In the sulfur dioxide test, a potassium permanganate solution in acidic conditions changes from purple to colorless. Which gas causes this change?

• A. Sulfate
• B. Sulfite
• C. Nitrate
• D. Chloride

Answer: (B)

Acidified potassium permanganate is a strong oxidizing agent. When a reducing gas is present, it donates electrons to MnO4−, which is reduced from purple MnO4− to colorless Mn2+. In the sulfur dioxide test, SO2 dissolves in water to form sulfurous acid, a good reducing agent. It donates electrons to the permanganate, turning the solution colorless as MnO4− becomes Mn2+, while SO3^2− is oxidized to sulfate. This specific decolorization happens because sulfite is readily oxidized by permanganate under acidic conditions.

Sulfate already exists in a high oxidation state and won’t provide the electrons needed to reduce MnO4− in this setup, so it doesn’t cause the color change. Nitrate and chloride don’t produce the same clear, characteristic decolorization in this test environment, so the sulfurous gas is the one that yields the observed effect.