A white precipitate was formed when BaCl 2(aq) was added to an aqueous solution of a salt X. The precipitate…

Step 1: Analyze the question Adding BaCl₂ to a solution of salt X gives a white precipitate. That precipitate dissolves in dilute HCl with rapid effervescence (fizzing). We need to find the anion in X.

Step 2: Concept Ba²⁺ forms white precipitates with several anions:

• SO₄²⁻ gives BaSO₄, which is insoluble in dilute HCl. This is the standard test for sulphate.
• SO₃²⁻ gives BaSO₃, which dissolves in dilute HCl, releasing SO₂ gas. SO₂ has a choking smell and turns acidified K₂Cr₂O₇ from orange to green.
• CO₃²⁻ gives BaCO₃, which dissolves in dilute HCl, releasing CO₂ gas. CO₂ is odourless and turns limewater milky.
• NO₃⁻ gives no precipitate, because all nitrates are soluble.

Step 3: Work through the options

• A. SO₄²⁻: BaSO₄ does not dissolve in HCl, so this is ruled out.
• B. SO₃²⁻: BaSO₃ does dissolve in HCl, but it gives SO₂, which is a gas with a pungent smell. Sulphite is a plausible candidate, and the question gives no test to tell the gas apart.
• C. NO₃⁻: There is no precipitate, so this is ruled out.
• D. CO₃²⁻: BaCO₃ is a white precipitate. It dissolves in dilute HCl with rapid effervescence of CO₂:

BaCO₃(s) + 2HCl(aq) → BaCl₂(aq) + H₂O(l) + CO₂(g)

Step 4: Choose between B and D Both carbonate and sulphite fit the observations. The question says "rapid effervescence" and mentions no pungent smell. WASSCE treats this as the classic carbonate test: a white precipitate that dissolves in dilute acid with effervescence.

Answer: D, CO₃²⁻ ions

Exam tip: Remember "BaSO₄ stays, BaCO₃ goes." A white precipitate that persists in acid means sulphate. One that dissolves with fizzing means carbonate (or sulphite, if the gas smells sharp). If the question mentions a choking smell, think sulphite.