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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?
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)
- 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.
Why can acetone and water be separated by distillation?
Acetone
Water
Boils at about
56 °C
Boils at
100 °C
Vaporises first when the mixture is heated
Still liquid while the acetone is boiling off
The vapour is cooled in the condenser and collected
Left behind in the distillation flask
Gap = 44 °C — comfortably above the ≈25 °C distillation needs
So the two are separated cleanly
- 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?
Fractional distillation
Fractional
split into several useful fractions
distillation
boil, then condense
↓
Fractional distillation
= for boiling points less than 25 °C apart. Crude oil → LPG, petrol, kerosene, diesel, bitumen.
Simple distillation
Fractional distillation
Boiling points differ by
at least about 25 °C
Boiling points differ by
less than 25 °C
Separates two liquids, or a liquid from a dissolved solid
Separates a mixture into many fractions at once
Acetone and water
Crude petroleum → petroleum gas, petrol, kerosene, diesel, lubricating oil, bitumen
- 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 effective
Why is distillation an effective method for separating a mixture of water and acetone?
View Answer →
๐ NCERT Question 8 — Two miscible liquids, A and
Two miscible liquids, A and B, are present in a mixture. The boiling point of A is 60 °C and the boiling point of B is 90 °C. Suggest a method to separate them. Also, draw a labelled diagram of the method suggested.
View Answer →
๐ NCERT Question 15 — Examine Fig. 5.26.
Examine Fig. 5.26.
(i) Identify the separation technique marked as ‘S’.
(ii) Label the apparatus A, B and C.
(iii) Which of the following mixtures can be separated by the technique identified above? Use the data given in Table 5.5. Mixtures:
(a) water — acetone (b) water — salt
(c) acetone — alcohol (d) sand — salt
(e) alcohol — chloroform (f) alcohol — benzene
View Answer →
(i) Identify the separation technique marked as ‘S’.
(ii) Label the apparatus A, B and C.
(iii) Which of the following mixtures can be separated by the technique identified above? Use the data given in Table 5.5. Mixtures:
(a) water — acetone (b) water — salt
(c) acetone — alcohol (d) sand — salt
(e) alcohol — chloroform (f) alcohol — benzene
