The decahydrate form of sodium carbonate i.e. washing soda on standing in air effloresces and crumbles to powder. The number of water molecule (s) present in the compound formed is :

Answer

To solve the question regarding the number of water molecules present in the compound formed when washing soda (sodium carbonate decahydrate) effloresces, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Compound**:
The compound in question is sodium carbonate decahydrate, which is commonly known as washing soda. Its chemical formula is \( \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \).

2. **Understand Efflorescence**:
Efflorescence is the process where a hydrated salt loses water of crystallization when exposed to air. This leads to the formation of a less hydrated form of the salt.

3. **Determine the Water Loss**:
When washing soda effloresces, it loses some of its water molecules. The question implies that after standing in air, washing soda crumbles to powder, indicating that it has lost water.

4. **Identify the Resulting Compound**:
After efflorescence, washing soda typically loses water and can become sodium carbonate monohydrate. The chemical formula for this compound is \( \text{Na}_2\text{CO}_3 \cdot \text{H}_2\text{O} \).

5. **Count the Water Molecules**:
In the resulting compound \( \text{Na}_2\text{CO}_3 \cdot \text{H}_2\text{O} \), there is only **1 water molecule** present.

### Final Answer:
The number of water molecules present in the compound formed after efflorescence is **1**.

---