The figure shows the corrosion of iron in the atmosphere.
Iron at the spots marked ‘Fe’ under the water drop gives the species shown by the arrows. What do these spots act as, and what takes place there?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Anode; iron is oxidised to Fe2+ ions, releasing electrons
The arrows in the figure show iron at these spots giving Fe2+ ions and electrons (2e−). Loss of electrons is oxidation, and the spot at which oxidation takes place behaves as the anode. The electrons released here move through the metal to another spot, where oxygen is reduced.
Anode: \( \ce{2Fe(s) -> 2Fe^2+ + 4e^-} \)
Why the other options are incorrect
Option (2) Cathode; oxygen is reduced to water — Incorrect. Oxygen is reduced to water at another spot, at the right of the drop, which receives the 4e− through the metal; that spot is the cathode, not the spots marked Fe.
Option (3) Cathode; Fe2+ ions take up electrons and are deposited as iron — Incorrect. The arrows run from Fe to Fe2+ and 2e−: iron passes into the drop and gives up electrons; it does not take them up.
Option (4) Anode; Fe2+ ions are oxidised by oxygen to Fe2O3 — Incorrect. The spots are anodic, but the change there is Fe → Fe2+. The Fe2+ ions are oxidised by atmospheric oxygen to rust (Fe2O3) later, at the left edge of the drop.
NCERT (2026-27 edition):
2.8 Corrosion – “In corrosion, a metal is oxidised by loss of electrons to oxygen and formation of oxides. …The chemistry of corrosion is quite complex but it may be considered essentially as an electrochemical phenomenon. At a particular spot (Fig. 2.13) of an object made of iron, oxidation takes place and that spot behaves as anode and we can write the reaction”
Anode: 2 Fe (s) → 2 Fe2+ + 4 e– Eo(Fe2+/Fe) = – 0.44 V
“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 with the reaction”
Cathode: O2(g) + 4 H+(aq) + 4 e– → 2 H2O (l)
“…The ferrous ions are further oxidised by atmospheric oxygen to ferric ions which come out as rust in the form of hydrated ferric oxide (Fe2O3. x H2O) and with further production of hydrogen ions.”
Fig. 2.13: Corrosion of iron in atmosphere