AQA A-Level Chemistry Paper 3, 2019: Question 19

AQA A-Level Chemistry • Inorganic Analysis

Detecting Carbonate Ions in the Presence of Excess Sulfate

What this question tests

This question assesses your understanding of inorganic qualitative tests for anions (specifically CO₃²⁻ and SO₄²⁻ ), reaction observations (gas evolution vs precipitate formation), and the ability to choose a reagent that avoids false positives when interfering ions are present in excess.

Question 19 Analysis

Multiple Choice Question (1 Mark)

✅ Correct Answer: B — dilute H₂SO₄(aq)

Mark Scheme Award: 1 mark for option B.

When dilute sulfuric acid is added to carbonate ions, effervescence is observed as carbon dioxide gas is evolved:

CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)

The excess sulfate ions already present in the mixture do not react with or mask the acid, nor do they produce a gas with H⁺(aq). Thus, effervescence is an unambiguous positive result for carbonate.

💡 Key Knowledge

Carbonate test: Addition of any dilute acid produces effervescence of CO₂(g), which turns limewater milky.
Sulfate test: Addition of acidified BaCl₂(aq) forms a dense white precipitate of BaSO₄(s).
Why Ba²⁺ fails here: Ba²⁺ forms insoluble white precipitates with both sulfate ( BaSO₄ ) and carbonate ( BaCO₃ ). Because sulfate is in excess, a thick white precipitate will always form regardless of carbonate presence.

Evaluating All Options

Option	Reagent	Outcome & Diagnostic Value	Verdict
A	dilute NaOH(aq)	Both sodium carbonate and sodium sulfate are soluble. Hydroxide ions do not react with carbonate or sulfate; no visible change occurs.	❌ Incorrect
B	dilute H₂SO₄(aq)	H⁺ reacts specifically with CO₃²⁻ to give brisk effervescence (CO₂ gas). Excess SO₄²⁻ does not interfere.	✅ Correct
C	BaCl₂(aq)	Forms an immediate white precipitate of BaSO₄ with the excess sulfate ions. It is impossible to tell if BaCO₃ also precipitated.	❌ Incorrect trap
D	NaCl(aq)	All sodium salts and all common chloride salts of these anions are soluble. No visible change occurs.	❌ Incorrect

❌ Common Errors & Misconceptions

Falling into the BaCl₂ reflex trap: Many students associate Group 2 / sulfate chemistry directly with BaCl₂ and choose C without thinking about the fact that BaCO₃ is also an insoluble white precipitate.
Thinking H₂SO₄ "contaminates" the mixture: Students sometimes avoid option B thinking: "We can't add sulfuric acid because sulfate is already in the mixture!" While you would avoid H₂SO₄ if you were trying to test for sulfate, you are testing for carbonate, where the diagnostic observation is bubbles of CO₂.

🧠 Exam Technique: Anion Testing Strategy

Always look for unique observations: Effervescence (gas evolution) is far easier to identify than trying to distinguish between two white precipitates.
Remember the standard sequence for testing anions:
1. Carbonate (CO₃²⁻): Add dilute acid (look for fizzing).
2. Sulfate (SO₄²⁻): Add acidified BaCl₂ (look for white precipitate).
3. Halides (Cl⁻, Br⁻, I⁻): Add acidified AgNO₃ (look for coloured precipitates).
Because carbonate testing only requires H⁺ ions, any strong dilute acid (HCl, HNO₃, or H₂SO₄) works!

Examiner Insight: In questions featuring mixtures of anions, always ask yourself: "What visible change will be masked by the excess ion?" Precipitation reactions are easily obscured by competing precipitates, whereas gas production cannot be masked by soluble spectator ions.