Question

Why a brown coating is formed on the iron rod kept dipped in a copper sulphate solution for some time?

Answer 1

because when the metal contact with water and air then this is has corrosion and shining surface is destroyed

Answer 2

This happens due to a displacement reaction, between Copper sulphate and Iron.

Since Iron is more reactive than copper, It displaces copper and Copper sulphate turns into Iron sulphate. And a brownish layer of copper gets deposited on the Iron Rod☺!