Edexcel A-Level Chemistry AS Paper 1, June 2024: Question 4

10 marks · Medium difficulty · Short Open Response

Describe the observations for the halide tests of potassium chloride, bromide, and iodide using silver nitrate and dilute nitric acid followed by ammonia solutions, and explain the role and reaction of nitric acid with carbonate impurities.

Practise this question

Question

An exam question with two parts about testing for halide and carbonate ions. Part (a)(i) asks for observations when dilute nitric acid and silver nitrate are added to KCl, KBr, and KI solutions (3 marks). Part (a)(ii) asks how to confirm the halide ions using ammonia solutions (3 marks). Part (b)(i) asks for the observation when silver nitrate is added to carbonate ions without nitric acid (1 mark). Part (b)(ii) asks for the observation when nitric acid is added to carbonate ions (1 mark). Part (b)(iii) asks for the ionic equation with state symbols for the reaction of nitric acid with carbonate ions (2 marks).
Question text

4 A student was provided with three aqueous solutions of potassium chloride,

potassium bromide and potassium iodide.

To identify the halide present, dilute nitric acid and silver nitrate solution were added

to each of the three solutions.

(a) (i) Give the observations that the student would make in each case.

(3)

KCl …

KBr …

KI …

(ii) Describe how the student could confirm the halide ions present in the

products of (a)(i) by adding ammonia solutions.

(3)

(b) Nitric acid removes other ions that would interfere with the halide test,*P76893A0324*

for example carbonate ions.

(i) State the observation when silver nitrate is added to a solution of

carbonate ions in the absence of nitric acid.

(1)

(ii) State the observation when nitric acid is added to a solution of

carbonate ions.

(1)

(iii) Write the ionic equation for the reaction of nitric acid with carbonate ions.

Include state symbols.

(2)

(Total for Question 4 = 10 marks)

Mark scheme

Show the mark scheme A mark scheme table detailing acceptable answers and additional guidance for questions 4(a)(i), 4(a)(ii), 4(b)(i), 4(b)(ii), and 4(b)(iii). It lists specific precipitate colours and solubilities in ammonia for the halide tests, observations for carbonate reactions including effervescence, and the correct ionic equation with state symbols.

Question

Acceptable Answer Additional Guidance Mark

Number

4(a)(i) An answer that makes reference to the following points: (3)

• (KCl forms a) white precipitate (1) Allow ppte / ppt / solid / crystals for precipitate

• (KBr forms an) off-white / cream / pale yellow precipitate (1) Penalise lack of ‘precipitate’ once only

• (KI forms a) yellow precipitate (1) Penalise incorrect initial colour once only

Question

Acceptable Answer Additional Guidance Mark

Number

4(a)(ii) An answer that makes reference to the following points: (3)

• (AgCl) (white) precipitate dissolves in dilute

(and concentrated aqueous) ammonia (1)

• (AgBr) (cream / off-white) precipitate does not dissolve in

dilute aqueous ammonia solution but dissolves in Allow AgBr only dissolves in concentrated

concentrated (aqueous) ammonia (1) ammonia solution

• (AgI) (yellow) precipitate does not dissolve in dilute or

concentrated (aqueous) ammonia (1) Allow AgI does not dissolve

Question

Acceptable Answer Additional Guidance Mark

Number

4(b)(i) An answer that makes reference to the following point: (1)

• white and precipitate forms Allow ppte / ppt / solid / crystals for precipitate

or Allow insoluble for precipitate.

brown and precipitate forms Do not award any additional incorrect observations e.g.

bubbles

Ignore incorrect formulae

Question

Acceptable Answer Additional Guidance Mark

Number

4(b)(ii) An answer that makes reference to the following point: (1)

• bubbles (of carbon dioxide) Accept effervescence / fizzing

Ignore just gas given off which turns limewater cloudy

Do not award any additional incorrect observations e.g.

