A solid compound `'X'` on heating gives `CO_(2)` gas and a residue. The residue mixed with water forms `'Y'`. On passing an excess of `CO_(2)` through `'Y'` in water , a clear solution, `'Z'` is obtained. On boiling `'Z'`, compound `'X'` is reformed. The compound `'X'` is

Options

A. `Na_(2)CO_(3)`
B. `K_(2)CO_(3)`
C. `Ca(HCO_(3))_(2)`
D. `CaCO_(3)`

Explanation

To solve the problem step by step, we will analyze the information given and deduce the identity of compound 'X'.

### Step 1: Identify the nature of compound 'X'
The problem states that a solid compound 'X' on heating gives CO₂ gas and a residue. This indicates that 'X' is likely a carbonate, as carbonates decompose upon heating to release carbon dioxide.

**Hint:** Think about compounds that release CO₂ when heated.

### Step 2: Analyze the residue
The residue left after heating 'X' must be a solid that can react with water to form a new compound 'Y'. This suggests that the residue is likely a metal oxide or hydroxide.

**Hint:** Consider what solid remains after heating a carbonate.

### Step 3: Forming compound 'Y'
When the residue is mixed with water, it forms 'Y'. If we assume that the residue is a metal oxide, it would likely form a metal hydroxide (for example, CaO + H₂O → Ca(OH)₂).

**Hint:** Think about how metal oxides react with water.

### Step 4: Passing CO₂ through 'Y'
The problem states that passing an excess of CO₂ through 'Y' in water gives a clear solution 'Z'. This suggests that 'Y' is a hydroxide that reacts with CO₂ to form a bicarbonate (for example, Ca(OH)₂ + CO₂ → Ca(HCO₃)₂).

**Hint:** Consider the reaction of hydroxides with carbon dioxide.

### Step 5: Reformation of compound 'X'
Finally, when 'Z' is boiled, compound 'X' is reformed. This indicates that 'Z' (the bicarbonate) decomposes back into the original carbonate upon heating (for example, Ca(HCO₃)₂ → CaCO₃ + CO₂ + H₂O).

**Hint:** Think about the thermal decomposition of bicarbonates.

### Conclusion
Based on the analysis, the only compound that fits all the criteria is calcium carbonate (CaCO₃). It decomposes upon heating to form calcium oxide (CaO) and carbon dioxide (CO₂), which then reacts with water to form calcium hydroxide (Ca(OH)₂), and subsequently reacts with CO₂ to form calcium bicarbonate (Ca(HCO₃)₂). When heated, calcium bicarbonate decomposes back to calcium carbonate.

Thus, the compound 'X' is **CaCO₃ (Calcium Carbonate)**.

### Summary of Steps:
1. Identify 'X' as a solid that decomposes to give CO₂.
2. Determine the residue as a metal oxide or hydroxide.
3. Form 'Y' as a hydroxide from the residue.
4. React 'Y' with CO₂ to form 'Z' (a bicarbonate).
5. Conclude that 'X' is reformed from 'Z' upon boiling.