Sodium and Potassium are highly reactive metals, they catch fire when kept in open
.and a lot of heat is given out which can prove harmful for us.
also their is low so they float on water which makes them to come in contact with air.
Since, the density of kerosene is also low both of them do not float on kerosene.
Kerosene is sooo saturated that they both do not react with kerosene.
So they are kept immersed in kerosene oil. sodium potassium are highly reactive metals.with reaction with H2O (water or moisture) they burn violently.as there is moisture in air to prevent their burning they are stored in kerosene as moisture cannot enter in it.