A compound X on heating gives a colourless gas. This residue is dissolved in water to obtain Y. excess `CO_(2)` is bubbled through aqueous solution of Y, Z is formed. Z on gentle heating give back X. the X is

Options

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

Answer

To solve the question step by step, we will analyze the information given about the compound X and its transformations.

### Step 1: Identify the initial compound X
The question states that compound X, when heated, gives a colorless gas. The possible candidates for X are:
1. Calcium Carbonate (CaCO₃)
2. Calcium Bicarbonate (Ca(HCO₃)₂)
3. Sodium Carbonate (Na₂CO₃)
4. Sodium Bicarbonate (NaHCO₃)

**Hint:** Think about the thermal decomposition of carbonates and bicarbonates to identify which compound releases a colorless gas upon heating.

### Step 2: Determine the gas produced upon heating
When we heat calcium carbonate (CaCO₃), it decomposes to produce calcium oxide (CaO) and carbon dioxide (CO₂), which is a colorless gas.

**Equation:**
\[ \text{CaCO}_3 \, (s) \, \xrightarrow{\text{heat}} \, \text{CaO} \, (s) + \text{CO}_2 \, (g) \]

**Hint:** Consider the products of heating carbonates and bicarbonates to see which one produces CO₂.

### Step 3: Identify the residue after heating
The residue after heating CaCO₃ is calcium oxide (CaO).

**Hint:** Remember that the solid left after the decomposition of a carbonate is typically an oxide.

### Step 4: Dissolve the residue in water to obtain Y
When calcium oxide (CaO) is dissolved in water, it forms calcium hydroxide (Ca(OH)₂), which is the compound Y.

**Equation:**
\[ \text{CaO} \, (s) + \text{H}_2\text{O} \, (l) \, \rightarrow \, \text{Ca(OH)}_2 \, (aq) \]

**Hint:** Think about the reaction of metal oxides with water to form hydroxides.

### Step 5: Bubbling excess CO₂ through Y to form Z
When excess CO₂ is bubbled through the aqueous solution of calcium hydroxide (Ca(OH)₂), it forms calcium bicarbonate (Ca(HCO₃)₂), which is compound Z.

**Equation:**
\[ \text{Ca(OH)}_2 \, (aq) + \text{CO}_2 \, (g) \, \rightarrow \, \text{Ca(HCO}_3)_2 \, (aq) \]

**Hint:** Consider how carbon dioxide reacts with hydroxides to form bicarbonates.

### Step 6: Gentle heating of Z to regenerate X
When calcium bicarbonate (Ca(HCO₃)₂) is gently heated, it decomposes back into calcium carbonate (CaCO₃), carbon dioxide (CO₂), and water (H₂O).

**Equation:**
\[ \text{Ca(HCO}_3)_2 \, (aq) \, \xrightarrow{\text{heat}} \, \text{CaCO}_3 \, (s) + \text{CO}_2 \, (g) + \text{H}_2\text{O} \, (g) \]

**Hint:** Remember that heating bicarbonates leads to the formation of carbonates and gases.

### Conclusion
Based on the steps above, we conclude that the compound X is calcium carbonate (CaCO₃).

**Final Answer:**
The compound X is calcium carbonate (CaCO₃).