Question

The zone refining process of metals is based on the principle of:
A. excess notable character of the liquid metal than that of impurity
B. lower melting point of the impurity than that of pure metal
C. greater solubility of impure metal than that of impurity
D. greater solubility of the impurities in the molten state than in the solid

Answer 1

Hint: Zone refining is one of the purifying methods to extract the pure form of metal from the ore such as silicon and germanium. This process is also known as zone melting, travelling melting zone and floating zone process.

Complete Step-by-Step Answer:
- Zone refining is the method which is used to purify the crystal. In this method, the thin part of the crystal undergoes the melting zone to purify.
- After passing through the melting zone, the impurities are collected at its advancing edge.
- Now, the purified crystal will undergo the process of recrystallization on the opposite edge.
- For example, Germanium was the first compound which was purified through the zone refining so that it can be used in the transistors.
- The principle on which this method works is the greater solubility of the impurities in the molten state so that when the crystal undergoes the melting zone all the impurities will melt and separate from the crystal.
-So, option D, is the correct answer.
-If the solubility of impure metal will be greater than the impurity then it will become difficult to separate the pure metal from the impure metal.
-That’s why option C, is an incorrect answer.
- If the solubility of impurity will be lesser than the pure metal then it will become difficult to recrystallize the pure metal.
-So, option B, is also an incorrect answer.
-In option A, it is said that the higher liquidity of metal than the impurity is the key principle but if the melting point of the metal will be high then it will be difficult to recrystallise the pure metal earlier than the impurity.
-So, option A is also an incorrect answer.

Therefore, option D is the correct answer.

Note: The greater solubility of the impurity is necessary so that after coming through the molten state when the solution is cooled only the pure crystal is converted into the solid whereas the impurity remains in the molten state and takes more time to convert into a solid.