Sulphur dioxide gas can be identified by which reaction with potassium manganate(VII) solution?

• A. Bubbling the gas turns purple manganate(VII) colourless
• B. Bubbling the gas turns the colourless solution purple
• C. Bubbling the gas turns green to yellow
• D. No change occurs

Answer: (A)

Sulphur dioxide acts as a reducing gas. Potassium manganate(VII) solution contains the purple MnO4− ion, a strong oxidizing agent. When SO2 is bubbled through this acidic solution, SO2 donates electrons and reduces MnO4− to Mn2+, which is colorless. Therefore the purple solution loses its color and becomes colorless. This observable decolorization is the distinctive test for SO2.

The other options don’t fit because they describe color changes that don’t occur with this redox pair in this setup, or no change at all.