In the metallurgy of silver and that of gold, the respective metal is leached with a dil. solution of \(NaCN\) from which silver and gold is later recovered by
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Replacement with \(Zn\)
Silver and gold are extracted by leaching (the cyanide process). The ore is treated with a dilute solution of NaCN or KCN in the presence of air (for \( \ce{O2} \)); the metal is oxidised and dissolves as a soluble cyanide complex:
\( \displaystyle \ce{4M(s) + 8CN-(aq) + 2H2O(aq) + O2(g) -> 4[M(CN)2]-(aq) + 4OH-(aq)}\quad(\text{M = Ag or Au}) \)
The metal is then recovered by replacement (displacement) with zinc, a more electropositive metal:
\( \displaystyle \ce{2[M(CN)2]-(aq) + Zn(s) -> [Zn(CN)4]^2-(aq) + 2M(s)} \)
Zinc acts as the reducing agent and goes into solution as the tetracyanozincate ion.
Why the other options are incorrect
Option (1) Vapour phase refining — Incorrect. A refining method in which the metal is converted into a volatile compound and then decomposed, e.g. Mond process for nickel (\( \ce{Ni(CO)4} \)) and van Arkel method for zirconium and titanium (\( \ce{ZrI4} \)).
Option (2) Distillation — Incorrect. Used to refine low-boiling metals such as zinc and mercury; it does not recover a metal from a cyanide solution.
Option (4) Zone refining — Incorrect. Used to obtain metals of very high purity, such as germanium, silicon and gallium for semiconductors, from an already extracted metal.
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):
Leaching – …
