Gold cannot be dissolved by any acid. However, it can be dissolved by aqua regia (kingly water) which is a mixture of:

A. Mineral water and iodine

B. nitric and hydrochloric acids

C. water and sulphur

D. alcohol and alum

Gold is indeed a unique metal, renowned for its resistance to corrosion and tarnishing. While it cannot be dissolved by most acids, it can be effectively dissolved by a specific mixture known as aqua regia. This term, which translates to "royal water," refers to a potent combination of two strong acids. The correct answer to your question is B: nitric and hydrochloric acids.

Understanding Aqua Regia

Aqua regia is a highly corrosive liquid that can dissolve noble metals like gold and platinum, which are typically resistant to other chemical reactions. The reason it can dissolve gold lies in the chemical properties of the acids involved.

The Components of Aqua Regia

• Nitric Acid (HNO3): This acid acts as an oxidizing agent. It helps to oxidize gold, converting it into gold ions.
• Hydrochloric Acid (HCl): This acid provides chloride ions, which then combine with the oxidized gold to form a soluble gold chloride complex.

When these two acids are mixed in a specific ratio, they create a solution that can effectively break down gold. The reaction can be summarized as follows:

The Chemical Reaction

When gold is introduced to aqua regia, the nitric acid oxidizes the gold, and the hydrochloric acid provides chloride ions that stabilize the gold ions in solution. The overall reaction can be represented as:

3 HCl + HNO3 + Au → AuCl3 + 2 H2O + NO

In this reaction, gold (Au) is transformed into gold chloride (AuCl3), which is soluble in the aqua regia solution.

Why Other Options Are Incorrect

Let’s briefly examine why the other options do not work for dissolving gold:

• A. Mineral water and iodine: This combination does not have the necessary chemical properties to dissolve gold.
• C. Water and sulphur: Neither water nor sulphur can react with gold in a way that would lead to dissolution.
• D. Alcohol and alum: While alcohol can dissolve some substances, it does not have the capability to dissolve gold.

Practical Applications

Aqua regia is not just a laboratory curiosity; it has practical applications in various fields. For example, it is used in:

• Gold Refining: Jewelers and refiners use aqua regia to recover gold from scrap materials.
• Analytical Chemistry: It is employed in the analysis of gold content in ores and other materials.

In summary, while gold is resistant to many acids, aqua regia, a mixture of nitric and hydrochloric acids, is the exception that allows for its dissolution. This unique property makes aqua regia an essential tool in both industrial and laboratory settings.