Question

Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer 1

Due to formation of Iron $\left(II\right)$ sulphate which is of green colour, copper sulphate solution changes its colour.

$\underset{\left(Blue\right)}{\underset{}{CuS{O}_4}}+F{e}^{2+}⟶\underset{\left(Green\right)}{\underset{}{FeS{O}_4}}+C{u}^{2+}$