Methods of Purification
(a)Describe methods of separation for the components of the following
types of mistures:
(i)solid-solid
(ii)solid-liquid
(iii)liquid-liquid(miscible)
Techniques to be covered for separations and purification include:
(i)Use of a suitable solvent, filtration and crystallization or evaporation
(ii)Distillation and fractional distillation
(iii)Paper chromatography
(b)
Pure Substances & Mixtures
A pure substance contains only one type
of substance, and is not mixed with
any other substance.
Are the following pure substances or mixtures?
 Clean tap water
 100% orange juice mixture
 Distilled water
mixture
Pure substance
Methods of Purification
Some terms :
•
•
•
•
Solute - the solid that dissolves
Solvent - the liquid that does the dissolving
Solution - solid + solvent
Residue - the insoluble solid trapped in the filter
paper
• Filtrate - the liquid that passes through the filter
paper
• Sublimate - the condensed solid produced
during sublimation
Methods of Purification
The particular method used for
separating any given mixture depends
on the nature (eg. solubility, physical
state) of its constituents.
Filtration
 Used to separate a solid from a liquid
in which the solid is insoluble
mixture of
solid and liquid
residue
filter funnel
filter paper
filtrate
Principle of Filtration
• The separation works only if the particles concerned
are of different sizes, such as larger particles of an
insoluble solid and smaller particles of a liquid or
solution.
Principle of Filtration
The filter material acts as a sieve which allows
the smaller particles to pass through and
keeps the bigger particles behind.
Principle of Filtration
Can a mixture of common salt and water
be separated by filtration? Why?
Filtration - large scale use
• Sand filters used in water treatment plants to
remove solid impurities
• Dissolved substances and bacteria will not be
removed
Decanting
• Separates an insoluble solid from a liquid
• liquid is carefully poured away from the solid
which is usually heavier and settles at the
bottom of the container.
Evaporation (Heating )to Dryness
 used to recover a soluble solid from its
solution
 Only for solids that will NOT decompose on
heating.
 Eg. sodium chloride (common salt) from salt
solution
Crystallisation
 used to recover a soluble solid from its solution
 for solids that decompose on heating
 Eg. Copper(II) sulphate and most other salts
Steps :
• The solution is heated (evaporated) to saturation
point OR ‘heated to remove most of the solvent’
• The saturated solution is left to cool; crystals are
formed.
• The crystals are removed by filtration. To purify
the crystals, they can then be washed with cold
distilled water and dried between filter papers.
Crystallisation - the Principle behind
 Substances are usually more soluble in hot water
than cold water eg. more copper(II) sulphate will
dissolve in water at 80ºC than at 30 ºC.
 When the hot saturated solution is cooled, the
cooled solution is unable to hold as much solute as
when it was hot. The extra solute that cannot
remain dissolved appears as crystals.
Solubility & Crystallisation
Solubility data for NH4Cl in grams/100 mL H2O
• 30oC - 41.4 g
• 50oC - 50.4 g
• 70oC - 60.2 g
• 90oC - 71.3 g
If the water was heated to 90°C, how much of the
solute will be able to dissolve?
If this hot solution is then cooled to 30 °C, how much
solute can the water contain now?
Hence what is the mass of crystals you would expect
to obtain?
Separating a mixture of solids
Method used depends on the nature of the solids :(1) If the mixture of solids behave differently in a
particular solvent , that is, one component is
soluble in it while the other is insoluble…
carefully choose a solvent that will dissolve only one of
the solids
Egs. common salt and sand;
Use water as solvent
naphthalene and sand
Use methylated
spirits as solvent
Separating a mixture of solids
Basic Steps :
Dissolution
Filtration
Evaporation
to dryness
OR
Crystallisation
Separating a mixture of solids
(2) Sublimation
used when one of the
solid sublimes
Eg.
• mixture of iodine +
copper;
• mixture of ammonium
chloride +
sodium chloride
Separating a mixture of solids
(3) Other methods :
 make use of the special properties of
the substance
Eg. mixture of iron filings and sulphur ….
…use a magnet
• Bring a magnet to the mixture.
• The iron filings will be attracted to the magnet
while the sulphur will be left behind.
Separating the Solvent from the Solution
Crystallisation
Evaporation
to dryness
solute
Distillation
solvent
solution
Simple Distillation
 Some Terms Used :
• Distillate
- the liquid that distils over
• Miscible liquids - liquids that mix
completely to form a single layer
• Immiscible liquids - liquids that do not mix
 A solvent can be separated from a solution
and collected by simple distillation.
Principle of Distillation
A liquid boils and turns into vapour at its boiling
point.
When the vapour is condensed, the (pure)
liquid is obtained again.
