Which of the following in insoluble in dil. `HNO_(3)` but dissolves in aquaregia ?

Options

A. `HgS`
B. `PbS`
C. `Bi_(2)S_(3)`
D. `CuS`

Answer

To solve the question "Which of the following is insoluble in dilute HNO3 but dissolves in aqua regia?", we can follow these steps:

### Step 1: Understand the properties of aqua regia
Aqua regia is a strong acidic solution made from a mixture of concentrated nitric acid (HNO3) and concentrated hydrochloric acid (HCl) in a ratio of 1:3. It is known for its ability to dissolve many metals and metal compounds that are typically resistant to other acids.

**Hint:** Remember that aqua regia is a powerful solvent for metals and can dissolve even noble metals.

### Step 2: Identify the solubility in dilute HNO3
We need to determine which compounds are insoluble in dilute nitric acid (HNO3). Some compounds, like certain sulfides, do not react with dilute nitric acid and remain insoluble.

**Hint:** Consider the common reactions of sulfides and other compounds with dilute acids.

### Step 3: Analyze the given options
We need to analyze each of the compounds provided in the question to see which one is insoluble in dilute HNO3 but soluble in aqua regia.

1. **HgS (Mercury(II) sulfide)**: This compound does not react with dilute HNO3 and remains insoluble. However, it can dissolve in aqua regia.
2. **PbS (Lead(II) sulfide)**: This compound reacts with dilute HNO3 to form lead nitrate, making it soluble.
3. **Bi2S3 (Bismuth(III) sulfide)**: This compound also reacts with dilute HNO3 to form bismuth nitrate, making it soluble.
4. **CuS (Copper(II) sulfide)**: This compound reacts with dilute HNO3 to form copper nitrate, making it soluble.

**Hint:** Look for compounds that do not react with dilute acids, particularly sulfides.

### Step 4: Conclusion
After analyzing the solubility of each compound in dilute HNO3 and their behavior in aqua regia, we find that **HgS (Mercury(II) sulfide)** is the only compound that is insoluble in dilute HNO3 but dissolves in aqua regia.

**Final Answer:** HgS (Mercury(II) sulfide) is the correct answer.