Question

In zone-refining method, the molten zone:
(A) consists of impurities only
(B) contains more impurity than the original metal
(C) contains the purified metal only
(D) moves to either side

Answer 1

Hint: Zone-refining method comes under the category of refining of the metal. Generally, whenever a metal extracted by any method is usually contaminated with some impurity. To obtain its more purified form, several techniques are used depending upon the differences in properties of the metal and the impurity mixed within it.

Complete step by step solution:
Zone refining is a very useful method to get metals with very high purity such as germanium and silicon. It is also referred to as zone melting, floating zone process and travelling melting zone. Zone refining refers to the method of purifying a given crystal wherein a thin region of the crystal undergoes melting. This zone, known as the molten zone, is now moved across the crystal.
The impurities in the metal are melted at the forward edge by the molten zone and move through the block of metal, leaving behind the solidified pure element. As they move through the block, the impurities in the metal are concentrated in the melt and are transported to one end of the given metal block.
The above discussion was about what zone-refining is. Let's check the process of zone refining:
In the zone refining process, a circular mobile heater is attached at one end of the metal rod which is made up of the impure metal. Now, the circular heater is moved slowly across the metal rod.
The impurities melt at the temporary position of this heater. The melt containing the impurities moves forward along with the heater through the metal rod. The pure metal is left to get solidified as the heater moves along the rod.
As the heater moves forward, the concentration of the impurities in the melt gets increased. This is because the impurities are more soluble in their corresponding molten state. Finally, the impurities are accumulated at one end of the metal rod.
The process described above is repeated a number of times in the same direction. The end of the rod in which the impurities are accumulated is cut off, leaving behind the pure metal.

Form the above discussion, we can easily say that the answer of the above question is (B)- contains more impurity than the original metal.

Note: Remember that zone refining process is an expensive process. Due to this reason, its application is limited to laboratory reagents and valuable chemicals only. Sometimes solid-liquid equilibria are not favourable for all impurities. In those cases, we have to combine zone refining with other techniques to achieve ultra-high purity.