The method of zone refining of metals is based on the principle of :

Options

A. Greater mobility of the pure metal than that of impurity
B. Higher melting point of the impurity than that of the pure metal
C. Greater noble character of the solid metal than that of the impurity
D. Greater solubility of the impurity in the molten state than in the solid

Answer

**Step-by-Step Solution:**

1. **Understanding Zone Refining**: Zone refining is a technique used to purify metals and semiconductors. It involves melting a small region of a solid material and moving this molten zone along the solid, which helps in separating impurities.

2. **Principle of Solubility**: The key principle behind zone refining is based on the solubility of impurities. In this method, it is observed that impurities tend to have a higher solubility in the molten state of the metal compared to its solid state.

3. **Analyzing the Options**:
- **Option 1**: Greater mobility of the pure metal than that of impurity – This is not the main principle of zone refining.
- **Option 2**: Higher melting point of the impurity than that of the pure metal – This is irrelevant to the process of zone refining.
- **Option 3**: Greater noble character of the solid metal than that of the impurity – This does not relate to the solubility aspect.
- **Option 4**: Greater solubility of the impurity in the molten state than in the solid state – This is the correct principle that zone refining is based on.

4. **Conclusion**: Since impurities are more soluble in the molten state than in the solid state, as the molten zone moves through the solid metal, the impurities are carried along with the molten metal, allowing for the purification of the solid metal.

5. **Final Answer**: Therefore, the method of zone refining of metals is based on the principle of **greater solubility of the impurity in the molten state than in the solid state** (Option 4).

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