John took two elements X and Y. He filled a small tub with water and dropped both the elements in water and observe the results

X and Y can be

With water sodium reacts rapidly.

$Na+H_{2}O\to NaOH+H_2\uparrow$

The metal catches fire rapidly with water and produces a characteristic orange fire.

Calcium metal floats on water, because the hydrogen bubble formed during the reaction of calcium with water stick on the metal and make it lighter.

$Ca+2H_{2}O\to Ca{\left(OH\right)}_2+H_2\uparrow$

The heat released is less since the reaction of calcium and water is less violent. Hence, hydrogen does not catch fire.