Why melting point of a substance is used as a criterion for testing the purity of the substance.

Explanation

### Step-by-Step Solution:

1. **Definition of Melting Point**: The melting point of a substance is defined as the temperature at which the solid and liquid phases of that substance are in equilibrium. At this temperature, the substance transitions from a solid to a liquid state.

2. **Purity of a Substance**: A pure substance has a specific melting point that is characteristic of that substance. For example, pure ice melts at 0°C, and pure gold has a melting point of 1064°C.

3. **Effect of Impurities**: When impurities are present in a substance, they disrupt the orderly arrangement of the molecules in the solid state. This disruption affects the energy required to break the solid structure, leading to changes in the melting point.

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