A diol is a compound containing two hydroxyl -OH groups. On oxidising an alkene with acidified potassium manganate(VII) two -OH groups are added across the double bond. The reaction is an addition and an oxidation. Water in the solution provides two hydrogen atoms and one oxygen atom whilst the potassium manganate(VII) provides the other oxygen atom. During the reaction, the potassium manganate(VII) solution turns from purple to colourless.The equation for the reaction isCH2=CH2 + [O] + H2O → CH2OH-CH2OH