What is the nature of elements which are purified by zone-refining method?

Explanation

**Step-by-Step Solution:**

1. **Understanding Zone Refining Method**: The zone refining method is a technique used to purify certain elements, particularly those that can be found in impure forms. It is based on the principle that impurities are more soluble in the molten state of a metal than in its solid state.

2. **Principle of Solubility**: In the zone refining process, when a metal is heated, it melts, and the impurities present in that metal tend to dissolve more readily in the molten state. This property is crucial for the purification process.

3. **Process Description**: The method involves a circular mobile heater that moves along the length of an impure metal rod. As the heater moves, it melts a small zone of the rod. The impurities in the molten zone dissolve into the liquid metal, while the pure metal solidifies behind the moving heater.

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