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2020 O Level Chem P1 Q18 — Redox / Oxidation and Reduction Solution

Redox / Oxidation and Reduction 2020 O Level Pure Chemistry · Paper 1 · Q18

Question

Colour changes are often observed when redox reactions occur. Which colour change is due to the reduction of the named reagent?

ABlack solid copper(II) oxide changes to blue aqueous copper(II) sulfate.
BColourless aqueous potassium iodide changes to become a brown solution.
CGreen aqueous iron(II) chloride changes to yellow/brown aqueous iron(III) chloride.
DPurple acidified potassium manganate(VII) changes to become colourless.

Correct answer: D

Video Solution

Explanation

Answer: D (purple acidified potassium manganate(VII) turning colourless). Reduction means a gain of electrons, shown by a decrease in oxidation state of the named reagent. In option D, the manganese in the manganate(VII) ion, MnO₄⁻, changes from +7 to +2 in the colourless Mn²⁺ ion, so the acidified KMnO₄ changes from purple to colourless as it is reduced. That is the reduction of the named reagent.

In option B, colourless aqueous potassium iodide turns brown as iodide ions, I⁻ (−1), are oxidised to iodine, I₂ (0) — oxidation, not reduction. In option C, green iron(II) chloride turns yellow-brown iron(III) chloride as Fe²⁺ is oxidised to Fe³⁺. In option A, copper stays at +2 throughout (oxide to sulfate), so that is not a redox change at all. The answer is D.

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