. When KMnO4 solution is added to oxalic acid solution, the decolourisation is slow in the beginning but becomes instantaneous after some time because

The reaction between KMnO4 (Potassium permanganate) and oxalic acid is a redox reaction. In the beginning, the reaction is slow because the reaction mixture is not homogeneous and the contact between the reactants is not sufficient.

Here is the reaction:

2KMnO4 + 5H2C2O4 + 3H2SO4 → K2SO4 + 2MnSO4 + 10CO2 + 8H2O

As the reaction proceeds, the oxalic acid starts to dissolve and the reaction mixture becomes homogeneous, increasing the contact between the reactants. This is why the decolourisation becomes instantaneous after some time.

Moreover, the reaction is catalyzed by the Mn2+ ions produced during the reaction. Initially, the concentration of these ions is low, so the reaction is slow. But as the reaction proceeds, more and more Mn2+ ions are produced, which act as a catalyst and speed up the reaction. This is another reason why the decolourisation becomes instantaneous after some time.