Question

Assertion: Gold and silver are extracted from their native ores by leaching.
Reason: Both silver and gold particles dissolve in a dilute solution of sodium cyanide in the presence of oxygen.
(A)- Both assertion and reason are correct and the reason is the correct explanation of assertion.
(B)- Both assertion and reason are correct but the reason is not the correct explanation of assertion.
(C)- Assertion is correct but the reason is incorrect.
(D)- Both assertion and reason are incorrect.

Answer 1

Hint: Metals occur in various forms, such as native form, carbonate form or oxide form. To get pure metal, these metals need to be extracted and purified. Gold and silver are often extracted from the same ores and have similar chemical properties, thus are hard to separate.

Complete answer:
-The process of separating gold from gold and silver is known as Gold parting.
-Silver is often found as sulphide ore, that is known as Argentite $(Ag{{S}_{2}})$, and gold occurs mainly as a grain of metal disseminated in quartz veins.
-The process of extraction of Au and Ag both begins by firstly crushing and concentrating it by froth floatation. Silver in this directly undergoes leaching while gold is leached after roasting.
-Leaching of silver ore is done by the cyanide process. In this process, silver ore is leached with NaCN or KCN solution in the presence of air which converts silver and its salts insoluble complex (Argento cyanide). The chemical reaction for this process is given as-
$A{{g}_{2}}SC{{N}^{-}}2{{[Ag{{(CN)}_{2}}]}^{-}}+{{S}^{2-}}$
-Oxygen air is then pumped to force the reaction to occur in the forward direction, that is
$4{{S}^{2-}}+5{{O}_{2}}+2{{H}_{2}}O\to 2S{{O}_{4}}^{2-}+2S\downarrow +4O{{H}^{-}}$
-Zinc dust is later then added to precipitate silver, whose chemical reaction is given as-
${{[Ag{{(CN)}_{2}}]}^{-}}+Zn\to Ag(Pure)\downarrow +{{[Zn{{(CN)}_{4}}]}^{2-}}$
-The process of leaching used for extraction of gold is known as MacArthur Forrest Cyanide process. In this process, concentrated ore of gold is taken in large vats made of cement or iron and is then treated with a dilute (0.5%)solution of KCN in the presence of air forming a soluble gold complex (potassium aurocyanide). The reaction for this is given as-
$4Au+8KCN+2{{H}_{2}}O+{{O}_{2}}\to 4K[Au{{(CN)}_{2}}]+4KOH$
-The gold obtained now still has some traces of impurities and to remove this, electrolytic refining is performed.

So, the statements mentioned in assertion and reason both are correct, therefore the correct answer is option A.

Note:
Silver can also be obtained from argentiferous lead by Parke’s process or Pattinson’s process. Gold and silver can also be extracted from anode mud formed during the electrolytic refining of copper. Another method of extraction is amalgamation, in which powdered ore is first converted to its amalgam and then distilled to remove mercury in the form of vapour leaving behind the pure metal.