The method of zone refining of metals is based on the principle of:
- Greater mobility of the pure metal than that of the impurity
- Higher melting point of the impurity than that of the pure metal
- Greater noble character of the solid metal than that of the impurity
- Greater solubility of the impurity in the molten state than in the solid
The Correct Option is D
Solution and Explanation
The method of zone refining of metals is based on the principle of the greater solubility of the impurity in the molten state than in the solid state. Let us explore this concept step-by-step.
Zone Refining Process
- The zone refining process is mainly used for the purification of metals, specifically semiconductor materials and other high-purity materials.
- In this process, a rod of impure metal is passed through an induction heater. This creates a narrow molten zone that moves along the length of the rod.
- As the molten zone moves, impurities preferentially dissolve in the liquid phase because they have greater solubility in the molten state compared to the solid state.
- Consequently, when the zone solidifies again, the impurities are left behind in the molten part and gradually migrate towards the end of the rod after repeated zone passes.
Explanation of Options
- Greater mobility of the pure metal than that of the impurity: This is incorrect. Zone refining does not rely on the mobility of the metal or the impurity but on differential solubility in molten vs. solid states.
- Higher melting point of the impurity than that of the pure metal: This is incorrect. Melting points do not play a direct role in the zone refining principle.
- Greater noble character of the solid metal than that of the impurity: This is incorrect. The process is not based on the noble character of the metal.
- Greater solubility of the impurity in the molten state than in the solid: This is the correct answer, as outlined above. The zone refining method relies on the difference in solubility between the solid and molten states to separate impurities.
Conclusion
Hence, zone refining is a crucial technique for producing high-purity metals and relies on exploiting the differing solubility characteristics of impurities in molten versus solid states.
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