AQA A-Level Chemistry AS Paper 1, June 2025: Question 6

7 marks · Medium difficulty · Long Answer

Identify two unknown inorganic compounds using test-tube reaction observations, write ionic equations for the reactions, and describe a test for hydroxide ions.

Practise this question

Question

Question text

06.1 A student completes a series of test-tube reactions on two solutions, A and B.

Each solution contains an unknown inorganic compound.

Table 4 shows the observations in these reactions.

Table 4

Solution A Solution B

Test 1

White precipitate

Effervescence

formed

Add H2SO4(aq)

Test 2

White precipitate

Effervescence

formed

Add HNO3(aq) followed by AgNO3(aq)

Test 3

Add NaOH(aq) and warm gently. Red litmus turns

No visible change

blue

Hold a piece of damp red litmus paper

over the end of the test tube.

Show how the observations can be used to identify the inorganic compound in

Solution A and the inorganic compound in Solution B.

Give ionic equations for the reactions that occur.

[6 marks]

06.2 Describe a test to confirm that a solution contains hydroxide ions.

[1 mark]

Mark scheme

Show the mark scheme

Question Marking guidance Additional Comments/Guidelines Mark

This question is marked using levels of response. Refer to the Mark Stage 1: Solution A

Scheme Instructions for Examiners for guidance on how to mark this

question. 2+ 2+

1a: (white ppt in) Test 1, so Ba (Sr )

Level 3: –

1b: (white ppt in) Test 2, so Cl

All stages are covered and the explanation of each stage is 1c: barium chloride / BaCl2 (strontium chloride

generally correct and virtually complete (ie at least two from / SrCl2)

stages 1, 2 and at least three from stage 3).

5-6 Stage 2: Solution B

Answer communicates the whole explanation, including

equations, coherently and shows a logical progression 2a: (effervescence in) Test 1 and/or Test 2, so

through all three stages. 2– –

CO3 / HCO3

+ 6

06.1 Level 2: 2b: (red litmus turns blue in) Test 3, so NH4

2c: ammonium carbonate / (NH4)2CO3 or (6 x AO3)

All stages are covered but the explanation of each stage

may be incomplete or may contain inaccuracies. ammonium hydrogencarbonate / NH4HCO3

OR Stage 3: Equations

2+ 2–

Two stages covered and the explanations are generally 3-4 3a: Ba + SO4 → BaSO4

2+ 2–

correct and virtually complete (ie two from stages 1 and/or 2 (Sr + SO4 → SrSO4)

or three from stage 3). + –

3b: Ag + Cl → AgCl

2– +

3c: CO3 + 2H → CO2 + H2O /

Answer is coherent and shows some progression through all – +– AS CHEMISTRY – 7404/1 –

HCO3 + H → CO2 + H2O

three stages. Some steps in each stage may be + –

3d: NH4 + OH → NH3 + H2O

incomplete.

Level 1:

Two stages are covered but the explanation of each

stage may be incomplete or may contain inaccuracies.

1-2

06.1

Only one stage is covered but the explanation is

(cont) 23

generally correct and virtually complete (i.e. two from

stages 1 and/or 2 or three from stage 3).

Answer shows some progression between two stages.

Level 0

Insufficient correct chemistry to gain a mark.

(Add) universal indicator and it turns blue / purple Add a named soluble magnesium salt and (slight)

6.2 or white precipitate formed

( x AO3)

(Add) red litmus and it turns blue

How to answer it

Identification of Unknown Inorganic Solutions & Ion Tests

📋 What this question tests

This question assesses your practical knowledge of qualitative inorganic analysis (Required Practical 4 / Group 2 and Group 7 chemistry):

Identifying cations (Ba²⁺/Sr²⁺ using sulfate, NH₄⁺ using warm NaOH and litmus).
Identifying anions (halides like Cl⁻ using acidified AgNO₃, carbonates/hydrogencarbonates using acids).
Constructing balanced, accurate ionic equations for precipitation and acid-base neutralization reactions.
Formulating concise practical confirmation tests for hydroxide ions (OH⁻).
Structuring a multi-stage extended response clearly to achieve Level 3 (5–6 marks).

