A student puts one big iron screw each in four test tubes containing aqueous solutions of aluminium sulphate, copper sulphate, ferrous sulphate and zinc sulphate. After some time he observed a reddish brown coating only on the surface of the screw which was put in the solution of

According to the reactivity series of metals (Table 3.2 in the NCERT textbook), iron ($Fe$) is more reactive than copper ($Cu$) but less reactive than aluminium ($Al$) and zinc ($Zn$).

• Why B is correct: Since iron is more reactive than copper, it displaces copper from the copper sulphate ($CuSO_4$) solution. The chemical equation for this displacement reaction is:

$$Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$$

The displaced copper metal is reddish-brown and deposits as a coating on the surface of the iron screw.

• Why A is incorrect: Aluminium is more reactive than iron ($Al > Fe$). Therefore, iron cannot displace aluminium from aluminium sulphate solution, and no reaction occurs.
• Why C is incorrect: Putting an iron screw in ferrous sulphate ($FeSO_4$) solution does not result in any displacement reaction.
• Why D is incorrect: Zinc is more reactive than iron ($Zn > Fe$). Therefore, iron cannot displace zinc from zinc sulphate solution, and no reaction occurs.