Rusting occurs in the presence of water and air.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Iron provides the anodic spot and gets oxidized
Rusting is an electrochemical process. On the surface of iron in contact with water and air, a tiny cell is set up:
Anode (iron): 2Fe(s) → \( \ce{2Fe^{2+}} \) + 4e− E°(\( \ce{Fe^{2+}} \)/Fe) = −0.44 V
Cathode: O2(g) + 4H+(aq) + 4e− → 2H2O(l) E° = 1.23 V
The spot where iron is oxidised acts as the anode; the electrons travel through the metal to another spot where oxygen is reduced (the cathode). \( \ce{Fe^{2+}} \) is further oxidised by air to \( \ce{Fe^{3+}} \), which appears as hydrated ferric oxide (rust), \( \ce{Fe2O3} \)·xH2O. Cell reaction:
\( \ce{2Fe(s) + O2(g) + 4H+(aq) -> 2Fe^2+(aq) + 2H2O(l)} \) E°cell = 1.67 V
Why the other options are incorrect
Option (2) Oxygen provides the anodic spot and gets reduced. — Incorrect. Oxygen is reduced, but reduction happens at the cathode, not at an anodic spot.
Option (3) Iron provides the cathodic spot and gets oxidized — Incorrect. Iron is oxidised, and oxidation takes place at the anode, not the cathode.
Option (4) Oxygen provides the cathodic spot and gets oxidized. — Incorrect. Oxygen gains electrons, so it is reduced, not oxidised.
NCERT (2026-27 edition):
Corrosion – “… Corrosion of iron (commonly known as rusting) occurs in presence of water and air. … At a particular spot (Fig. 2.13) of an object made of iron, oxidation takes place and that spot behaves as anode … Electrons released at anodic spot move through the metal and go to another spot on the metal and reduce oxygen in the presence of H+ … This spot behaves as cathode …”
