When a nitrate is warmed with zinc powder and an NaOH solution, a gas is evolved. Which of the following reagents will be turned brown by the gas?

Options

A. Sodium nitroprusside
B. Sodium cobaltinitrite
C. Nessler's reagent
D. Barium chloride

Answer

To solve the question, we need to analyze the reaction that occurs when a nitrate is warmed with zinc powder and sodium hydroxide (NaOH) solution.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
When a nitrate (NO3-) is heated with zinc powder in the presence of NaOH, a reduction reaction occurs. Zinc acts as a reducing agent and reduces the nitrate ion to ammonia (NH3) gas.

2. **Identifying the Gas Produced**:
The gas evolved in this reaction is ammonia (NH3). This is a crucial point because we need to find out which of the given reagents will react with ammonia to produce a brown color.

3. **Analyzing the Given Options**:
- **Option 1: Sodium nitrobutyrate**: This compound does not react with ammonia to produce a brown color.
- **Option 2: Sodium cobalt nitrate**: This reagent is used for the identification of potassium ions and does not turn brown with ammonia.
- **Option 3: Nessler's reagent**: This reagent is known to react with ammonia. When ammonia reacts with Nessler's reagent (which contains potassium tetraiodomercurate(II)), it forms a complex that has a chocolate brown color.
- **Option 4: Barium chloride**: This reagent is used for the identification of sulfate ions and does not react with ammonia to produce a brown color.

4. **Conclusion**:
Based on the analysis, the reagent that will turn brown when exposed to the gas (ammonia) produced in the reaction is **Nessler's reagent**. Therefore, the correct answer is **Option 3**.

### Final Answer:
**Option 3: Nessler's reagent will be turned brown by the gas (ammonia).**

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