AQA A-Level Chemistry Paper 3, 2024: Question 22

Carbonate + Al³⁺ in water: predict precipitate and gas

What this question tests 1 mark • AO2

Recognising that CO₃²⁻ behaves as a base in water and can produce CO₂ with acidic metal aquo ions.
Knowing Al³⁺ exists as an acidic hexaaqua complex in solution and forms Al(H₂O)₃(OH)₃(s) on reaction with base.
Applying “what actually happens in aqueous solution” rather than simple double displacement rules.

Part (a) (1 mark): Formulas of the precipitate and the gas formed

Mark scheme answer: Option D (1 mark, AO2): Al(H₂O)₃(OH)₃ and CO₂

✅ Correct answers (what to write)

Precipitate: Al(H₂O)₃(OH)₃(s)

Gas: CO₂(g)

How the mark is awarded: This is a 1-mark MCQ. You must select the option that contains both correct formulas.

💡 Key knowledge (why these are formed)

Al³⁺ in water is acidic: it exists as an aquo ion (often written as [Al(H₂O)₆]³⁺) which can donate H⁺ (via hydrolysis).
Carbonate is a base: CO₃²⁻ removes H⁺ in aqueous reactions, producing H₂CO₃ which decomposes to CO₂ + H₂O.
Aluminium “carbonate” doesn’t persist in water: instead of forming Al₂(CO₃)₃(s), the chemistry is dominated by hydrolysis → aluminium hydroxide/hydroxide-aquo precipitate.
The precipitate is commonly described as Al(OH)₃(s), but the exam expects the hydrated form: Al(H₂O)₃(OH)₃ .

🧠 Exam technique (how to spot it fast)

When you see CO₃²⁻ plus a small, highly charged metal ion (Al³⁺, Fe³⁺), think: CO₂ gas is likely (carbonate acting as base against an acidic aquo ion).
For Al³⁺ in water, the precipitate in “base added” conditions is an aluminium hydroxide-type solid (often written with coordinated water).
In MCQs, eliminate options containing SO₂ immediately unless you have a sulfite/reducing agent or acidified sulfite—sulfate does not give SO₂ here.

❌ Common errors (what loses marks)

Choosing Al₂(CO₃)₃: treating it as a simple precipitation by ion exchange. In aqueous solution, Al³⁺ hydrolyses and carbonate is protonated → CO₂ + Al(OH)₃-type solid.
Picking SO₂: confusing sulfate (SO₄²⁻) with sulfite (SO₃²⁻). Sulfate is not readily converted to SO₂ under these conditions.
Writing only “Al(OH)₃”: chemically acceptable in many contexts, but this MCQ’s correct option uses Al(H₂O)₃(OH)₃ . In non-MCQ questions, check what form the mark scheme expects.

📐 Calculations / equations you can use (no maths needed, but logic matters)

Acidic aquo ion hydrolysis (concept): [Al(H₂O)₆]³⁺ behaves as an acid (produces H⁺ in water).
Carbonate + acid → CO₂: CO₃²⁻ + 2H⁺ → CO₂ + H₂O
Formation of hydroxide-type precipitate: Al³⁺ + 3OH⁻ (+ 3H₂O) → Al(H₂O)₃(OH)₃(s)

Examiner insight (what distinguishes top responses): The best answers treat this as a reaction in water, recognising hydrolysis and acid–base behaviour, not just swapping ions.

Quick self-check

💡 One-sentence memory hook

Al³⁺ is acidic in water → carbonate gets protonated giving CO₂, and aluminium ends up as an Al(OH)₃ / Al(H₂O)₃(OH)₃ precipitate.

🧠 MCQ elimination checklist

Any option with SO₂ here? Cross it out.
Any option claiming stable Al₂(CO₃)₃ in water? Usually wrong for Al³⁺.
Left with: hydroxide-type solid + CO₂.