In the following question, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices:

Assertion : Au and Ag are extracted by leaching their ores with a dilute solution of NaCN.

Reason: Impurities associated with these ores dissolve in NaCN.

Assertion is true but the reason is false.

The correct answer is option 3. Assertion is true but the reason is false.

Assertion: Au (gold) and Ag (silver) are extracted by leaching their ores with a dilute solution of NaCN.

This statement is true. The extraction of gold and silver using cyanide leaching is a well-known method. Gold and silver are dissolved in a dilute solution of sodium cyanide (NaCN) through the following reactions:

\(4 \text{Au} + 8 \text{NaCN} + \text{O}_2 + 2 \text{H}_2\text{O} \rightarrow 4 \text{Na[Au(CN)}_2\text{]} + 4 \text{NaOH}\)

\(2 \text{Ag} + 4 \text{NaCN} + \text{O}_2 + 2 \text{H}_2\text{O} \rightarrow 2 \text{Na[Ag(CN)}_2\text{]} + 2 \text{NaOH}\)

Reason: Impurities associated with these ores dissolve in NaCN.

This statement is false. The purpose of leaching with NaCN is to selectively dissolve gold and silver from their ores, not the impurities. Most of the impurities do not dissolve in NaCN and remain as solid residues, which can be separated from the cyanide solution. This selective dissolution is why cyanide leaching is effective for extracting gold and silver. The primary impurities like silicates, oxides, and sulfides generally do not dissolve in the cyanide solution.

Thus, while the assertion is correct in stating that gold and silver are extracted by leaching with NaCN, the reason given is incorrect because impurities do not generally dissolve in NaCN.