Viva Questions with Answers on Purification of Chemical Substances by Crystallisation

1. Define the term ‘crystallisation’.
Ans. The substances when present in well-defined geometrical shapes are called crystals. These are formed when a hot saturated solution of the salt is allowed to cool slowly and undisturbed. This process is termed as crystallisation.
2. What is solubility ?
Ans. It is the amount of the solute which when dissolved in 100 gms of the solvent provides a saturated solution.
3. Why is crystallisation done ? 
Ans. Crystallisation enables to prepare a substance in state of highest purity.
4. What is filtration ?
Ans. It is a process of separating insoluble substances by passing the solution through a filter paper.
5. What is Kipp’s waste ?
Ans. It is a’ mixture left behind after production of H2S gas by reaction between FeS and dilute H2S04. It mainly contains FeS04 and unreacted dilute H2S04.
6. What is meant by the term, ‘water of crystallisation’ ? 
Ans. Water of crystallisation is the definite number of water molecules that is present in lose combination with one formula unit of the compound.
7. Explain the term—saturated solution.
Ans. A solution in which no more of solute can be dissolved at a particular temperature is known as saturated solution.
8. Why is solution not heated to dryness to get crystals ?
Ans. Heating the solution to dryness will not remove soluble impurities and crystals of very poor quality are obtained.
9. What is characteristic of crystals ?
Ans. Crystals have well defined geometry and shape.
10. Why is the hot saturated solution not cooled suddenly ?
Ans. By allowing saturated solution to cool slowly, crystals grow in size. It helps in their better separation as units rather than giving a massy substance of no proper geometry.
11. What is the term ‘seeding’ ?
Ans. Sometimes on cooling the saturated solution, crystallisation does not occur. A crystal of same substance is placed in the saturated solution which causes seeding. It helps in quick separation of crystals from saturated solution.
12. What is green vitriol ?
Ans. It is hydrated ferrous sulphate \(FeSO_{4}.7H_{2}O\).
13. What is mother liquor ?
Ans. The liquid left behind after the separation of crystals from a saturated solution is known as mother liquor.
14. Name the different steps involved in the process of crystallisation ?
Ans. The various steps are :
(i) Preparation of the solution. (ii) Filtration of the solution.
(iii) Concentration of the solution. (iv) Cooling of the solution slowly.
(v) Separation and drying of the crystals.
15. What are the formulae of blue vitriol, potash alum and green vitriol crystals ?
Ans. The formulae are (\(CuSO_{4}.5H_{2}O\)),(\({K_2}S{O_4}.A{l_2}{(S{O_4})_3}.24{H_2}O\)),\(FeSO_{4}.7H_{2}O\).
16. What is the formula of benzoic acid ?


17. What happens when the following crystals are heated separately ?
(i) Blue vitriol (ii) Potash alum (iii) Benzoic acid
Ans. (i) It changes into white powder due to loss of water of crystallisation.
(ii) It changes into fluffy white mass.
(iii) It undergoes sublimation.

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