The gas which turns mercurous nitrate paper black is

Options

A. `NH_(3)`
B. `CI_(3)`
C. `SO_(2)`
D. `SO_(3)`

Answer

To solve the question "The gas which turns mercurous nitrate paper black is:", we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Given Options**:
The options provided are:
- A) NH3 (Ammonia)
- B) Cl3 (Chlorine)
- C) SO2 (Sulfur Dioxide)
- D) SO3 (Sulfur Trioxide)

2. **Understand the Reaction**:
We need to determine which of these gases can react with mercurous nitrate (Hg2(NO3)2) to produce a black color. Mercurous nitrate paper is used to test for the presence of certain gases.

3. **Analyze Each Gas**:
- **NH3 (Ammonia)**: When ammonia gas reacts with mercurous nitrate, it forms mercury (Hg) and ammonium nitrate (NH4NO3), along with an insoluble complex of HgNH2NO3. The finely divided mercury produced is what turns the paper black.
- **Cl3 (Chlorine)**: Chlorine does not react with mercurous nitrate to produce a black color.
- **SO2 (Sulfur Dioxide)**: Sulfur dioxide does not produce a black color when reacting with mercurous nitrate.
- **SO3 (Sulfur Trioxide)**: Sulfur trioxide also does not react to produce a black color.

4. **Conclusion**:
Based on the analysis, the gas that turns mercurous nitrate paper black is **NH3 (Ammonia)**. Therefore, the correct answer is option A.

### Final Answer:
The gas which turns mercurous nitrate paper black is **NH3 (Ammonia)**.

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