Question

What is the principle of zone refining of metals?
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

Zone refining or zone melting is a group of similar methods of purifying metals, in which a narrow region of a crystal is melted, and this molten zone is moved along with the crystal. It is dependent on the solubility of metal.

Complete answer:
In the zone refining process, a circular mobile heater is fixed at one end of the metal rod that is made of the impure metal. Now the circular mobile heater is moved slowly across the metal rod. The metallic impurities melt at the temporary position of this heater. As the impurities through the block, they are concentrated in the melt and are thus transported to the end of the block. The soft pure metal or metalloid is left behind to solidify as the heater moves along the rod. The principle on which the process works is that the impurities in an ore or an ingot are more soluble in the molten state than in the solid state.

Hence, the correct answer to the question is option D.

Note:
Zone refining is a useful method of getting high purity elements such as silicon, gallium and germanium, which are used for application in solar cells. This method is also used for the refining of high purity metals. A thin region of the crystal undergoes melting and this molten zone is now moved across the crystal. In impurities that are present in the metal are thus taken forward by the molten zone and move through the metallic block, leaving behind the solidified pure metal.