mark] (AO1 recall)

The added dilute nitric acid (HNO₃) reacts with and destroys interfering anions such as carbonate (CO₃²⁻) and hydroxide (OH⁻), preventing them from producing insoluble silver precipitates like Ag₂CO₃ or Ag₂O/AgOH that would yield a false positive.

💡 Key Knowledge

  • The Halide Test Sequence:
    1. Add dilute HNO₃(aq)
    2. Add aqueous AgNO₃(aq)
  • Interfering reactions removed by acid:
    2H⁺(aq) + CO₃²⁻(aq) → H₂O(l) + CO₂(g)
    H⁺(aq) + OH⁻(aq) → H₂O(l)
  • What happens without acid:
    Silver carbonate ( Ag₂CO₃ ) forms as an off-white precipitate, masking or mimicking the white precipitate of silver chloride ( AgCl ).

❌ Common Errors & Distractor Analysis

  • Distractor A: Suggests forming a complex anion [Ag(NO₃)₂]⁻ . Nitric acid does not form stable nitrate complexes with silver; silver nitrate dissociates into free ions Ag⁺ and NO₃⁻ .
  • Distractor B: AgCl is already extremely insoluble ( Ksp ≈ 1.8 × 10⁻¹⁰ mol² dm⁻⁶ ). Adding HNO₃ has no common ion effect with AgCl and does not decrease its solubility.
  • Distractor C: Adding an acid provides H⁺ ions, which decreases the pH, not increases it.
  • Alternative Acid Mistake: Examiners often highlight that students mistakenly suggest using HCl or H₂SO₄ instead of HNO₃. Using HCl introduces Cl⁻ ions (giving an immediate false positive), while H₂SO₄ can precipitate insoluble silver sulfate ( Ag₂SO₄ ).

🧠 Exam Technique & Examiner Tips

  • Standard phrase to memorise: Whenever asked "Why is dilute nitric acid added before silver nitrate?" in free-response questions, the standard mark-winning answer is:
    "To react with / remove carbonate ions (or hydroxide ions) that would form other insoluble silver precipitates / give a false positive."
  • Look for the function: The acid acts as a "cleaning" reagent rather than an active participant in precipitate formation.
  • Speed strategy for MCQs: Eliminate C immediately based on basic acid-base chemistry (acids lower pH). Eliminate A as nitrate ions are spectator ions. Between B and D, remember that qualitative tests always guard against interfering ions.

Topics

Inorganic Chemistry · Required Practicals · 3.2.3 Group 7(17), The Halogens · 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 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.