Explain why, the blue colour of copper sulphate solution fades when an iron nail is kept dipped in it. Write chemical equation of the reaction involved. What does this reaction tell us about the relative reactivities of copper and iron?

When an iron nail is put in the blue colour copper sulphate solution, then the iron displaces copper from copper sulphate and forms green coloured iron sulphate solution, that's why the blue colour of the solution fades. The reaction involved is as follows:

$\underset{\mathrm{grey}}{\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\mathrm{blue}}{{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)}\to \underset{\mathrm{light}\mathrm{green}}{{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{brown}}{\mathrm{Cu}\left(\mathrm{s}\right)}$

As iron displaces copper from its sulphate solution, so, we can conclude that iron is more reactive than copper.