Edexcel A-Level Chemistry Paper 3, June 2025: Question 3

6 marks · Medium difficulty · Extended Writing

Describe qualitative tests, observations, and deductions using only the provided reagents to identify ammonium carbonate, potassium carbonate, and potassium sulfate solutions.

Practise this question

Question

Question 3 asks students to describe qualitative tests to positively identify either an anion or cation in three unlabelled bottles containing ammonium carbonate (NH4)2CO3(aq), potassium carbonate K2CO3(aq), and potassium sulfate K2SO4(aq). A list of available resources is provided: test tubes, a water bath, and solutions of AgNO3, BaCl2, Ca(OH)2, CuSO4, HCl, HNO3, MgCl2, NaOH, along with red and blue litmus paper. The response requires describing the qualitative tests carried out, expected observations, and appropriate deductions for 6 marks.
Question text

A student carries out a series of qualitative tests to identify the contents of three

unlabelled bottles. The bottles are known to contain the following solutions.

• ammonium carbonate, (NH4)2CO3(aq)

• potassium carbonate, K2CO3(aq)

• potassium sulfate, K2SO4(aq)

The student has access to just test tubes and a water bath, plus only the

reagents shown.

AgNO3(aq) BaCl2(aq) Ca(OH)2(aq) CuSO4(aq)

HCl(aq) HNO3(aq) MgCl2(aq) NaOH(aq)

red litmus paper blue litmus paper

The student carries out qualitative tests to positively identify either an anion or a

cation in each compound and so identify the contents of each bottle.

Describe the qualitative tests carried out, the expected observations and

appropriate deductions.

(6)

… *P77834A0836*

(Total for Question 3 = 6 marks)

Mark scheme

Show the mark scheme Mark scheme for Question 3 detailing a 6-mark level of response grid. Indicative points include: IP1 for testing carbonate by adding HNO3(aq) or HCl(aq) to samples; IP2 for observing effervescence to identify carbonate ions in (NH4)2CO3 or K2CO3; IP3 for testing ammonium ions by adding NaOH(aq) and gently heating in a water bath; IP4 for observing gas that turns damp red litmus paper blue to confirm (NH4)2CO3; IP5 for testing sulfate by adding BaCl2 and HCl or HNO3; and IP6 for observing a white precipitate confirming K2SO4.

Question

Answer Additional Guidance Mark

Number

This question assesses a student’s ability to show a Guidance on how the mark scheme should be applied: (6)

coherent and logically structured answer with linkages and fully-

sustained reasoning. The mark for indicative content should be added to the

Marks are awarded for indicative content and for how the answer mark for lines of reasoning.

is structured and shows lines of reasoning.

For example, an answer with five indicative marking

The following table shows how the marks should be points, which is partially structured with some linkages

awarded for indicative content. and lines of reasoning, scores 4 marks (3 marks for

indicative content and 1 mark for partial structure and

Number of indicative marking Number of marks awarded for some linkages and lines of reasoning).

points seen in answer indicative marking points

64 If there are no linkages between points, the same five

5–4 3 indicative marking points would yield an overall score of 3

3–2 2 marks (3 marks for indicative content and no marks for

11 linkages).

In general it would be expected that 5 or 6 indicative

The following table shows how the marks should be awarded for points would get 2 reasoning marks, and 3 or 4 indicative

structure and lines of reasoning. points would get 1 mark for reasoning, and 0, 1 or 2

indicative points would score zero marks for reasoning.

Number of marks

awarded for structure of If there is any incorrect chemistry, deduct mark(s) from the

answer and sustained line reasoning. If no reasoning mark(s) awarded do not deduct

of reasoning mark(s).

Answer shows a coherent and logical

structure with linkages and fully

sustained lines of reasoning

demonstrated throughout.

Answer is partially structured with

some linkages and lines of reasoning.

Answer has no linkages between

points and is unstructured.

