Question
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Zn-Ag alloy formed in the upper layer of molten lead is skimmed off from the surface of the molten lead by perforated ladles. This alloy contains lead as impurity. This impurity of Pb is removed by:
(A) Distillation process
(B) Cupellation
(C) Liquation
(D) Bett’s electrolysis

Answer
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Hint: Zn and Ag are low melting metals. This means that they will have a lower melting point than their impurities. This process is based on the principle in which metals like silver or gold does not oxidise but other metals which are present as impurity react and oxidised at high temperature and are skimmed off from the surface

Complete step by step answer:
- All the above given options are methods of refining metals.
- Let us first understand what is meant by refining a metal.
- When a metal is extracted by any method, it will surely be contaminated with some impurities.
- Removal of these impurities for getting metals with high purity is called refining.
- Let us look at all the given types of refining methods.
- The distillation process is usually used for metals with low boiling point. We evaporate the impure metal. Hence, we will obtain the pure metal as a distillate.
- The liquation process is a process in which a metal with a low melting point is made to flow on a sloping surface. Hence, the metal will be separated from the impurities which have a higher melting point. The impurities will remain there and the pure metal will flow down the slope.
- Cupellation is a process in which metal is separated from impurities by melting the impure metal in a cupel, which is a flat, porous dish. The cupel should have high-temperature-resistance. After melting, a blast of hot air is directed onto it in a special furnace. The impurities present in the metal will oxidize, partly vaporize and will partly be absorbed into the pores of the cupel.
- The Bett’s electrolysis is a process for the purification of pure lead metal from the impure metal. Thin sheets of pure lead are used as cathode and the impure lead to be purified is used as anode. Pure metallic lead will be obtained at the cathode. The impurities will remain in the solution.
- Now that we know the refining techniques, we can now decide the best technique for removing the lead impurity.
- We can find that the best technique would be to use the liquation method. This is because the liquid method is the best method to remove metals with low melting point and both Ag and Zn are low melting metals.
Hence, the correct answer is option B.

Note:
Bett’s electrolysis method is a very expensive one. And hence is only used when lead is needed to be obtained in an extremely pure form. We can predict which method is the best one to refine a metal by considering both its properties (low melting or low boiling) and its cost.