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๐ฌ Think about it
When you evaporate salt water you get the salt, but the water escapes. What if you want the water back too? Or want to separate two mixed liquids like acetone and water? There is a method that recovers both. Let us learn distillation.
What is distillation?
- It separates two miscible liquids by heating the mixture.
- The liquid with the lower boiling point vaporises first.
- The vapour is cooled and turned back into liquid.
- It works when boiling points differ by at least about 25 °C.
How Distillation Works
Heat the liquid mixture
↓
Lower-boiling liquid vaporises
It turns to vapour first.
↓
Cool in condenser
Vapour condenses back to pure liquid.
↓
Collect the pure liquid (distillate)
Why can acetone and water be separated by distillation?
- Acetone boils at about 56 °C and water at 100 °C.
- This is a large difference in boiling points.
- So acetone vaporises well before water does.
Example:
On heating the mixture, acetone (56 °C) escapes as vapour first and is collected, leaving water behind.
What is fractional distillation?
- It separates components whose boiling points differ by less than 25 °C.
- It is used to refine crude petroleum into fractions.
- Products include petroleum gas, petrol, kerosene and diesel.
- The gaseous fraction is liquefied under pressure as LPG.
Table 5.5: Boiling points of some compounds
| Solvent | Water | Acetone | Alcohol | Chloroform | Benzene |
|---|---|---|---|---|---|
| Temperature (°C) | 100 °C | 56 °C | 78 °C | 61 °C | 80 °C |
Important Points
- Distillation separates miscible liquids by boiling and condensing.
- It needs a boiling-point difference of about 25 °C or more.
- Fractional distillation handles smaller boiling-point gaps, as in oil refining.
๐ India's Scientific Contributions
- Distillation has long been used to extract fragrances from flowers.
- In Kannauj, the earthy smell after rain is captured as Mitti ka Ittar.
- It uses the traditional Deg-Bhapka distillation method.
- This perfume is in great demand in India and abroad.
๐ Ready to Go Beyond
- A petroleum refinery processes crude oil into useful products.
- Crude petroleum is separated by fractional distillation.
- Products include petroleum gas, petrol, kerosene and diesel.
- The gaseous fraction is liquefied under pressure as LPG.
โ Test Yourself
-
What kind of mixture does distillation separate?
View Answer
Two miscible liquids with a boiling-point difference of about 25 °C or more. -
Why does acetone separate before water?
View Answer
Acetone has a much lower boiling point (about 56 °C versus 100 °C). -
When is fractional distillation used?
View Answer
When boiling points differ by less than 25 °C, as in refining crude oil.
Important Definitions
- Distillation — separating miscible liquids by boiling and then condensing the vapour.
- Fractional distillation — separating liquids with small boiling-point differences (less than 25 °C).
๐ NCERT Question 12 — Why is distillation an
Why is distillation an effective method for separating a mixture of water and acetone?
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๐ NCERT Question 8 — Two miscible liquids, A
Two miscible liquids A (b.p. 60 °C) and B (b.p. 90 °C) — suggest a method to separate them.
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๐ NCERT Question 15 — Examine Fig. 5.26 and
Examine the distillation set-up in Fig. 5.26, identify technique 'S', label A, B, C, and use Table 5.5 to choose which mixtures it can separate.
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