Question

What are the characteristics of the solvent used during purification of copper sulphate by crystallization?
a.) Not dissolved the substance at room temperature.
b.) Dissolved the impurities at room temperature.
c.) Does not dissolve the impurities at high temperature.
d.) All the above.

Answer 1

Hint: Crystallization is mainly based on the difference in solubility of compound and impurities in the solvent. This is a specialized laboratory method for the purification of solid, in which normally water with sulphuric acid is used as a solvent.

Complete Solution :
- In Crystallization a saturated solution of impure substance is prepared in a hot solvent. The impure substance is dissolved in a suitable solvent to reach its nearly saturated solution at high temperature in respect to room temperature. On increasing temperature solubility of solute is increased and a small amount of solvent is required to dissolve the solute in respect to room temperature. When the solution is cooled the pure substance crystalizes and impurities left behind the solution.
- Success of this process depends on the selection of the solvent. So a solvent should have following
characteristics in the Crystallization process -

(A) -The impurities must be least soluble in the solvent at high temperature and more soluble in low temperature.
(B) -The solubility of solute increases in solvent by increasing the temperature of the solution.
(C) - Not dissolve the substance or solute at room temperature and dissolve all impurities at room temperature, so that it can be easily separated from impurities.
(D) - At high temperature it dissolves all the solute and not dissolves the impurities, so that at high temperature impurities will not react with solute particles and are easily separated from the solution by filtration.
So, the correct answer is “Option D”.

Note: A saturated solution is the solution in which the maximum amount of solute can be dissolved in the solvent
Quick cooling gives tiny pure crystals of copper sulphate while slow cooking gives the bigger and impure crystal of copper sulphate.