Question 4
Why is sodium metal stored under paraffin oil or kerosene?
The right answer is the one marked above.
Because sodium is so highly reactive that it must be kept where air cannot reach it. It reacts rapidly with the oxygen of the air to form the oxide of the metal.
You can see it happen. A piece taken out of the oil has no shine, but cut it and the fresh surface has a silvery metallic lustre — and within a short time that lustre gradually fades, as the air gets to work on it.