When we evaporate a salt solution, we get the salt — but the water is gone.
What if we want the solvent back as well?
And what if a student accidentally mixes acetone and water, two liquids that mix completely? Evaporation cannot help there.
What is distillation and where is it used?
Distillation is used to separate
two miscible liquids
.
Miscible liquids
are liquids that mix into each other uniformly — such as water and acetone.
It can also recover the
solvent
, and can separate a liquid from a solution containing dissolved solids.
Definition
Distillation
is the process of separating a homogeneous mixture of two miscible liquids by heating the mixture until the liquid with the lower boiling point vaporises, and then cooling that vapour back into a liquid.
What idea does distillation work on?
It works on the
difference in boiling points
.
It works when the two miscible liquids differ in boiling point by at least about
25 °C
.
A large difference lets one liquid vaporise before the other has begun to form vapour in any significant amount.
Acetone
Water
Boiling point about 56 °C
Boiling point 100 °C
Boils first and turns to vapour
Stays behind in the distillation flask
Difference in boiling points = 100 °C − 56 °C =
44 °C
, which is comfortably more than 25 °C.
How does distillation separate the two liquids?
Distillation of an acetone and water mixture
Heat the mixture in the distillation flask
acetone has the lower boiling point
↓
Acetone boils first at about 56 °C
it turns into vapour
↓
The vapour passes through the condenser
cooled by circulating water or air
↓
It condenses into pure liquid acetone
collected in a separate conical flask
↓
Water, with its higher boiling point of 100 °C, stays behind in the distillation flask
The pure liquid collected after condensing is called the
distillate
.
What are the two special kinds of distillation?
Two special kinds
Distillation
Separating by boiling point
Steam distillation
Also called hydro-distillation
Used for:
Perfume from flowers
Fractional distillation
For boiling points less than 25 °C apart
Used for:
Refining crude oil
What is steam distillation, or the Deg-Bhapka method?
This process takes the fragrance out of flowers and turns it into perfume.
Two vessels are used — the
Deg
and the
Bhapka
— which is why it is called the
Deg-Bhapka method
.
Deg-Bhapka method
Flowers and water go into a large copper vessel (Deg)
sealed tightly with clay
↓
Heat from below
water and perfume oil boil into vapours
↓
Vapours travel through a bamboo pipe
into a second vessel (Bhapka) kept in a cold water tank
↓
The cold turns the vapour back into liquid
perfume oil is lighter than water, so it floats on top
↓
The floating oil is taken off separately as ittar
๐ฎ๐ณ India's Scientific Contributions
Distillation has long been used to extract fragrances from flowers to make perfumes.
You must have noticed the pleasant earthy smell that rises from the ground after the first rain. In
Kannauj
, a town in Uttar Pradesh known as the perfume capital of India, this fragrance is captured and turned into a natural perfume called
Mitti ka Ittar
.
It is made using the traditional
Deg-Bhapka
distillation method, passed down through generations in Kannauj, and is in great demand in India and around the world.
The
Fragrance and Flavour Development Centre
in Kannauj separates flavour and fragrance components from flowers, leaves and other plant parts, supports farmers in growing fragrance-producing plants, and helps people set up their own perfume businesses.
What is fractional distillation?
Fractional distillation
separates the components of a mixture whose boiling points differ by
less than 25 °C
.
The components separate into different
fractions
, which is where the name comes from.
Refining crude oil in a petroleum refinery is the standard example.
Fractions from crude oil, top to bottom
Petroleum gas — highest in the tower
filled into cylinders under high pressure as LPG
↓
Petrol
for cars
↓
Aviation fuel (kerosene)
for planes
↓
Diesel and lubricating oil
for trucks and machines
↓
Bitumen — the heaviest, left at the very bottom, used for making roads
๐ Ready to Go Beyond
A
petroleum refinery
is an industrial unit where crude oil is processed into useful petroleum products.
Crude petroleum is extracted from the Earth's crust and separated into fractions by fractional distillation.
Crude oil is heated in a furnace and sent into the tower; the lightest fraction rises highest and the heaviest stays lowest.
The gaseous fraction is filled into steel cylinders under high pressure, where it liquefies, and is used as domestic fuel in the form of
LPG
.
๐ Summary - What we have learnt
Distillation
separates two miscible liquids using the difference in their boiling points.
The boiling points must differ by at least about
25 °C
; acetone (56 °C) and water (100 °C) differ by 44 °C.
The lower-boiling liquid vaporises first, condenses in the condenser, and is collected separately.
Steam distillation
(Deg-Bhapka) makes ittar from flowers;
fractional distillation
refines crude oil.
CA Maninder Singh is a Chartered Accountant with 16+ years of practical experience and 20+ years of teaching experience. At Teachoo, he simplifies Accounts, Tax and GST with step-by-step examples so students can apply concepts confidently in exams and real life.
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