Question

Answer Additional Guidance Mark

Number

Indicative content: Only listed reagents can be used. (6)

contd Ignore tests carried out to identify potassium

ions / flame tests

Ignore minor slips e.g. missing charges on ions,

wrong formulae of reagents, missing state

symbols that have been listed in the question.

Ignore flame tests

Ignore equations even if incorrect

For all tests listed the reagent and the ion test and

• IP1 Carbonate (test) linked to the result must be dependent on the correct

Add HNO3(aq) / HCl(aq) (to all 3 samples) reaction except for the limited exceptions that

are listed in the additional guidance.

• IP2 Carbonate result Ignore just ‘produce a gas’

Solutions that effervesce / bubble / fizz Allow gas turns Ca(OH)2(aq) cloudy

(must be a carbonate) Ignore gas turns limewater cloudy

/ either (NH4)2CO3 or K2CO3 Do not award subsequent inappropriate gas tests

e.g. pop test

• IP3 Ammonium ion test linked to

Add NaOH(aq) gently heat / warm / put in water bath Do not award Bunsen burner for heating

• IP4 Ammonium ion result Do not award dip red litmus paper into the

(produces a gas) turns solution

(damp) red litmus blue (when held over Do not award bleaches litmus.

the mouth of the test tube) proves (NH4)2CO3 Allow a near miss for IP4 if the ammonium

carbonate solution has been heated without

NaOH being added / adding sodium hydroxide

IP5 and IP6 on next page. without heating

Ignore white smoke seen with a glass rod

dipped in HCl(aq) and held above the tube

Indicative content:

contd

• IP5 Sulfate ion test (linked to)

Add BaCl2 and HCl / HNO3 (in either order) Do not award H2SO4

• IP6 Sulfate ion result Allow if acid is missing from IP5

Formation of white ppt / solid confirms it is Do not award if BaCl2 is missing

K2SO4 Allow goes milky / cloudy

(Total for Question 3 = 6 marks)

How to answer it

Identifying Unknown Ionic Solutions: (NH₄)₂CO₃, K₂CO₃, and K₂SO₄

📋 What this question tests

This 6-mark extended response question tests your practical qualitative analysis skills and logical deductive reasoning under strict constraints:

  • Inorganic Qualitative Tests: Testing for carbonate ions (CO₃²⁻), sulfate ions (SO₄²⁻), and ammonium ions (NH₄⁺).
  • Adhering to Constraints: Using only the reagents and apparatus specified in the prompt (e.g. water bath instead of a Bunsen burner, no flame test wire).
  • Lines of Reasoning: Linking reagents, precise observations (effervescence, precipitates, litmus colour changes), and clear chemical deductions for all three unknown bottles.

Question Structure & Assessment Strategy

6 Marks: Level of Response (4 Marks Indicative Content + 2 Marks Reasoning)

💡 Indicative Marking Points (IP1 – IP6)

  • IP1 (Carbonate Test): Add HNO₃(aq) or HCl(aq) to samples.
  • IP2 (Carbonate Result): Effervescence / bubbling confirms carbonate; identifies (NH₄)₂CO₃ or K₂CO₃.
  • IP3 (Ammonium Test): Add NaOH(aq) and warm gently in a water bath.
  • IP4 (Ammonium Result): Pungent gas evolved turns damp red litmus paper blue when held over the mouth of the test tube; confirms (NH₄)₂CO₃ (and thus distinguishes it from K₂CO₃).
  • IP5 (Sulfate Test): Add acidified BaCl₂(aq) (with HCl or HNO₃).
  • IP6 (Sulfate Result): White precipitate formed confirms K₂SO₄.

