When a clean iron nail is dipped in a blue copper sulphate solution, which observation confirms that a displacement reaction has occurred?

Correct answer: A reddish-brown copper coating forms on the nail and the solution turns green.

Iron is more reactive than copper, so it displaces copper from copper sulphate: \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). The displaced copper forms a reddish-brown coating on the nail, and the iron sulphate formed makes the solution green. In contrast, an unchanged blue solution would not indicate displacement.