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 substance 'A' is:

Options

A. `NH_(4)Cl`
B. `NH_(4)NO_(3)`
C. `NH_(4)NO_(2)`
D. `NaNO_(3)`

Explanation

To determine the identity of the white crystalline solid 'A', let's analyze the information given step-by-step.

### Step 1: Identify the Reaction with Caustic Soda
The question states that a white crystalline solid 'A' on boiling with caustic soda (NaOH) gives a gas 'B'. This suggests that 'A' is likely an ammonium salt, as ammonium salts release ammonia gas when treated with a strong base like NaOH.

**Hint:** Consider common ammonium salts that are white and crystalline.

### Step 2: Determine the Gas 'B'
The gas 'B' produced from the reaction is likely ammonia (NH3). This can be confirmed by the fact that ammonia can be detected using Nessler's reagent (potassium tetraiodomercurate (II) solution), which gives a brown precipitate when reacting with ammonia.

**Hint:** Think about the properties of ammonia and how it reacts with Nessler's reagent.

### Step 3: Analyze the Precipitate Formation
When gas 'B' (ammonia) is passed through the alkaline solution of potassium tetraiodomercurate (II), it forms a brown precipitate. This confirms the presence of ammonia, supporting the idea that the original solid 'A' is an ammonium salt.

**Hint:** Recall that the brown precipitate indicates the presence of ammonium ions.

### Step 4: Heating Substance 'A'
The question mentions that substance 'A' on heating evolves a neutral gas 'C' that is inert at room temperature and reactive in the presence of a catalyst. This suggests that 'C' is nitrogen gas (N2), which is known to be inert under standard conditions but can react in the presence of a catalyst.

**Hint:** Consider the decomposition reactions of ammonium salts upon heating.

### Step 5: Confirm the Identity of Substance 'A'
The only ammonium salt that fits all the criteria provided is ammonium nitrite (NH4NO2). When ammonium nitrite is heated, it decomposes to produce nitrogen gas (N2) and water (H2O). This aligns with the description of gas 'C'.

**Hint:** Check the thermal decomposition products of ammonium salts to find the one that produces nitrogen gas.

### Conclusion
Based on the analysis, the substance 'A' is **ammonium nitrite (NH4NO2)**.

### Summary of Steps:
1. Identify that 'A' is likely an ammonium salt.
2. Recognize that gas 'B' is ammonia (NH3).
3. Confirm the formation of a brown precipitate with Nessler's reagent.
4. Conclude that gas 'C' is nitrogen (N2) from the decomposition of 'A'.
5. Identify 'A' as ammonium nitrite (NH4NO2).