In the oxidation–reduction test, what color change occurs when sulfur dioxide reduces acidified potassium permanganate solution?

• A. Purple turns colourless.
• B. Purple remains purple.
• C. Color changes to green.
• D. Purple turns pink.

Answer: (A)

At the heart of this test is a redox color change. Acidified potassium permanganate is a deep purple oxidizing agent, and sulfur dioxide acts as a reducing agent. When SO2 donates electrons, permanganate is reduced from MnO4− (manganese in +7) to Mn2+ (manganese in +2). In strongly acidic solution, Mn2+ is essentially colorless, so the purple color of the permanganate fades away. That’s why the correct description is purple turning colorless. The other options don’t fit because the permanganate doesn’t stay purple under reacting conditions, and the specific color changes mentioned don’t occur in this acidic redox system.