A student observed that when an iron nail was placed in a blue copper sulphate solution, the solution gradually turned green and a reddish-brown coating formed on the nail. What type of reaction is this?

Correct answer: Displacement reaction

Iron is more reactive than copper, so it displaces copper from copper sulphate solution:
\(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\).
The \(\mathrm{FeSO_4}\) solution is green, and the reddish-brown coating is copper. This is a displacement reaction because a more reactive metal removes a less reactive metal from its salt solution; it is not a combination reaction, in which reactants form a single product.