When an iron nail is dipped in copper sulphate solution, it becomes brownish in colour because iron has replaced copper from its solution.
Thus the amount of copper sulphate decreases and its colour fades.
\(\mathrm{Fe}_{(a)}+\mathrm{CuSO}_{4(a q)} \rightarrow \mathrm{FeSO}_{4(a q)}+\mathrm{Cu}_{(a)}\)
This is a displacement reaction.