$A$ catalyst is a substance that ......

$A$ catalyst is a substance which

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 :

How is Raney nickel,which is used as a catalyst,prepared?

In the oxidation of oxalic acid by $KMnO_4$,the color of $KMnO_4$ disappears slowly at the beginning of the reaction,but then it disappears very rapidly. This is an example of .......

Catalysts in the following reactions are:
$(I) CH_3COOCH_{3(l)} + H_2O_{(l)} \rightarrow CH_3COOH_{(aq)} + CH_3OH_{(l)}$
$(II) 2SO_{2(g)} + O_{2(g)} \rightarrow 2SO_{3(g)}$ (Contact Process)
$(III) 2SO_{2(g)} + O_{2(g)} \rightarrow 2SO_{3(g)}$ (Lead Chamber Process)
$(IV) N_{2(g)} + 3H_{2(g)} \rightarrow 2NH_{3(g)}$ (Haber Process)