Why does borax swell up on heating?

Explanation

### Step-by-Step Solution:

1. **Understanding Borax Composition**:
- Borax is chemically represented as Na₂B₄O₇·10H₂O, which indicates that it contains sodium (Na), boron (B), oxygen (O), and water molecules (H₂O).

2. **Heating Borax**:
- When borax is heated, it undergoes a thermal decomposition reaction. The heat causes the water molecules (10H₂O) associated with borax to evaporate.

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