Question

why sodium metal is stored under kerosene

Answer 1

Sodium is a highly reactive metal that catches fire when exposed to atmospheric oxygen. So to keep it away from oxygen it is kept under kerosene . It is so reactive that even in water it ablaze vigorously.

Answer 2

Sodium metal is stored in the kerosene because Sodium is highly reactive with the oxygen present around it. To avoid the reaction between the oxygen and the sodium it is stored in the kerosene.