To a solution of a substance gradual addition of ammonium hydroxide result in a black precipitate which does not dissolve in excess of `NH_(4)OH` however when HCI is added to the original solution a white precipitate is formed .The solution containe

Options

A. Lead salt
B. Silver salt
C. Mercurous salt
D. Copper salt

Explanation

To solve the question, we need to analyze the behavior of the given solution when ammonium hydroxide (NH4OH) and hydrochloric acid (HCl) are added. The observations made during the experiment will help us identify the cation present in the solution.

### Step-by-Step Solution:

1. **Identify the Reaction with Ammonium Hydroxide:**
- When ammonium hydroxide is added gradually to the solution, a black precipitate is formed. This indicates that the cation present in the solution could potentially be from a salt that produces a black precipitate upon reaction with NH4OH.

2. **Evaluate Possible Cations:**
- **Lead (Pb²⁺):** When Pb²⁺ reacts with NH4OH, it forms a white precipitate of lead hydroxide (Pb(OH)₂). This is not consistent with the observation of a black precipitate.
- **Silver (Ag⁺):** The reaction of Ag⁺ with NH4OH produces a brown precipitate of silver oxide (Ag₂O). This also does not match the observation of a black precipitate.
- **Mercurous (Hg₂²⁺):** The reaction of mercurous ions with NH4OH produces a black precipitate, specifically a mixture of mercury(I) oxide (HgO) and other compounds. This matches the observation of a black precipitate that does not dissolve in excess NH4OH.
- **Copper (Cu²⁺):** The reaction of Cu²⁺ with NH4OH leads to the formation of a blue precipitate of copper hydroxide (Cu(OH)₂), which does not match the observation.

3. **Identify the Reaction with Hydrochloric Acid:**
- When HCl is added to the original solution, a white precipitate is formed. This indicates that the cation must also react with HCl to form a white precipitate.
- For mercurous salts, the reaction with HCl can lead to the formation of mercurous chloride (Hg₂Cl₂), which is a white precipitate.

4. **Conclusion:**
- Based on the observations, the only cation that fits both criteria (formation of a black precipitate with NH4OH and a white precipitate with HCl) is **mercurous salt (Hg₂²⁺)**.

### Final Answer:
The solution contains **mercurous salt (Hg₂²⁺)**.

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