If you put an iron nail in copper sulfate solution, what will you observe? Why?

Observation: The blue color of copper sulfate solution gradually fades, and a reddish-brown layer of copper forms on the iron nail.

Reason: Iron is more reactive than copper, so it displaces copper from copper sulfate solution:

$$Fe + CuSO₄ → FeSO₄ + Cu$$

The fading of blue color is due to the formation of iron sulfate (greenish), while copper gets deposited on the nail.