cloudy solution

Do not award incorrect gas identified

Question

Acceptable Answer Additional Guidance Mark

Number

4(b)(iii) Example of ionic equation (2)

• correct species and balancing (1) 2H+(aq) + CO 2−(aq) → CO (g) + H O(l)

32 2

• state symbols (1) Allow state symbols for near miss equation e.g. non ionic

equation

(Total for Question 4 = 10 marks)

How to answer it

Testing for Halides and Interfering Ions

What this question tests

This question assesses your practical knowledge of qualitative inorganic analysis, specifically the halide test using acidified silver nitrate followed by differential solubility in aqueous ammonia. It also tests your understanding of why acidification is necessary by exploring interfering ions like carbonates, requiring observations and precise ionic equations with state symbols.

Question 4 (a) (i)

Precipitation Reactions of Halides with Silver Nitrate

✅ Correct Answers (3 Marks)

  • KCl: White precipitate (1 mark)
  • KBr: Off-white / cream / pale yellow precipitate (1 mark)
  • KI: Yellow precipitate (1 mark)

💡 Key Knowledge

When silver nitrate ( AgNO₃ ) is added to aqueous halide ions ( X⁻ ), insoluble silver halide precipitates form:

Ag⁺(aq) + X⁻(aq) → AgX(s)

Colour trends down Group 7: White (chloride) → Cream (bromide) → Yellow (iodide).

❌ Common Errors

  • Omitting the word "precipitate" (penalised once across the whole paper).
  • Confusing the cream precipitate of silver bromide with simple white or distinct yellow.
Mark breakdown: 3 marks total (1 mark per correct halide observation).
Question 4 (a) (ii)

Confirming Halides using Ammonia Solutions

✅ Correct Answers (3 Marks)

  • AgCl precipitate: Dissolves in both dilute and concentrated aqueous ammonia. (1 mark)
  • AgBr precipitate: Insoluble in dilute ammonia, but dissolves in concentrated aqueous ammonia. (1 mark)
  • AgI precipitate: Insoluble in both dilute and concentrated aqueous ammonia. (1 mark)

🧠 Exam Technique

Be extremely precise with your phrasing. Use terms like "dissolves" or "soluble" and clearly distinguish between dilute and concentrated ammonia. Vague statements like "it disappears" will lose marks.

Mark breakdown: 3 marks total (1 mark for each precipitate's behaviour with ammonia).
Question 4 (b) (i) & (ii)

Interfering Ions: Carbonates

✅ Correct Answers

  • (b)(i) Without acid: White precipitate forms (when silver nitrate is added to carbonate ions). (1 mark)
  • (b)(ii) With acid: Bubbles / effervescence / fizzing observed (when nitric acid is added to carbonate ions). (1 mark)

💡 Key Knowledge

Silver carbonate ( Ag₂CO₃ ) is insoluble and also forms a white precipitate. Nitric acid is added before silver nitrate to react with and remove carbonate ions, preventing a false positive test for halides.

Mark breakdown: 2 marks total (1 mark for 4bi, 1 mark for 4bii).
Question 4 (b) (iii)

Ionic Equation with State Symbols

✅ Correct Answer (2 Marks)

2H⁺(aq) + CO₃²⁻(aq) → CO₂(g) + H₂O(l)

  • Mark 1: Correct species and balanced stoichiometry.
  • Mark 2: Correct state symbols included for all species.

🧠 Exam Technique

Remember that nitric acid provides hydrogen ions ( H⁺(aq) ) and carbonates are CO₃²⁻(aq) . Carbon dioxide is a gas ( g ) and water is a liquid ( l ).

❌ Common Errors

Including spectator ions (like NO₃⁻ or K⁺ ) when an ionic equation is specifically requested.

Mark breakdown: 2 marks total (1 for correct species/balancing, 1 for state symbols).

Topics

Inorganic Chemistry · Topic 4: Inorganic Chemistry and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.