2023 O Level Chem P1 Q27 — Qualitative Analysis Solution

Answer: B. The observation — damp red litmus paper turning blue — is the test for ammonia. Ammonia must be released only on adding aluminium foil to the mixture; before that, no visible reaction occurs.

Aluminium in alkaline solution reduces nitrate ions to ammonia (the Devarda-type test for nitrate): \(3\text{NO}_3^- + 8\text{Al} + 5\text{OH}^- + 18\text{H}_2\text{O} \rightarrow 3\text{NH}_3 + 8[\text{Al(OH)}_4]^-\). So the mixture must be alkaline and contain nitrate — and must not visibly react on its own before Al is added.

A — HCl + Zn — gives \(\text{H}_2\) bubbles (visible reaction). ✗
B — KOH + \(\text{KNO}_3\) — alkaline, contains \(\text{NO}_3^-\), no visible reaction on its own. Al releases \(\text{NH}_3\). ✓
C — \(\text{Na}_2\text{CO}_3\) + \(\text{H}_2\text{SO}_4\) — gives \(\text{CO}_2\) bubbles. ✗
D — \(\text{NH}_4\text{Cl}\) + \(\text{ZnCl}_2\) — no alkali, no nitrate. ✗