Final answer:
The addition of sodium sulphite to KMnO4 under alkaline conditions causes a color change from purple to brown due to the reduction of KMnO4 to MnO2.
Step-by-step explanation:
When sodium sulphite is added to KMnO4 under alkaline conditions, the color change observed is due to the reduction of the purple KMnO4 to brown MnO2. This reaction can be represented as:
2MnO4¯ + 5SO3²¯ + 6OH¯ → 2MnO2 + 5SO4²¯ + 3H2O
The sodium sulphite (SO3²¯) acts as a reducing agent, causing the KMnO4 to be reduced to MnO2. The alkaline conditions provided by the OH¯ ions in the reaction allow for this reduction to occur. The change in color from purple to brown is a visual indicator of the reduction reaction taking place.