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

The reaction between an iron nail and copper sulphate solution is a classic example of a displacement reaction. In this type of chemical reaction, a more reactive element displaces a less reactive element from its compound.

• Chemical Process: When an iron nail (Fe) is placed in a blue-colored copper sulphate (CuSO4) solution, iron, being more reactive than copper, displaces copper from the solution.
• Chemical Equation: Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s).
• Observations: The blue color of the solution fades and gradually turns light green due to the formation of ferrous sulphate (FeSO4). The brownish coating on the iron nail is the metallic copper that has been displaced and deposited on the nail's surface.

Because one element (iron) replaces another (copper) in a compound, it is categorized as a displacement reaction, making Option D correct.