Chapter 3: Separation
Techniques
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3.1 Chromatography
• Chromatography is a technique of using a
solvent to separate a mixture into its
components
• Example: Paper Chromatography
• Paper chromatography can be use to
separate dyes in ink, amino acids obtain
from proteins, identify toxics
How does chromatography works?
• A spot of food colouring is applied to
chromatography paper
• Once the paper is dipped in ethanol, it
soaks up.
• Ethanol travel up the paper, carrying dyes
along
• Dye that is not very soluble > carried not
far
• Dye that is soluble > Carried far
How can we identify a banned substance
present in food colouring?
• Chromatography was performed on a sample of
food colouring (‘X’) and 4 banned dyes (‘A’, ‘B’,
‘C’, ‘D’)
• If X contains any of the 4 banned dyes, itt is not
safe to be consumed.
Interpreting Chromatography
• Chromatography paper with separated
component is known as chromatogram
Solvent front
The chromatogram shows the
food colouring used in the
experiment was a mixture of two
dyes.
Coloured
spots
Interpreting Chromatography
• Chromatography paper with separated
component is known as chromatogram
Solvent front
The chromatogram has only a
spot, this dye is made up of a
dye. Thus, it’s pure.
What conclusion can be drawn from this
chromatogram?
X
A
B
C
D
• Identical dyes produce spots at same height
• Sample X does not contains the banned dyes A,B,D
• However, X contains the banned dye. Therefore, it must
not be consumed
What conclusion can be drawn from this
chromatogram?
X
A
B
C
D
• Dyes A&D are pure
• Both dye B & C are mixture of 2 different dyes
• Sample X is a mixture of 3 dyes
How do we identify colourless substances?
Chromatography can also be used for colourless
substances such as amino acids. To separate
and analyse colourless substance, we apply a
locating agent on a chromatogram.
If no dye appear after using locating agent, this
means that solvent can’t dissolve the dye
Uses of Chromatography
•
•
•
•
Separate the components in a sample
Identify number of components in a sample
Identify the components present in a sample
Determine whether if a sample is pure
Take Note:
Pencil line
• Why do we use pencil?
– Pencil is insoluble in solvent
– Solvent used cannot above the pencil line.
Separation Techniques
Besides chromatography, there are other
techniques to separate mixtures.
a)A solid from liquid – sand + water : Filtration
b)Solids – Sand + Iron Fillings : Magnetic
Attraction
c) Liquids – Fractional Distillation
d)A liquid form a solution – Distillation
Separating insoluble solid from liquid Filtration
We can use filtration to separate solid particles
from a liquid (often it is water)
Residue
Filter Paper
Filtrate
Example of Small Solid
particles includes sand,
clay, dust particles and
precipitates
Separating insoluble solid from liquid –
Evaporation to Dryness
To recover salt from salt solution, we evaporate
the solution to dryness
Salt Solution
Evaporating Dish
Separating insoluble solid from liquid –
Crystallisation
When a substance decomposes on heating or can
lose its water of crystallisation, we do not
evaporate it to dryness, we crystallise it.
Sugar Decomposes
on heating
How do we purify by crystallisation?
In crystallisation, water is removed by
heating the solution. Heating is stopped at
the stage when a hot saturated solution is
form. If the resulting solution is allowed to
cool to room temperature, the dissolved
solid will be formed as pure crystals.
How do we test for a saturated solution?
• A clean glass rod can be used to test whether
a solution is saturated.
• It is dipped into the solution and removed.
There will be a small amount of solution on
the rod
• If small crystals form on the rod as the
solution cools, the solution is saturated
• We say the solution is at is saturated point or
crystallisation point
• Saturated points = maximum amount of
solute in solvent at certain room temperature.
Separating Solids (E.g.: Salt and Sand)
Methods: By adding a suitable solvent and filtration
Sand (Residue)
Salt (dissolved) +
Water [Filtrate]
1. Pour some distilled
water (dissolve the
salt) into mixture of
common salt and sand.
Stir and warm (to
speed up whole
dissolving processes)
2. Filter the mixture
Separating Solids (E.g.: Salt and Sand)
Methods: By adding a suitable solvent and filtration
Sand (Residue)
Salt (dissolved) +
Water [Filtrate]
Note: If is sugar, continue up till
step 3. As for step 4, it will be
crystallisation.
3. Wash the residue
(remove salt solution,
get rid of salt) with a
little distilled water to
remove all salt solution
from it. Residue =
Sand
4. Evaporate the filtrate to
dryness. Salt is
recovered
Separating a Liquid from a solution
• When a solid dissolves in a solvent, a solution
is formed (i.e.: Salt solution)
• A pure solvent can be separated from a
solution by simple distillation
• Distillation is the process of boiling liquid and
condensing vapour.
• For example, pure water can be obtained
from salt solution by this process
• To collect solute from solvent, we evaporate
solvent.
How does simple distillation work?
1. In the distillation flask,
water boils and enter
condenser
2. In the condenser, water
vapour condenses.
3. Pure water is collected
as the distillate
Boiling chips in the
flask is used to
smoothen boiling.
Things to take note when doing..
• Thermometer: Should be placed beside the
side arm of distillation flask. (Temperature of
thermometer = Boiling point of solvent)
• Condenser: Consists of 2 tubes : Inner tube
and outer wet jacket. Cold running water enters
from bottom of condenser and leaves from top.
If the distillate is volatile, the receiver
can be put in a large container filled
with ice which helps keep temperature
of distillate so that it remains in liquid
state.
Separating Miscible Liquids – Fractional
Distillation
• Unlike oil and water, ethanol and water mix
together completely to form a solution.
They are said to be miscible
• If two liquids are miscible, they must be
separated by fractional distillation.
Separating Miscible Liquids – Fractional
Distillation
A fractionating column, is attached to the round-bottomed
flash and the condenser for fractional distillation. Many glass
beads in the fractionating column provide a large surface area
for vapour to condense on. Other than glass beads, a
fractionating column may be filled with plates or a spiral
Example of mixture of 2 liquids
-
-
Ethanol and water are soluble in each other
To separate these 2 miscible liquid, we use
fractional distillation
The boiling point of ethanol is 78 ℃while the
boiling point of water is 100 ℃
In fractional distillation, liquid with lower
boiling point(ethanol) will be collected first
How does fractional distillation work?
1. In the distillation flask, the
ethanol and water boils and
liquids may distil together
2. Vapours of liquids with
higher boiling points
condense along
fractionating column and reenter the round-bottomed
flask
3. Ethanol distils over. In
condenser, ethanol vapour
condenses.
4. Ethanol is collect as
distillate.
How does temperature change as solution of
ethanol and water undergoes fractional distillation?
Temperature (℃)
1. The temperature of mixture increases as it’s heated
2. At 78 ℃, ethanol distils over. The temperature remains
constant until all ethanol has distilled out of roundbottomed flask
3. Temperature then increased until 100 ℃. At 100 ℃,
water distils over. Temperature remains unchanged as
water is being distilled.
Industrial applications of fractional
distillation
-
-
Fractional distillation is used in industries to
obtain nitrogen, argon, oxygen from air
Fractional distillation can be used to separate
mixtures of liquids such as crude oil
Ethanol is formed when glucose solution
undergoes fermentation in presence of yeast.
We can separate ethanol from glucose solution
by fractional distillation