Q62 of 66 Page 156

Benzoic acid is an organic compound. Its crude sample can be purified by crystallisation from hot water. What characteristic differences in the properties of benzoic acid and the impurity make this process of purification suitable?

According to the principle of crystallisation, we know that the solute must be less soluble in the cold water and more soluble in hot water, or any other solvent like water.

Now, Benzoic acid is soluble in water in lesser amount as it is an organic compound and is less polar, whereas the water molecule is very polar. But on increasing the temperature of the water we observed that the benzoic acid is more soluble in hot water as compared to cold water, and the other reason is that the impurities are almost insoluble in water or completely soluble in cold water, so that they either can be filtered from the benzoic acid solution (if not soluble) or they remain dissolved even after cooling the solution(if completely soluble in cold water), and do not interfere with recrystallization of the benzoic acid, as we can discard the extra solution once the crystallisation process is complete and we obtain the purified benzoic acid.


Hence the characteristic properties of Benzoic acids are:


Benzoic acid is relatively more soluble in hot water as compared tothe cold water.
(ii) Impurities present in benzoic acid are insoluble in water or more soluble in water so that they remain in the solution.


More from this chapter

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60

In the following questions a statement of Assertion (A) followed by a statement of

Reason (R) is given. Choose the correct option out of the choices given below each question.


Assertion (A): Components of a mixture of red and blue inks can be separated by distributing the components between stationary and mobile phases in paper chromatography.


Reason (R): The coloured components of inks migrate at different rates because paper selectively retains different components according to the difference in their partition between the two phases.


(i) Both A and R are correct and R is the correct explanation of A.


(ii) Both A and R are correct but R is not the correct explanation of A.


(iii) Both A and R are not correct.


(iv)A is not correct but R is correct.


 61

What is meant by hybridisation? Compound CH2 = C = CH2 contains sp or sp2 hybridised carbon atoms. Will it be a planar molecule?

 63

Two liquids (A) and (B) can be separated by the method of fractional distillation. The boiling point of liquid (A) is less than boiling point of liquid (B). Which of the liquids do you expect to come out first in the distillate? Explain.

 64

You have a mixture of three liquids A, B and C. There is a large difference in the boiling points of A and rest of the two liquids i.e., B and C. Boiling point of liquids B and C are quite close. Liquid A boils at a higher temperature than B and C and boiling point of B is lower than C. How will you separate the components of the mixture. Draw a diagram showing set up of the apparatus for the process.