When kept open in air, the crystals of washing soda lose 9 molecules off water to form a monohydrate.
`Na_(2)CO_(3)*10H_(2)O underset("to air")overset("exposed")toNa_(2)CO_(3)*H_(2)O+9H_(2)O` this process is called

Options

A. efflorescence
B. deliquescence
C. dehydration
D. hydration

Answer

To solve the question regarding the process that occurs when washing soda (sodium carbonate decahydrate) loses 9 molecules of water to form a monohydrate, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Compound**: The compound in question is sodium carbonate decahydrate, represented as \( \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \). This indicates that it has 10 water molecules associated with it.

2. **Write the Reaction**: When exposed to air, the washing soda loses 9 water molecules, resulting in the formation of a monohydrate. The reaction can be written as:
\[ \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \xrightarrow{\text{exposed to air}} \text{Na}_2\text{CO}_3 \cdot \text{H}_2\text{O} + 9\text{H}_2\text{O} \]

3. **Understand the Process**: The process involves the loss of water molecules from the hydrate. This is a specific type of reaction where a hydrated compound loses its water of crystallization when exposed to air.

4. **Define the Process**: The term used to describe the loss of water from a hydrate is "efflorescence." Efflorescence occurs when a hydrate loses water to the atmosphere, often resulting in a powdery residue.

5. **Conclusion**: Therefore, the process by which washing soda loses 9 molecules of water to form a monohydrate is called **efflorescence**.

### Final Answer:
The process is called **efflorescence**.