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2019 O Level Chem P1 Q21 — Qualitative Analysis / Identifying Ions Solution

Qualitative Analysis / Identifying Ions 2019 O Level Pure Chemistry · Paper 1 · Q21

Question

Y is a white solid mixture that has the following properties. Y dissolves in water. When dilute hydrochloric acid is added to an aqueous solution of Y, bubbles are produced. When chlorine is bubbled through an aqueous solution of Y the solution turns red-brown. What is present in Y?

Acalcium carbonate, CaCO₃, and potassium bromide, KBr
Bcopper(II) carbonate, CuCO₃, and magnesium iodide, MgI₂
Csodium carbonate, Na₂CO₃, and sodium bromide, NaBr
Dsodium nitrate, NaNO₃, and ammonium bromide, NH₄Br

Correct answer: C

Video Solution

Explanation

Answer: C (sodium carbonate, Na₂CO₃, and sodium bromide, NaBr). Work through the three clues.

Y dissolves in water, so any insoluble compound is ruled out: calcium carbonate and copper(II) carbonate are both insoluble, eliminating options A and B. With dilute hydrochloric acid, Y gives bubbles (effervescence), which is carbon dioxide from a carbonate: \(\text{CO}_3^{2-} + 2\text{H}^+ \to \text{CO}_2 + \text{H}_2\text{O}\). So Y must contain a carbonate, which rules out the nitrate/ammonium bromide option D.

When chlorine is bubbled through aqueous Y the solution turns red-brown, which is bromine, Br₂, formed because chlorine displaces bromide: \(\text{Cl}_2 + 2\text{Br}^- \to 2\text{Cl}^- + \text{Br}_2\). So Y also contains bromide. The only option with a soluble carbonate and a soluble bromide is sodium carbonate with sodium bromide, option C.

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