What do you observe when : Glauber's salt exposed to air.

Explanation

### Step-by-Step Solution:

1. **Identify Glauber's Salt**:
- Glauber's salt is the common name for sodium sulfate decahydrate, which has the chemical formula Na2SO4·10H2O. This means it contains 10 molecules of water of crystallization.

2. **Understand the Property of Efflorescence**:
- Efflorescent substances are those that lose their water of crystallization when exposed to air. This property is crucial in understanding what happens to Glauber's salt.

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