AQA A-Level Chemistry AS Paper 1, June 2023: Question 19
1 mark · Medium difficulty · Multiple Choice
Identify which unknown solution contains carbonate ions based on test-tube observations.
Practise this questionQuestion
Question text
Use this information for Questions 19 to 23.
A student completes some test-tube reactions on five solutions, P, Q, R, S and T.
The student completes each test on separate samples of each solution.
Observations are shown in the table.
Test 1 Test 2 Test 3 Test 4
Solution Add a few drops of Add HNO3(aq) then Add a few drops of Add a few drops of
H2SO4(aq) a few drops of NaOH(aq) Cl2(aq)
AgNO3(aq)
P White precipitate Cream precipitate No visible change Orange solution
Effervescence and
Q Effervescence No visible change No visible change
white precipitate
R No visible change No visible change White precipitate No visible change
S White precipitate No visible change Dark brown solution
T No visible change White precipitate White precipitate No visible change
Use the information in the table to answer Questions 19 to 23.
19 Which solution contains carbonate ions?
[1 mark]
A Solution P
B Solution Q
C Solution R
D Solution S
Mark scheme
Show the mark scheme
19 B 1 (AO3) Solution Q
How to answer it
Qualitative Analysis: Identifying Carbonate Ions
What this question tests
This question assesses practical inorganic analysis (Required Practical 4 / AO3 skills), specifically the deductive identification of unknown inorganic solutions using chemical tests for aqueous anions (carbonate, CO₃²⁻). It requires linking characteristic observations—such as effervescence of CO₂ gas on reaction with dilute acid—to the presence of a specific ion.
Question 19
Identifying the solution containing CO₃²⁻ ions [1 mark]
✅ Correct Answer
B — Solution Q
💡 Key Knowledge: Anion Testing
- Test for Carbonate (CO₃²⁻): Add a dilute acid, such as HCl(aq) or HNO₃(aq).
- Positive observation: Rapid effervescence (bubbling / fizzing) as carbon dioxide gas is released.
- Gas verification: Bubbling the resulting gas through limewater (Ca(OH)₂(aq)) causes it to turn cloudy/milky due to the formation of a CaCO₃ precipitate:
CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
🧠 Exam Technique & Deduction Strategy
- Link observation to deduction: In qualitative analysis problem sets with unknown solutions (P, Q, R, S), always construct a mini grid of observations:
• Acid added → Fizzing = Carbonate (CO₃²⁻)
• Acidified BaCl₂ → White precipitate = Sulfate (SO₄²⁻)
• Acidified AgNO₃ → Precipitate (white/cream/yellow) = Halide (Cl⁻/Br⁻/I⁻) - Elimination: Cross out any solution that does not react with acid or gives only a precipitation reaction with other reagents without gas evolution.
❌ Common Errors & Pitfalls
- Confusing gas evolution: Mistaking the evolution of hydrogen (from acid + metal) or ammonia (from ammonium + warm NaOH) for carbon dioxide.
- Order of testing: In mixed qualitative analysis, testing for sulfate (Ba²⁺) before testing for carbonate can cause false positives because BaCO₃ is also an insoluble white precipitate. Dilute acid must always be added first.
- Vague observations: Recording "gas produced" instead of specific observations like "effervescence" or "fizzing".
Topics
Inorganic Chemistry · Required Practicals · Required Practical 4: Carry out simple test-tube reactions to identify Cations and Anions · 3.2.6 Reactions of Ions in Aqueous Solution
Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.