Extraction of gold and silver involves leaching the metal with \(CN^-\) ion. This is also __________ for gold or silver
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Oxidation reaction
In the extraction of gold and silver, the metal is leached with a dilute solution of NaCN or KCN in the presence of air:
4Au(s) + 8CN−(aq) + 2H2O(aq) + O2(g) → 4[Au(CN)2]−(aq) + 4OH−(aq)
The metal goes from 0 to +1 (Au → \( \ce{Au^{+}} \), Ag → \( \ce{Ag^{+}} \)), so for gold or silver this step is an oxidation reaction. The metal is later recovered by displacement with zinc, which is the reduction step.
Why the other options are incorrect
Option (2) Reduction reaction — Incorrect. The metal loses electrons in leaching; it is reduced only later, when zinc displaces it from [M(CN)2]−.
Option (3) Redox reaction — Incorrect. The whole equation does involve \( \ce{O2} \) being reduced, but the question asks what the step is for gold or silver, and for the metal it is oxidation.
Option (4) Disproportionation reaction — Incorrect. In disproportionation the same element is both oxidised and reduced; here the metal is only oxidised.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Oxidation Reduction – “… As studied earlier, extraction of gold and silver involves leaching the metal with CN–. This is also an oxidation reaction (Ag → Ag+ or Au → Au+). The metal is later recovered by displacement method.”