2019 O Level Chem P1 Q19 — Redox / Oxidation and Reduction Solution

Answer: B (adding potassium iodide to X gives a brown colour). Reduction is the gain of electrons, so X is reduced when X gains electrons from something else.

In B, adding aqueous potassium iodide to a solution of X produces a brown colour, which is iodine, I₂, formed when iodide ions are oxidised. For the iodide to be oxidised (lose electrons), X must gain those electrons, so X is reduced. Correct.

In A, X decolourises acidified potassium manganate(VII): the purple manganate(VII) is reduced to colourless Mn²⁺, meaning X donates electrons and is oxidised, not reduced. In C, \(\text{X} \to \text{X}^{2+} + 2\text{e}^-\), X is clearly losing electrons, so it is oxidised. In D, \(\text{XO}^- \to \text{XO}_3^-\), the oxidation number of X rises from +1 to +5, so X is oxidised. Only B shows X being reduced.