Chemistry Handout 1 REF #: 001
Separation Methods and Techniques
Method: Filtration
Use: to separate suspended or settled solids (insoluble)
from a liquid
Basis for Separation: differences in particle size of the
various components
Retains: can be used to keep either the liquid or
solids of even both
Examples: sand and water, wood chips and water,
Styrofoam particles from water
Method: Evaporation
Use: to separate solid solutes from a solution. The
solute must not:
1. Decompose on heating
2. Contains water of crystallization
Basis for Separation: Application of heat which
results in the evaporation of the solvent
Retains: the solid solute
Examples: salt and water, does sucrose contain water
of crystallization
Method: Crystallization
Use: to separate solid solutes from a solution
especially if the solid contains water of
crystallization.
Basis for separation:
Retains: Solid solute
Examples: Copper sulphate from copper sulphate
solution
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Chemistry Handout 1 REF #: 001
Method: Separating funnel
Use: to separate immiscible liquids
Basis for Separation: immiscibility of liquids
Retains: can retain all components
Examples: oil and water
Method: Paper Chromatography
Use: to separate several solutes, usually
coloured, present in a solution. Paper
Chromatography involves a stationary
phase (the paper) and a mobile phase
(the solvent).
Basis for Separation: The solvent moves
up the paper separating the dyes
according to:
Their solubility in the solvent-
most soluble moves fastest
Their attraction for the paper-
least attracted moves fastest
Retains: solutes
Examples: dyes in black ink, pigments in chlorophyll
Method: Solvent Extraction
Use: to separate solutes from a solution when one of the solutes is soluble in a second immiscible
solvent.
Basis for Separation:
The desired component being more soluble in one solvent than in the other
The immiscibility of the liquids
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Chemistry Handout 1 REF #: 001
Retains: solutes
Examples: sodium chloride and iodine in trichloroethane and water (iodine is not very soluble in water)
Method: Sublimation
Use: to separate a solid which sublimes from a mixture of
solids.
Basis for Separation: One solid sublimes and the other does
not
Retains: solutes
Examples: E.g, to separate iodine or ammonium chloride
from a mixture containing either substance.
Method: Simple Distillation
Use: to separate and retain
the solvent from a solution.
Basis for Separation: Wide
difference in boiling point
between solute and solvent
Retains: both solute and
solvent
Example: to obtain distilled
water from sea water
Method: Fractional
Distillation
Use: to separate a solution
of liquids
Basis for Separation: the
miscible liquids have small
differing boiling points
(Miscible liquids are liquids
which mix completely)
Retains: all components
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Chemistry Handout 1 REF #: 001
Example: E.g ethanol (BP-78°C) and water (BP-100°C).
The Extraction of Sucrose from Sugar Cane
Sucrose is a carbohydrate, a disaccharide consisting
of one molecule of glucose joined to one molecule
of fructose by a glycosidic bond, its chemical
formula is C12H22O11.
The cane is cut from the stalk in the fields, and enters the factory.
It is then cut by knives and crushed in a series of mills to release the juice.
Water is then sprayed on the cut and crushed cane to dissolve the sugar and concentrated juice from the cells.
The fibres are removed and burnt in the boiler furnace, which converts water into steam to drive turbines to
generate electricity.
The extracted mixture of dilute juice contains sugars and other soluble, partially soluble and insoluble materials. It
is allowed to settle in a tank.
Lime, Ca(OH)2 is added to the settling tank to:
- Neutralize the acids, this prevents the
inversion of sucrose to glucose and fructose
- Precipitate impurities- the lime causes the
insoluble and sparingly soluble substances to
coagulate so the settle out. Adding lime to
the mixture clarifies it.
The mixture is filtered in a rotary filter and
the mud is washed to extract all the sucrose.
The mud is returned to the fields
The clear juice is heated in a series of
evaporators at reduced pressure. This:
- Makes the sugar solution concentrated and
by the end it is very concentrated and is
called syrup.
- The steam from the boiler furnace is used
to heat the first evaporator.
- The steam from the first evaporator is used to heat the second and so on.
Vacuum distillation (distillation at low pressure which reduces the boiling point and thus the temperature) is then
used to concentrate the syrup even further by removing almost all of the water. This produces a heavy, viscous (thick and
sticky) substance called massecuite. The low pressure in vacuum distillation prevents charring and caramelisation.
The massecuite is allowed to cool and as it does, crystals of sugar form. This is called crystallization.
