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Question
What is the difference between distillation, distillation under reduced pressure and steam distillation ?
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Solution
The differences among distillation, distillation under reduced pressure, and steam distillation are given in the following table.
| Distillation | Distillation under reduced pressure |
Steam distillation |
| It is used for the purification of compounds that are associated with non-volatile impurities or those liquids, which do not decompose on boiling. In other words, distillation is used to separate volatile liquids from non-volatile impurities or a mixture of those liquids that have sufficient difference in boiling points. | This method is used to purify a liquid that tends to decompose on boiling. Under the conditions of reduced pressure, the liquid will boil at a low temperature than its boiling point and will, therefore, not decompose. | It is used to purify an organic compound, which is steam volatile and immiscible in water. On passing steam, the compound gets heated up and the steam gets condensed to water. After some time, the mixture of water and liquid starts to boil and passes through the condenser. This condensed mixture of water and liquid is then separated by using a separating funnel. |
| Mixture of petrol and kerosene is separated by this method. | Glycerol is purified by this method. It boils with decomposition at a temperature of 593 K. At a reduced pressure, it boils at 453 K without decomposition. | A mixture of water and aniline is separated by steam distillation. |
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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 |
Assertion (A): Simple distillation can help in separating a mixture of propan-1-ol (boiling point 97°C) and propanone (boiling point 56°C).
Reason (R): Liquids with a difference of more than 20°C in their boiling points can be separated by simple distillation.
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.
