Rhubarb sticks contain oxalic acid, which is a reducing agent. When oxalic acid reacts with potassium permanganate solution, it decolorizes the solution. The following chemical reaction takes place:
`2MnO_4^(2-) + 5C_2H_2O_4 + 6H_3O^(+) -> 2Mn^(2+) + 10CO_2 + 14H_2O`
In this reaction, potassium permanganate gets reduced to the Mn2+ ion and this causes the solution to lose its color. The potassium permanganate has a purple color and in the reaction with oxalic acid, it gets decolorized.
The rate of this reaction, that is, how fast the solution loses its color, is dependent on the amount of oxalic acid in the solution. The amount of oxalic acid is varied by either increasing the amount of rhubarb sticks or by increasing the surface area of the rhubarb sticks (for example, instead of one large piece of stick, we can cut it into smaller portions and then add it). A larger surface area ensures easier access to oxalic acid and thus speeds up the reaction.
Hope this helps.
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