Question

compare the reactivity of sodium and iron towards water​

Answer 1

Put sodium in water and lead in water.

Oxygen must also be present for lead to mix with water where lead hydroxide is formed.

Sodium will react violently with water forming sodium hydroxide and hydrogen gas which can (will) burn. All group 1 elements release heat in their reactions with water. Lithium releases the most heat while Caesium has the most violent reaction with water (explosion).

This is a way to prove the reactivity. I hope this answers your question.