🧠 How to Score the 2 "Structure & Reasoning" Marks

  • Sustained Logic: Structure your answer test-by-test. Never state just an observation without stating the conclusion/identity it leads to.
  • Elimination: Identifying (NH₄)₂CO₃ via the ammonium test automatically leaves the other effervescing solution as K₂CO₃.
  • Zero Contradictions: Any incorrect chemical theory or invalid reagent voids reasoning marks.
Mark Allocation Guide:
• 6 indicative points + sustained logical structure = 6/6
• 5–6 indicative points + partial structure = 5/6 (or 4/6 if no linking structure)
• 3–4 indicative points + partial structure = 3/6

Complete Model Answer & Deduction Plan

Systematic Flow to Positively Identify All Three Bottles

✅ Test 1: Testing for the Carbonate Ion, CO₃²⁻ (IP1 & IP2)

Procedure: Take a small portion of each unknown solution in separate test tubes. Add dilute nitric acid, HNO₃(aq) [or dilute hydrochloric acid, HCl(aq)].

Observations:

  • Two solutions produce effervescence (bubbles of colourless gas).
  • One solution produces no reaction (no effervescence).

Deductions: The two solutions producing bubbles contain carbonate ions and are (NH₄)₂CO₃(aq) and K₂CO₃(aq). The unreactive solution must be K₂SO₄(aq).

Ionic equation: 2H⁺(aq) + CO₃²⁻(aq) → H₂O(l) + CO₂(g)

✅ Test 2: Testing for the Ammonium Ion, NH₄⁺ (IP3 & IP4)

Procedure: To fresh samples of the two carbonate solutions, add aqueous sodium hydroxide, NaOH(aq). Place the test tubes into a warm water bath. Hold a piece of damp red litmus paper over the mouth of each test tube.

Observations:

  • One solution releases a pungent alkaline gas that turns damp red litmus paper blue.
  • The other solution shows no change to the litmus paper.

Deductions: The solution evolving ammonia gas is positively identified as (NH₄)₂CO₃. Consequently, the other carbonate solution is positively identified as K₂CO₃.

Ionic equation: NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

✅ Test 3: Confirming the Sulfate Ion, SO₄²⁻ (IP5 & IP6)

Procedure: To a fresh sample of the remaining bottle (or all three), add dilute hydrochloric acid, HCl(aq) [or HNO₃(aq)], followed by aqueous barium chloride, BaCl₂(aq).

Observations: A dense white precipitate forms.

Deductions: The formation of barium sulfate (BaSO₄) positively confirms the presence of sulfate ions, confirming the bottle contains K₂SO₄.

Ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Examiner Insight: Pitfalls & Mistakes to Avoid

❌ Common Errors (Lost Marks)

  • Adding sulfuric acid: Acidifying the sulfate test with H₂SO₄ introduces sulfate ions from the reagent, creating a false-positive white precipitate!
  • Inventing equipment: Suggesting a Bunsen burner or a flame test wire. The question explicitly states you have access to "just test tubes and a water bath, plus only the reagents shown". Mentioning flame tests is strictly ignored or penalised.
  • Dipping litmus paper: Stating to "dip the litmus paper into the solution" instead of holding damp litmus paper in the gas at the mouth of the tube. Dipping it into NaOH directly tests the reagent, not the evolved NH₃ gas!
  • Vague gas descriptions: Stating merely "a gas is given off" instead of specifically describing effervescence, fizzing, or bubbles.

🧠 Exam Technique Checklist

  • Damp paper is essential: Always state damp red litmus paper. Ammonia gas (NH₃) must dissolve in moisture on the paper to form OH⁻ ions and turn the indicator blue.
  • State the acid for BaCl₂: Always acidify with HCl or HNO₃ before or with BaCl₂ to eliminate false positives from any remaining carbonate ions (which would form insoluble BaCO₃).
  • Explicit link to bottle identity: Ensure you finish by linking each result directly to the name/formula of the bottle: (NH₄)₂CO₃, K₂CO₃, and K₂SO₄.

Topics

Inorganic Chemistry · Core Practicals · Topic 15: Transition Metals · Core Practical 7: Identify unknown organic liquids and inorganic solids · Core Practical 15: Analyse organic and inorganic unknowns

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