Question

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

• A. Greater mobility of the pure metal than that of the 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 1

The Correct Option is D

The method of zone refining of metals is a sophisticated technique used to obtain metals of very high purity. This method is particularly beneficial for producing semiconductor-grade materials like silicon or germanium. The principle underlying this process involves the difference in solubility of impurities in the solid and molten states of a metal.

1. Principle of Zone Refining:
   • The correct principle for the method of zone refining is: "Greater solubility of the impurity in the molten state than in the solid." This means that when a metal is in its liquid (molten) form, impurities are more soluble compared to when the metal is in its solid form.
2. Process Explanation:
   • During zone refining, a small region of the metal rod is melted with the help of a heating coil. As this molten zone is slowly moved along the length of the metal rod, impurities concentrate in the liquid phase.
   • This movement allows the molten zone to sweep impurities to one end of the rod, thereby purifying the remaining solid metal.
3. Justification of the Correct Option:
   • Greater solubility of the impurity in the molten state than in the solid: This is the correct answer as it describes the key physical property exploited in the zone refining process.
   • Greater mobility of the pure metal than that of the impurity: This option is inaccurate in the context of zone refining, as the process does not primarily depend on the mobility of atoms.
   • Higher melting point of the impurity than that of the pure metal: Although this might be true for some impurities, it is not the principle behind zone refining.
   • Greater noble character of the solid metal than that of the impurity: This option is unrelated to the principle of zone refining.

In conclusion, the effectiveness of zone refining is based on the differential solubility of impurities between the solid and molten states of the metal, a principle that is utilized to achieve significant purification.