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Purification and Separation Techniques

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Title: Purification and Separation Techniques


1
Purification and Separation Techniques
  • Purity

2
Determining Purity
A pure substance is made up of only one type of
substance. A pure substance contains only one
type of particles (i.e. one type of atom or one
type of molecule). Thus, elements compounds
are pure substances!
Air is a mixture. However, diamond and graphite
are pure elements.
3
Air A mixture of Elements Compounds
  • Elements
  • Nitrogen, N2 (g)
  • Oxygen, O2 (g)
  • Argon (Noble gases)
  • Compounds
  • Carbon dixoife, CO2 (g)
  • Water vapour, H2O (g)

4
Diamond Graphite To be considered as an
element (Carbon)
5
Diamond Graphite To be considered as an
element Carbon
  • Reason
  • Because both are made of carbon atoms only.
  • One type of atoms present throughout the entire
    structure

6
Diamond Graphite To be considered as an
element Carbon
  • However, the carbon atoms are arranged
    differently in their structures.
  • Thus, they are allotropes of carbon.

Allotropes are same element with different
structures / different arrangements of atoms.
7
Why is it important to determine purity of a
substance?
  • Impurities in drugs must be detected as they may
    cause side effects.
  • Chemicals are often added to food and beverages.
  • It is important to ensure that only the pure
    chemicals are added.
  • In the electronic industry, microchips have to be
    made form pure silicon.
  • Any impurity in the chips can greatly reduce
    their effectiveness.

8
How do we determine the purity of a substance?
  • We can do so by doing one of the following
  • Checking the melting point of a solid
  • Checking the boiling point of a liquid
  • Performing chromatography.

9
Determining Purity by Melting and Boiling Points
A pure solid has an exact and constant melting
point.
A pure liquid has an exact and constant boiling
point.
10
Effect of Impurities on Melting Points
  • Impurities affect the melting point of a
    substance
  • in two ways
  • 1. They lower the melting point (m.p.).
  • The greater the amount of impurities, the lower
    the melting point of the substance.
  • 2. They cause melting to take place over a range
    of temperatures.

11
Effect of Impurities on Melting Points
Example Addition of salt to ice
  • Pure ice melts at exactly 0C (at atmospheric
    pressure).
  • When salt is added it lowers the m.p. to values
    between 5C to 25C, depending upon the amount
    of salt added.
  • Application Salt is spread on icy roads in
    winter to melt the ice.
  • Also, this explain why it is very rarely cold
    enough for sea water to freeze.

12
Effect of Impurities on Boiling Points
  • If a liquid is impure, its boiling point will
    increase.
  • The liquid will also boil over a range of
    temperatures.
  • The greater the amount of impurities, the higher
    the boiling point of the liquid.

13
Effect of Impurities on Boiling Points
Example Addition of salt to water
  • Pure water boils at exactly 100C (at atmospheric
    pressure).
  • When salt is added, it increases the b.p. of
    water.

14
Purification and Separation Techniques
  • Chromatography

15
Determining Purity - Chromatography
Chromatography is a technique of using a solvent
to separate a mixture into its components.
16
Chromatography - Procedure
  1. Draw a pencil line on a piece of chromatography
    paper about 2 cm from the end of the paper.
  2. Put a tiny spot of black ink on the pencil line.
    Allow the spot the dry.
  3. Suspend the paper inside a glass tank containing
    a small volume of ethanol (solvent).
  4. Ensure that the ink spot is 1 cm above the level
    of ethanol.
  5. Observe the solvent front carefully as it travels
    up the paper.
  6. Remove the paper when the solvent front reaches
    nearly to the top of paper.

17
How does chromatography work?
  • To determine the purity of a food colouring
    (which may be made up of different dyes).
  • A spot of the food colouring is applied to the
    chromatography paper.
  • The chromatography paper is dipped in ethanol
    (the solvent).
  • Ethanol travels up the paper, carrying the dyes
    along.
  • A dye that is more soluble in the solvent will
    travel further.