The massecuite is put into perforated vessels which are spun at very high speed. The liquid part (molasses) is flung
through the perforations, leaving the sugar crystals inside. This separation process is called centrifugation.
The molasses may:
- Returned for further crystallization
- The final molasses is used to manufacture rum, alcohol, animal feed, etc.
The unrefined sugar (refined-extremely processed with no nutritional value) then sent to the local market or
exported.
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Chemistry Handout 1 REF #: 001
MCQ- Circle the correct answer
1. The separation technique shown in the diagram is:
A. Filtration C. Distillation
B. Fractional Distillation D. Chromatography
2. Which of the following techniques would be best to separate sand and water?
A. Evaporation B. Crystallization C. Filtration D. Separating funnel
3. Which of the following methods would not be used to separate an insoluble solid
and a liquid? A. Evaporation B. Chromatography C. Decanting D.
Filtration
4. Which of the following pairs of separating techniques will both separate salt from a
mixture of salt and water?
A. Chromatography and evaporation C. Decanting and distillation
B. B. Decanting and filtration D. Distillation and evaporation
5. Which of these methods can be used to separate a soluble solid from an insoluble solid?
A. Mixing the mixture with water and then filtering it. Then we could evaporate the water from the resulting
liquid.
B. Mixing the mixture with sand and then filtering it. Then we could evaporate the sand from the resulting
solid.
C. Mixing the mixture with hydrogen gas, and then filtering it. Then we could evaporate the solids out.
D. None of the above
6. If you want to make pure drinking water from sea water, what process would you use?
A. Filtration B. Distillation C. Evaporation D. Separating Funnel
7. In chromatography, where are the spots of coloured substances placed?
A. Randomly on a piece of paper C. On a horizontal line on the paper
B. On a vertical line on the paper D. None of the above
8. What is the correct order for obtaining salt from a mixture containing sand and salt?
A. Dissolving in water filtration evaporation C. Filtration dissolving in water evaporation
B. Evaporation filtration dissolving in water D. Filtration evaporation dissolving in water
9. Pure water can be separated from inky water by simple distillation because:
A. Water and ink have different boiling points C. Ink evaporates leaving the water behind
B. Water evaporates leaving the ink particles behind D. All of the above
10. Which of the following diagrams represents a mixture of elements?
(Note: each ball represents an atom)
Structured Responses
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Chemistry Handout 1 REF #: 001
1. Describe how you would separate the following:
a) A suspension of chalk in water d) Two miscible liquids, one which boils at 60 0C and the other of which boils at
1000C
b) Powdered iodine and sand e) Tetrachloromethane and water (tetrachloromethane does not dissolve in
water)
c) Powdered copper sulphate and powdered copper f) Pigments obtained from various crushed petals
2. (Experiment) The figure below shows the apparatus which is used to measure the solubility of a solute
(potassium nitrate) in water at a particular temperature.
Procedure: i) The water bath is kept as close as possible to the
temperature at which the solubility is to be measured.
ii) Solute is added to the water in the test tube, and the solution
is stirred. Solute is added until no more will dissolve, no matter
how much the solution is stirred.
iii) An evaporating basin is weighed.
iv) Some of the solution from step ii) is carefully taken from the
test tube with a warm pipette. It is transferred to the
evaporating basin and weighed immediately.
v) The solution in the basin is carefully evaporated until all the
water has gone and only dry solute remains.
vi) It is weighed again
a) Referring to step step i) above, state two things the
experimenter can do to keep the temperature of the water
bath as close as possible to the required level.
b) Referring to step ii) above, what is the name given to the
solution formed?
The results from the experiment are summarized below:
Temperature at which experiment was performed = 40 0C Mass of evaporating basin = 30.0 g
Mass of Basin + solution = 46.4 g Mass of basin + solute after evaporation = 36.4 g
Mass of solute = _______________________ g Mass of water in solution = ____________________________ g
c) Fill in the blanks above. (Show working)
d) Determine the solubility of the potassium nitrate at the required temperature.
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Chemistry Handout 1 REF #: 001
3. The figure shows a chromatogram. Study it and answer the questions below.
i) Why is there only one spot for the green pigment and for the yellow pigment?
ii) The brown pigment is made up of three colours. Name them
iii) Calculate the Rf value for the red, blue, green and yellow pigments.
iv) Account for the difference in the four R f values calculated above
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