Black precipitate of copper sulphide dissolves in:

Options

A. `KCN` solution
B. sodium sulphide solution
C. sodium hydroxide
D. boiling dilute `(M)` sulphuric acid.

Answer

To determine in which solution the black precipitate of copper sulfide (CuS) dissolves, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Black Precipitate**:
- The black precipitate in question is copper sulfide (CuS).

2. **Consider Possible Solvents**:
- We need to evaluate the options provided (though not listed in the question, we will assume common reagents such as KCN, NaOH, HCl, etc.).

3. **Analyze the Reaction with KCN**:
- When CuS is treated with potassium cyanide (KCN), the cyanide ions (CN⁻) can react with copper ions (Cu²⁺) to form a soluble complex.
- The reaction can be represented as:
\[ \text{CuS} + 4 \text{CN}^- \rightarrow \text{[Cu(CN)_4]}^{3-} + \text{S}^{2-} \]
- This indicates that CuS will dissolve in KCN, forming a soluble complex.

4. **Conclusion**:
- Based on the reaction with KCN, we conclude that the black precipitate of copper sulfide dissolves in KCN solution.

### Final Answer:
- The black precipitate of copper sulfide (CuS) dissolves in KCN solution.