For gases: Place a piece of filter paper soaked with acidified potassium manganate (VII), KMnO4, at the mouth of the test tube | Filter paper turns from purple to colourless | The manganate (VII) ion, MnO4-, is reduced to manganese (II) ion, Mn2+ (colourless) |
For solutions: Add acidified potassium manganate (VII) to unknown solution | Colour changes from purple to colourless | The manganate (VII) ion, MnO4-, is reduced to manganese (II) ion, Mn2+ (colourless) |