A compound X of sodium forms a white powder .It is a consituent of baking powder and used in some antacids .when heated it gives a compound Y which is anhydrous and absorbs water to become hydrated salt .When this salt kept open in air looses water molecules in a process called efflorescence .When dissolved in water it forms a strong base and a weak acid Z.
What happens when sodium carbonate hydrate is exposed to air ?

Options

A. It loses one molecule of water .
B. It loses ten molecules of water .
C. It loses nine molecules of water .
D. It dissociates to give `CO_(2)` .

Explanation

To solve the question, we need to analyze the information given and break it down step by step.

### Step-by-Step Solution:

1. **Identify Compound X**:
- The question states that compound X is a sodium compound that forms a white powder, is a constituent of baking powder, and is used in some antacids.
- The compound that fits this description is **Sodium Bicarbonate (NaHCO₃)**.

**Hint**: Think about common compounds used in baking and antacids that contain sodium.

2. **Heating Compound X**:
- When sodium bicarbonate (NaHCO₃) is heated, it decomposes to form:
\[ \text{2 NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{CO}_2 + \text{H}_2\text{O} \]
- The product formed is **Sodium Carbonate (Na₂CO₃)**, which is anhydrous (Y).

**Hint**: Remember that heating can cause decomposition reactions, especially with bicarbonates.

3. **Formation of Hydrated Salt**:
- Sodium carbonate (Na₂CO₃) can absorb water from the environment to form a hydrated salt, specifically **Sodium Carbonate Decahydrate (Na₂CO₃·10H₂O)**.

**Hint**: Hydrated salts contain water molecules in their structure.

4. **Efflorescence**:
- When sodium carbonate hydrate is exposed to air, it loses water molecules in a process called **efflorescence**.
- For sodium carbonate decahydrate, it loses 9 water molecules to become sodium carbonate monohydrate (Na₂CO₃·H₂O):
\[ \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \xrightarrow{\text{exposed to air}} \text{Na}_2\text{CO}_3 \cdot \text{H}_2\text{O} + 9\text{H}_2\text{O} \]

**Hint**: Efflorescence is a common phenomenon for hydrated salts when they are left in open air.

5. **Dissolving in Water**:
- When sodium carbonate (Na₂CO₃) is dissolved in water, it dissociates to form:
\[ \text{Na}_2\text{CO}_3 \rightarrow 2\text{NaOH} + \text{H}_2\text{CO}_3 \]
- Here, sodium hydroxide (NaOH) is a strong base, and carbonic acid (H₂CO₃) is a weak acid (Z).

**Hint**: Consider the properties of the ions produced when salts dissolve in water.

6. **Conclusion**:
- When sodium carbonate hydrate is exposed to air, it loses 9 water molecules, forming sodium carbonate monohydrate. Therefore, the answer to the question is that it loses 9 molecules of water.

**Final Answer**: Sodium carbonate hydrate loses 9 molecules of water when exposed to air.