Question

The colour of copper sulphate solution change when an ____ nail is dipped into it.

Answer 1

When Iron nail (Ferrum) is dipped in Copper sulfate $\left(CuS{O}_4\right)$ there takes reaction between them and copper sulfate changes its color from blue to light green.

Reaction:

$CuS{O}_4+F{e}^{+2}⟶FeS{O}_4+C{u}^{+2}$

This shows Iron is more reactive than copper, it can replace Copper from $CuS{O}_4$

$CuS{O}_4$ is in blue Color and $FeS{O}_4$ is in light green color.

So, the correct answer is iron.