Question

Crystallisation is better than simple evaporation because:
A. some impurities may remain in the solution after filtration
B. some solids decompose on heating
C. both A and B
D. none of the above

Answer 1

Hint: During evaporation, some impurities may remain intact even after filtration and some solids decompose too on heating. This point should be kept in mind while comparing the two processes, crystallisation and evaporation.

Step by step answer:
The crystallization technique is better than evaporation because the extent of purification with crystallization is very high as it does not require very high-temperature conditions.

Hence, there is no need to boil the solvent. The technique does not require a very high apparatus setup or very costly apparatus setup for performing this purification process. If the compound is soluble in a single solvent and less soluble or insoluble in any other solvents, crystallization can be done by mixing the two solvents.

Only in the crystallization technique, solids do not get separated with solvent and they also do not decompose or get charred on heating to dryness. But, in case of evaporation technique, solids get evaporated with solvent. Impurities also remain dissolved in the solution after filtration, solids might even decompose or might get charred on heating to dryness.

Some impurities contaminate the solid in evaporation technique as we usually allow the solution to heat and to make the solvent evaporated, but this might degrade the solute into some other compound. This does not happen with crystallization. Hence, none of the options satisfy the given statement.

Therefore, we can conclude that Option D is the correct answer.

Note: In crystallization, one does not boil the solvent. It is a natural or artificial process with which a solid is formed. The atoms and molecules are highly organized into crystal structures. They form with precipitation, freezing or rare deposition from a gas.