Which among the following can be explained by adsorption theory?

When acidified $KMnO_4$ is added to hot oxalic acid solution,the decolourization is slow in the beginning,but becomes very rapid after some time. This is because :

When $KMnO_4$ solution is added to hot oxalic acid solution,the decolorization is slow in the beginning but becomes instantaneous after some time. This is because

Which of the following reactions requires a catalyst?

Wilhelm Ostwald redefined the action of

Give four examples of heterogeneous catalysis.