When MnO2 is fused with KOH in the presence of an oxidising agent such as KNO3, a coloured compound is formed. The product and its colour are:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → K2MnO4, dark green
Fusing MnO2 with KOH and an oxidising agent (KNO3, or air) is the alkaline oxidative fusion used in making KMnO4. Manganese is oxidised from +4 to +6, giving potassium manganate:
\( \displaystyle \ce{2MnO2 + 4KOH + O2 -> 2K2MnO4 + 2H2O} \)
Potassium manganate is dark green.
Why the other options are incorrect
Option (2) KMnO4, purple — Incorrect. Permanganate (+7) is formed only in a later step, when the manganate disproportionates in a neutral or acidic solution (or is oxidised electrolytically).
Option (3) Mn2O3, brown — Incorrect. Mn here is +3, lower than +4; an oxidising fusion cannot lower the oxidation state.
Option (4) Mn3O4, black — Incorrect. Its average oxidation state of Mn is +8/3, also lower than +4, so it is not an oxidation product of MnO2.
NCERT (2026-27 edition):
4.4.1 Oxides and Oxoanions of Metals – “Potassium permanganate KMnO4
Potassium permanganate is prepared by fusion of MnO2 with an alkali metal hydroxide and an oxidising agent like KNO3. This produces the dark green K2MnO4 which disproportionates in a neutral or acidic solution to give permanganate.”