Answer: D. A redox reaction involves changes in oxidation state — one species is oxidised, another reduced.
1: \(\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}\) is acid–alkali neutralisation. No oxidation states change. Not redox.
2: \(\text{CuSO}_4 + \text{Zn} \rightarrow \text{Cu} + \text{ZnSO}_4\). Zn → \(\text{Zn}^{2+}\) (oxidised, 0 to +2); \(\text{Cu}^{2+}\) → Cu (reduced, +2 to 0). Redox.
3: \(\text{Pb}^{2+} + 2\text{Cl}^{-} \rightarrow \text{PbCl}_2\). Precipitation only — no oxidation state change. Not redox.
4: \(\text{Fe} + 2\text{HCl} \rightarrow \text{FeCl}_2 + \text{H}_2\). Fe → \(\text{Fe}^{2+}\) (oxidised); \(\text{H}^{+}\) → \(\text{H}_2\) (reduced). Redox.
Reactions 2 and 4 → answer D.
Every O Level Pure Chemistry TYS MCQ solution video, plus full Sec 3 / Sec 4 topic recordings.
Find out more