Question

sodium and potasium metals are kept under mathane oil

Answer 1

Sodium and potassium metals are both kept in methane or in dry mineraloil. ... Sodium and potassium are among the most aggressive metalsand react vigorously with the oxygen of air. Now take the case of kerosene it is cheaper than any other natural oils. So it is preferred to keep them in methane
to avoid reaction .

Answer 2

sodium and potassium metals are kept under methane oil because these metals are highly reactive to water or moisture and catch fire. so in order to prevent them from moisture, they are kept under methane oil.

I hope it will help you.