Question

Which method is used to obtain copper sulphate from an impure sample?
A.Distillation
B.Fractional distillation
C.Crystallization
D.None of the above

Answer 1

Hint: To answer this question, you must recall the various methods of separation or purification. The method of separation depends on the nature of the substance. For instance, distillation is used to separate substances with lower boiling points and crystallization is used to separate crystalline compounds from impurities. Crystalline solids are separated by crystallization.

Complete step by step answer:
Copper sulphate is a crystalline inorganic compound and is known as copper sulphate pentahydrate. It is blue in colour due to the five water molecules present in its crystal. Thus, it is also known as blue vitriol commonly. On dehydration, it forms grey coloured anhydrous copper sulphate.
Since we know that copper sulphate is a crystalline solid, thus, it can be separated easily from an impure sample by the process of crystallization. The pure crystals of copper sulphate pentahydrate are crystallized out, separating it from the mother liquid containing impurities.

Thus, the correct option is C.
Note:
The crystallization of copper sulphate is a commonly used process for its preparation. The compound is made by electrolytic processes. Sulphuric acid is used in the electrolyte solution and the two electrodes are made up of copper. The anode consists of impure copper while the cathode is pure copper. Copper ions from the impure anode come into the solution, which are then reduced at the cathode to form pure metallic copper. A small amount of sulphuric acid is added which stops the hydrolysis of copper sulphate. From that solution, copper sulphate pentahydrate is crystallized leaving behind the impurities. Then the solution is filtered to obtain the pure crystals of copper sulphate pentahydrate.