Separation and Purification
Techniques
Soft Toys
Games
Books
But what if…
Sugar
Sand
How to separate them?
Lesson Objectives
At the end of the lesson, students should
be able to:
 Name the 3 methods in which we can
test for pure substances
 Identify the effect of impurities on a
substance.
 Choose suitable separation techniques to
separate a given mixture.
 Explain the working principles behind
each separation technique.
To test for Purity of a Substance
Methods of Testing
Observation
Melting point
If a substance is pure, it will melt
at a fixed temperature.
Boiling point
If a substance is pure, it will boils
at a fixed temperature.
Use of chromatography
If a substance is pure, it will
show only one spot on the
chromatogram.
Effect of impurities in a substance
Attributes
Effect of impurities
Melting point
A substance melts below its
melting point.
Boiling point
A substance boils higher than
its boiling point.
Gives rise to more than one
Use of chromatography
spot on the chromatogram.
Physical Methods of Separation
Only separate the different substances in
a mixture.
 No new substance is formed.

The choice of separation
technique depends on the
nature of the mixture.
Common physical methods to separate a
mixture
◦
◦
◦
◦
Paper chromatography
Filtration
Crystallisation
Simple and fractional distillation
FILTRATION
Solid-Liquid Mixture
Filtration
Solid-liquid mixture (insoluble solids) E.g. sand
in water
A filter paper is used because it contains very tiny
pores.

Mixture of solid
and liquid
Applications

NEWater – purification of water using
microfiltration to remove impurities.
CRYSTALLISATION
Separating soluble solids from a solution
Crystallisation

To separate dissolved pure solid from a
Maximum amount
solution.
of solid dissolved
in a given solvent.
2. Saturated solution
Evaporation of solution
3. Pure Crystals formed.
1. Heat the solution
until saturated.
How to test for saturated solution?

Dip a glass rod into the solution and
removed.

If crystals are formed on the glass rod, it
means that the solution is saturated.

This is the saturation point or
crystallisation point.
Think!!!

Why can’t we just evaporate the solution
to dryness to obtain the crystals?

For some substances, they will
decompose when heated.  charring

When water is removed, any soluble
impurities will be left on the crystal 
not pure.
Mixture of Sand and Sugar

How do I get sand and sugar back?

Filtration  Sand is the residue, filtrate is
the sugar solution

Crystallisation  pure sugar crystals

Is the filtrate always a pure liquid?
SIMPLE DISTILLATION
Separating liquid from a solution
Distillation
To separate a liquid from a solution.
E.g. salt solution, sugar solution
 Involves two physical state changes.

Set-up for distillation
1
Bulb of thermometer placed beside
the side arm of the distillation flask to
ensure accurate measurement of
boiling point.
Liquid is heated until
its boiling point and
changes to vapour.
2
Vapour is cooled
and changes to
liquid (distillate).
Mixture
To ensure smooth
boiling.
Other examples:
1. Marble chips
2. Porcelain chips
Main concept
Boiling point of the liquid
Boiling chips
To ensure smooth boiling
 During boiling of water, big air bubbles
can be seen.
 These air bubbles causes ‘bumping’.
 Boiling chips can reduce this ‘bumping’
effect.

Temperature Profile
Temperature remains unchanged
until all the liquid boils off.
Temperature as
solution is heated
Disadvantage of Simple Distillation

Unable to separate liquids who boiling
point differ by less than 20°C.
Use fractional distillation!
FRACTIONAL
DISTILLATION
Separating miscible liquids
Fractional distillation
For liquids that are miscible and having
different boiling points.
 Miscible – mix together completely to
form a solution. E.g. water and ethanol
 Liquid with lower boiling point distill
over first.
 Liquids with higher boiling point will
return back into the round-bottomed
flask.

Set-up of fractional distillation
Thermometer
Water outlet
Main concept
Boiling point of the liquids
where the liquid with the
lowest boiling point
will distill over first.
Condenser
Fractionating
column
Round-bottomed flask
Liquid-liquid mixture
Water inlet
Distillate
Boiling chips
During fractional distillation,
• The liquid with lowest boiling point will
distill over to the condenser first.
•
The vapour of liquids with higher boiling
point condenses along the fractionating
column and re-enter the round-bottomed
flask.
Fractionating Column

Packed with glass beads

Provide large surface area for
evaporation and condensation of liquids
with different boiling points  for
effectively separation.
Temperature Profile
Liquid with lowest boiling
point will distill over first
Second liquid distill
over.
First liquid distill over
upon reaching its
boiling point
Industrial Applications
Separation of liquid air
 Separation of crude oil
 Separation of alcohol from fermented
solution.

Think!!!

What is the difference between
distillation and fractional distillation?
Simple distillation
Fractional
Distillation
No fractionating
Has a fractionating
column
column
Separate liquids whose Able to separate
boiling points differs
liquids whose boiling
point differs by less
more than 20°C
than 20°C
Think!!

Is it possible to obtain salt from seawater?
Yes

However, industrially, we use reverse
osmosis to obtain salt (Desalination).
What is the reason?
Distillation exhaust a lot of energy,
resulting in high production cost.
TYPES of MIXTURES
MIXTURE
SUBSTANCES TO
OBTAIN
METHODS
Salt mixed with
broken glass pieces
Salt crystals
Evaporation
Sea Water
Pure Salt
Desalination/
Distillation
Copper (II) Sulphate
solution
Copper (II) Sulphate
Crystals
Crystallisation
Ink
Pure Water
Paper
Chromatography
Wine
Ethanol
Fractional Distillation
Check your Understanding
Which process is used to separate

Three water-soluble dyes?

Two miscible liquids with boiling points of
78°C and 100°C? Fractional distillation

Water containing an insoluble solid?
Chromatography
Filtration

Water containing a dissolved solid?
Crystallisation
Check your Understanding

A mixture contains the following three liquids that are
completely miscible:
Liquid
propanone
Ethanol
Water
Boiling point (°C )
48
78
100

The liquid can be separated by fractional distillation.
State, with a reason, which liquid will distill over first.
Propanone. It has the lowest boiling point among the 3
liquids.

Name an industrial process that involves fractional distillation.
Fractional distillation of crude oil
What did you learnt today?
3 methods to test for purity
 Effect of impurities in a substance
 3 separation techniques and their working
principles

Summary
Fractional distillation
Simple distillation
To separate miscible
liquids
To separate solvent
from solution
Separation
technique
Filtration
To separate
solid from a
liquid
To separate solids
(only one is
soluble in a
solvent)
To separate
solid from a
solution
Crystallisation
Online Quiz
http://www.sciencequiz.net/jcscience/jcchemis
try/septechniques/septechniques1a.htm