Simple Distillation
2. .. vapourise.
The vapour rises
up the flask
3. the condenser is
cold, so the vapour
thermometer condenses to liquid
water.
flask
sea water
condenser
Boiling chips
1. Solution is
heated, causing the
solvent to …
distillate
4. Pure water
drips into the
beaker. It is
distilled water.
Simple Distillation
thermometer
Water out
flask
sea water
condenser
Boiling chips
To maintain even
boiling, with not
too much bumping
Water in
Liebig Condenser
Vapour enters
Condensed vapour in liquid
form (distillate) leaves
Direction of water flow
Water
out
Cold
water in
Water flows in anti-current to the flow of vapour.
Liebig Condenser
Vapour enters
Condensed vapour in liquid
form (distillate) leaves
Direction of water flow
Water
Cold
Thisout
is to make sure the coldest part
of the
water
in
condenser is just before the vapour escapes.
Simple Distillation
Simple distillation can be used to obtain
water from salt solution
•
water from copper(II) sulphate solution
•
ether from a solution of sugar in ether
•
Note :
The liquid that distils over is called the
distillate .
The solid that remains in the flask is called the
residue .
Simple Distillation
Qns. : Where is the thermometer placed?
What is the reason for this?
thermometer
flask
sea water
condenser
Boiling chips
distillate
Simple Distillation
Thermometer placed at the side arm of the flask so
that it records the temperature of the vapour as it
enters the condenser.
thermometer
flask
sea water
condenser
Boiling chips
distillate
Separating miscible liquids Fractional distillation
• miscible liquids can only be separated
by fractional distillation if they have
different boiling points .
• Eg. mixture of ethanol and water.
Separating miscible liquids Fractional distillation
Fractionating
column
flask
Boiling chips
thermometer
Water out
condenser
Water in
mixture of ethanol
and water
distillate
Separating miscible liquids Fractional distillation
2. The fractionating
column is packed with
glass beads to increase
its surface area.
Vaporisation followed by
condensation takes place
many times as the
vapour is swept
upwards.
3. Eventually, the
liquid with the
lower boiling point
reaches the top and
distils over.
1. When heated, the
liquid with the lower
boiling point will
vaporize more readily.
Separating miscible liquids Fractional distillation
4. The temperature
stays constant at
78°C. When all
the ethanol has
distilled over, the
temperature reading
rises above 78°C.
At 100°C, water
starts to distil over.
5. The receiver is
changed to collect each
distillate separately.
Separating miscible liquids Fractional distillation
Sketch a graph of temperature versus time to
show the changes in temperature readings
throughout the distillation.
temperature
100°
C
78°
C
time
Separating miscible liquids Fractional distillation
Note :
• The glass beads in the
fractionating column provides a
large surface area so that
condensation occurs more
readily.
• The liquid with the lower
boiling point distils over first,
followed by the liquid with the
next higher boiling point.
glass
bead
s
Separating miscible liquids Fractional distillation
Note :
• If the liquids in the mixture
have the same boiling point,
fractional distillation is not
possible.
• If the difference in boiling point
is great, fractional distillation
occurs readily.
glass
bead
s
Fractional distillation - applications
 Fractional distillation can be used to
separate :• nitrogen and oxygen from liquid air
• the components of crude oil
• ethanol from fermented liquor
Fractional distillation of crude oil
Separating immiscible liquids
 A separating funnel can
be used to separate two
immiscible liquids.
• Eg. water and petrol.
 This method can be used
to separate :
• a mixture of petrol and
water
• engine oil and water
The main points so far…
Mixtures
solid + liquid
(Solid INSOLUBLE in
liquid)
Filtration
Residue
(solid)
Filtrate
(liquid)
liquid + liquid
solid + solid
The main points so far…
Mixtures
solid + liquid
(Solid SOLUBLE
in liquid)
Simple
Distillation
Residue
(solid/
solute)
Distillate
(liquid/
solvent)
liquid + liquid
solid + solid
The main points so far…
Mixtures
YES
solid + solution
Filtration
Residue
(insoluble
solid)
Add suitable
solvent
Filtrate
(solution)
liquid + liquid
solid + solid
Difference
in
solubility?
Crystallisation
Crystals
(soluble
solid)
The main points so far…
Mixtures
liquid + liquid
solid + solid
YES
solid + solution
Filtration
Add suitable
solvent
Difference
in
solubility?
NO
Sublimation
Crystallisation
Other physical
difference?
Egs.
Using
magnet
The main points so far…
Mixtures
Fractional
distillation
liquid + liquid
miscible
liquids
(different
boiling
pts.)
immiscible
liquids
Separating
funnel