Question

Why is potassium stored under paraffin

Answer 1

Potassium is the most reactive metal on this earth. It reacts very explosively with both water and air that is why it is stored in kerosene or paraffin instead. ... Since potassiumis highly reactive in nature, it reacts vigorously even with air (oxygen).

Answer 2

Potassium can be  also stored in kerosene to keep air away from it. But since it's extremely reactive, it will react even with impurities and oxygen dissolved in the kerosene, forming a rather thick layer of potassium oxides on its surface.