2018 O Level Chem P1 Q3 — Qualitative Analysis / Identifying Ions Solution

Answer: C (W = lead(II) nitrate, X = zinc sulfate). Work through the tests for each solution.

W: a white precipitate with dilute \(\text{H}_2\text{SO}_4\) points to an insoluble sulfate (\(\text{Pb}^{2+}\) or \(\text{Ba}^{2+}\)); a white precipitate with aqueous ammonia that is insoluble in excess fits \(\text{Pb}^{2+}\); and warming with \(\text{NaOH}\) and aluminium foil gives a gas that turns red litmus blue (ammonia), the test for nitrate. So W is \(\text{Pb(NO}_3)_2\).

X: a white precipitate with \(\text{Ba(NO}_3)_2\) shows sulfate; a white precipitate with ammonia that dissolves in excess to a colourless solution shows \(\text{Zn}^{2+}\). So X is \(\text{ZnSO}_4\). Answer C.