A white crystalline solid 'A' on boiling with caustic soda solution gives a gas 'B', which on passing through an alkaline solution of ptassium tetraiodomercurate (II) solution gives a brown ppt. the substance 'A' on heating eveolves a neutral gas 'C', which is inert at room temperature and reactive a presence of catalyst and does ont give grown fumes with nitric oxide.
Q. The gas 'B' is:

Options

A. `H_(2)S`
B. `NH_(3)`
C. `HCl`
D. `CO_(2)`

Explanation

To solve the question step by step, we will analyze the information provided and deduce the required gas 'B'.

### Step 1: Identify the white crystalline solid 'A'
The problem states that a white crystalline solid 'A' reacts with caustic soda to produce gas 'B'. A common white crystalline solid that fits this description is ammonium nitrite (NH4NO2).

**Hint:** Look for common white crystalline solids that can react with caustic soda.

### Step 2: Reaction of 'A' with caustic soda
When ammonium nitrite (NH4NO2) is boiled with caustic soda (NaOH), it decomposes to produce ammonia (NH3) gas, sodium nitrite (NaNO2), and water (H2O). The reaction can be summarized as follows:

\[ \text{NH}_4\text{NO}_2 + \text{NaOH} \rightarrow \text{NH}_3 (g) + \text{NaNO}_2 + \text{H}_2\text{O} \]

Thus, gas 'B' produced in this reaction is ammonia (NH3).

**Hint:** Identify the products of the reaction when a known compound reacts with a base.

### Step 3: Confirm gas 'B' using Nessler's reagent
The problem states that gas 'B' (NH3) is passed through an alkaline solution of potassium tetraiodomercurate (II) (K2HgI4), which is known as Nessler's reagent. The presence of ammonia in the gas will lead to the formation of a brown precipitate. This confirms that gas 'B' is indeed ammonia.

The reaction with Nessler's reagent can be summarized as follows:

\[ \text{NH}_3 + \text{K}_2\text{HgI}_4 \rightarrow \text{Brown precipitate} \]

**Hint:** Use chemical tests (like Nessler's reagent) to confirm the presence of specific gases.

### Step 4: Analyze the gas 'C' evolved on heating 'A'
The problem also mentions that when solid 'A' is heated, it evolves a neutral gas 'C' that is inert at room temperature but reactive in the presence of a catalyst. This gas does not give brown fumes with nitric oxide. This description fits nitrogen gas (N2), which is neutral and inert under normal conditions.

**Hint:** Consider the properties of gases evolved during thermal decomposition.

### Conclusion
From the analysis, we conclude that the gas 'B' produced when the white crystalline solid 'A' (NH4NO2) reacts with caustic soda is ammonia (NH3).

### Final Answer
The gas 'B' is: **Ammonia (NH3)**.