Which of the following statements about gold (Au) element are correct?

(A) Gold occurs in nature in native state.

(B) Gold dissolves in aqua-regia.

(C) Gold is extracted by leaching the metal with dilute NaCN.

(D) Leaching is followed by replacement reaction.

Choose the correct answer from the options given below:

(A), (B), (C) and (D)

The correct answer is option 4. (A), (B), (C) and (D).

Let us go through each statement in detail to understand why all of them are correct regarding gold (Au).

(A) Gold occurs in nature in native state:

Gold is one of the few metals that occurs naturally in its elemental or metallic form, which means it is found as a pure metal, not as a compound. This makes it a native metal. Gold deposits are found in riverbeds, quartz veins, and other geological formations, often in its metallic form or as small nuggets. Native gold is chemically inert, meaning it doesn’t easily form compounds, which is why it remains in its pure state in nature.

(B) Gold dissolves in aqua regia:

Aqua regia is a highly corrosive mixture of nitric acid \((HNO_3)\) and hydrochloric acid \((HCl)\) in a 1:3 ratio. Although gold is resistant to most acids, it dissolves in aqua regia due to the combined action of nitric acid and hydrochloric acid. The nitric acid oxidizes the gold, forming gold ions \((Au^{3+})\), while the chloride ions from the hydrochloric acid form a soluble complex \((AuCl_4^-)\). The overall reaction is

\(Au(s) + 3HNO_3(aq) + 4HCl(aq) \longrightarrow HAuCl_4(aq) + 3NO_2(g) + H_2O(l)\)

This is why aqua regia is used to dissolve gold, even though gold is generally resistant to corrosion.

(C) Gold is extracted by leaching the metal with dilute NaCN:

The cyanide process, also called cyanidation, is used to extract gold from its ores. In this process, finely ground gold ore is mixed with a dilute solution of sodium cyanide \((NaCN)\). Gold reacts with the cyanide ions to form a soluble complex, sodium dicyanoaurate \((Na[Au(CN)_2])\). The reaction is:

\(4Au(s) + 8NaCN(aq) + O_2(g) + 2H_2O(l) \longrightarrow 4Na[Au(CN)_2](aq) + 4NaOH(aq)\)

This method allows the gold to be dissolved and separated from the ore. The cyanide process is one of the most efficient and commonly used methods for gold extraction.

(D) Leaching is followed by a replacement reaction:

After gold is leached from the ore using sodium cyanide, the gold is recovered through a process called precipitation or replacement. This involves adding a more reactive metal, typically zinc (Zn), to the gold solution. Zinc displaces the gold from the complex ion because zinc is higher in the reactivity series than gold. The reaction is:

\(2Na[Au(CN)_2](aq) + Zn(s) \longrightarrow 2Au(s) + Na_2[Zn(CN)_4](aq)\)

This reaction causes the gold to precipitate out as a solid, which can then be collected.

Thus, the correct answer is (4) (A), (B), (C), and (D).