A colourless water soluble solid (X) on heating gives equimolar quantities of (Y) and (Z). Y gives dense white fumes with HCl and Z does so with `NH_(3)`. Y gives brown ppt. with `K_(2)HgI_(4)` (Nessler's reagent) and Z gives white precipitate with nitrates of `Ag^(+), Pb^(2+) and Hg^(+)`. What is X ?

Options

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

Explanation

To solve the problem, we need to analyze the information given about the colorless water-soluble solid (X) and the products (Y and Z) formed upon heating.

### Step-by-Step Solution:

1. **Identify the Products (Y and Z)**:
- The solid (X) on heating produces equimolar quantities of (Y) and (Z).
- Y gives dense white fumes with HCl.
- Z gives dense white fumes with NH3.

**Hint**: Dense white fumes are typically indicative of the formation of ammonium chloride (NH4Cl) when NH3 reacts with HCl.

2. **Analyze the Reactions**:
- Since Y gives dense white fumes with HCl, it is likely to be NH3 (ammonia).
- Since Z gives dense white fumes with NH3, it is likely to be HCl (hydrochloric acid).

**Hint**: Think about the common reactions that produce dense white fumes, especially involving NH3 and HCl.

3. **Check the Precipitation Reactions**:
- Y (NH3) gives a brown precipitate with K2HgI4 (Nessler's reagent).
- Z (HCl) gives white precipitates with AgNO3, Pb(NO3)2, and Hg(NO3)2.

**Hint**: Remember that NH3 reacts with Nessler's reagent to form a brown precipitate, while HCl reacts with silver, lead, and mercury nitrates to form white precipitates.

4. **Determine the Original Solid (X)**:
- Since heating (X) produces NH3 and HCl in equimolar quantities, we can deduce that (X) is likely to be NH4Cl (ammonium chloride).
- Upon heating NH4Cl, it decomposes into NH3 and HCl.

**Hint**: Look for a compound that can yield both NH3 and HCl when heated.

5. **Final Conclusion**:
- The colorless water-soluble solid (X) is NH4Cl (ammonium chloride).
- This satisfies all conditions: it is colorless and water-soluble, and upon heating, it produces equimolar amounts of NH3 and HCl, which behave as described in the problem.

### Answer:
The solid (X) is **NH4Cl (ammonium chloride)**.

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