Question 06.1: Identifying Unknowns A & B and Ionic Equations

Extended Response (Level of Response) • 6 Marks

💡 Key Knowledge & Test Logic

Solution A:
- Test 1 (H₂SO₄ added): White ppt indicates an insoluble sulfate → Ba²⁺ (or Sr²⁺ ).
- Test 2 (HNO₃ then AgNO₃ added): White ppt indicates silver chloride → Cl⁻ .
- Identity A: Barium chloride, BaCl₂ (or SrCl₂).
Solution B:
- Tests 1 & 2: Effervescence with acid (H₂SO₄ or HNO₃) indicates release of CO₂ gas → CO₃²⁻ or HCO₃⁻ .
- Test 3 (NaOH, gentle heat): NH₃ gas evolved turns damp red litmus blue → NH₄⁺ .
- Identity B: Ammonium carbonate, (NH₄)₂CO₃ (or NH₄HCO₃).

✅ Model Answer: 3-Stage Structure

Stage 1: Identification of Solution A

Test 1 produces a white precipitate of insoluble sulfate, confirming presence of Ba²⁺ (or Sr²⁺).
Test 2 produces a white precipitate with acidified silver nitrate, confirming presence of Cl⁻.
Therefore, Solution A is barium chloride / BaCl₂ (or strontium chloride / SrCl₂).

Stage 2: Identification of Solution B

Test 1 / Test 2 produces effervescence (bubbles of CO₂), confirming CO₃²⁻ (or HCO₃⁻).
Test 3 turns damp red litmus paper blue due to alkaline NH₃ gas released, confirming NH₄⁺.
Therefore, Solution B is ammonium carbonate / (NH₄)₂CO₃ (or ammonium hydrogencarbonate / NH₄HCO₃).

Stage 3: Ionic Equations

Test 1 with A: Ba²⁺ + SO₄²⁻ → BaSO₄ (or Sr²⁺ + SO₄²⁻ → SrSO₄ )
Test 2 with A: Ag⁺ + Cl⁻ → AgCl
Effervescence in B: CO₃²⁻ + 2H⁺ → CO₂ + H₂O (or HCO₃⁻ + H⁺ → CO₂ + H₂O )
Test 3 with B: NH₄⁺ + OH⁻ → NH₃ + H₂O

🧠 Exam Technique & Mark Scheme Criteria

This is a Levels of Response question scored across 3 stages:

Level 3 (5–6 marks): All 3 stages covered. Must include at least 2 points from Stage 1, 2 points from Stage 2, and at least 3 correct ionic equations from Stage 3. Well organized and logically explained.
Level 2 (3–4 marks): Covers all stages with omissions/minor errors, OR covers two stages completely (e.g. 2 deductions per solution or 3 equations).
Level 1 (1–2 marks): Covers two stages partially or one stage virtually completely.

Examiner Tip: Always group your answer into headings: "Solution A", "Solution B", and "Ionic Equations". This guarantees clear communication and helps the marker quickly tick off stages.

❌ Common Errors to Avoid

Full equations instead of ionic equations: The question specifically asked for ionic equations. Writing full molecular equations (e.g. BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl) will penalise Stage 3.
Unbalanced charges or missing atoms: Forgetting the stoichiometric 2H⁺ when reacting with carbonate: CO₃²⁻ + 2H⁺ → CO₂ + H₂O .
Omitting spectator ion cancellation: Do not include Na⁺ or NO₃⁻ in the silver halide or ammonium equations.
Forgetting to identify the whole compound: Naming only individual ions (e.g., "contains Ba²⁺ and Cl⁻") without concluding that the compound is "barium chloride" (BaCl₂).

Question 06.2: Test for Hydroxide Ions (OH⁻)

Short Answer • 1 Mark

✅ Accepted Answers (Any ONE)

Indicator method 1: Add red litmus paper → turns blue.
Indicator method 2: Add Universal Indicator (solution or paper) → turns blue / purple.
Precipitation method: Add a named soluble magnesium salt (e.g. MgCl₂ / MgSO₄) → a (slight) white precipitate forms (of Mg(OH)₂).

Mark allocation: 1 mark for naming both the reagent and the correct observation.

❌ Common Errors

Giving an incomplete observation (e.g. stating "turns blue" without stating that red litmus was used).
Stating "blue litmus turns red" (that is the test for an acid).
Giving "magnesium" rather than a magnesium salt solution (magnesium metal reacts slowly with water, which is not the standard analytical test).

Topics

Inorganic Chemistry · Required Practicals · 3.2.2 Group 2, The Alkaline Earth Metals · 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, AS Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.