An iron nail dipped in copper sulphate solution turns brown. This is due to which one of the following types of reactions?

The correct answer is D, Displacement reaction. Iron is more reactive than copper, so it pushes copper out of copper sulphate, and the freed copper coats the nail brown.

The reaction is Fe + CuSO₄ → FeSO₄ + Cu. A reddish-brown layer of copper forms on the nail, and the blue colour of the solution slowly fades to pale green as iron sulphate forms. A more reactive metal always displaces a less reactive one from a solution of its salt, which is why the reactivity series matters. The reverse does not happen: a copper wire placed in iron sulphate solution shows no change.

A, an addition reaction, joins two molecules into one, as when hydrogen adds to a vegetable oil; nothing is added here.

B, a decomposition reaction, breaks one compound into simpler substances, as when limestone is heated; no compound breaks apart on its own here.

C, substitution, is the term used for organic compounds in which one atom replaces another, as when chlorine replaces hydrogen in methane.

Exam tip: K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au; a metal higher in the series displaces one lower down.