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Question
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.
Solution
When the difference in boiling points of two liquids is not much then simple distillation cannot be used to separate them. Vapours of such liquids are formed within the same temperature range and are condensed simultaneously. The technique of fractional distillation is used in such cases. In this technique, vapours of a liquid mixture are passed through a fractionating column before condensation. The fractionating column is fitted over the mouth of the round bottom flask. The liquid (A) with low boiling point will distill first.
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Define Distillation
Match the type of mixture of compounds in Column I with the technique of separation/purification given in Column II.
| Column I | Column II |
| (i) Two solids which have different solubilities in a solvent and which do not undergo reaction when dissolved in it. | (a) Steam distillation |
| (ii) Liquid that decomposes at its boiling point | (b) Fractional distillation |
| (iii) Steam volatile liquid | (c) Simple distillation |
| (iv) Two liquids which have boiling points close to each other | (d) Distillation under reduced pressure |
| (v) Two liquids with large difference in boiling points. | (e) Crystallisation |