18
How does chromatography work?
  • The principle involved depends upon the relative
    solubilities of the solutes in the dyes in
    solvent.
  • If a dye is put in small spots at the bottom of
    the paper, and another solvent is soaked up the
    paper, the solutes present in the dye dissolve to
    different extent.
  • Some are more soluble in the mobile solvent
    moving up the paper.
  • However, for those that are less soluble do not
    travel very far up the paper.
  • This difference in solubility allows the
    different pigments in the dye to be separated.

19
How do we interpret the result of chromatography?
The chromatography paper with the separated
components is called a chromatogram.
The chromatogram shows that the food colouring is
a mixture of two dyes. It is not pure.
20
How does the chromatogram of a pure substance
look like?
However, if there is only one spot on the
chromatogram, it means the substance is pure.
21
Conduct an online chromatography experiment.
1
  • http//www.sambal.co.uk/chromatography.html

2
Click on me to find out more!!!
22
What are Rf values?
Solvent front
The positions of the solvent front and spot on a
chromatogram depend on how long the experiment
was allowed to run.
Solvent Front The wet moving edge of the
solvent that progresses along the paper
Chromatogram after a period of time.
Chromatogram after a longer period of time.
23
What are Rf values?
The ratio between the distance travelled by the
substance and the distance travelled by the
solvent is a constant. This ratio is called the
Rf value of the substance.
24
How do we measure the distances?
ANSWER Use a ruler to measure the distances for
calculation of Rf value
25
Examples notice the same Rf values
Solvent Front
Solvent Front
Rf 3 cm 4.5 cm 0.667 (3 s.f.)
Rf 3.5 cm 5.2 cm 0.667 (3 s.f.)
Starting Line
Starting Line
Chromatogram after a period of time.
Chromatogram after a longer period of time.
26
Rf values
The Rf value of a substance DOES NOT change as
long as chromatography is carried out under the
same conditions (i.e. same solvent and same
temperature). This property allows us to easily
identify a substance on a chromatogram.
27
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
    and D).
  • If X contains any of the 4 banned dyes, it is not
    safe to be consumed.

28
What conclusions can be drawn from this
chromatogram?
  • Identical dyes produce spots at the same height
    (due to same Rf values).
  • Sample X does not contain the banned dyes A, B
    and D.
  • However, X contains the banned dye C. Therefore,
    it must not be consumed.

29
What other conclusions can be drawn from this
chromatogram?
Unknown dye
  • Dyes A and D are pure.
  • Both dye B and dye C are mixtures of two
    different dyes.
  • Sample X is a mixture of three dyes - two are
    being identified (from dye C) but one dye is
    unknown.

30
How do we identify colourless substances?
Chromatography can also be used for colourless
substances such as amino acids. To separate
and analyse colourless substances, we apply a
locating agent on a chromatogram.
31
What are the uses of chromatography?
  • Chromatography is used to
  • separate the components in a sample,
  • identify the number of components in a sample,
  • identify the components present in a sample,
  • determine the purity of a sample.

32
Purification and Separation Techniques
  • Separating Solids from Liquids

33
Separation Techniques
Besides chromatography, there are other
techniques to separate mixtures.
  • You will learn how to separate the following
  • A solid from a liquid
  • Solids
  • A liquid from a solution
  • Liquids

34
Separating insoluble solid from liquid -
filtration
We can use filtration to separate solid particles
from a liquid (often it is water).
Examples of small solid particles include sand,
clay, dust particles and precipitates.
35
Filtration
Upon filtration, the solid that remains on the
filter paper is called the residue.
The liquid or solution that passes through the
filter paper is called the filtrate.
36
Separating soluble solid from liquid
Evaporation to Dryness
To recover salt from salt solution, we evaporate
the solution to dryness.
37
Separating soluble solid from liquid
Crystallisation
  • Many substances decompose when they are heated
    strongly.
  • E.g. sugar will be decomposed into water and
    carbon when it is being heated strongly.
  • Most crystals, e.g. when heated, blue hydrated
    copper(II) sulfate crystals, CuSO4.5H2O, give off
    water of crystallisation to become white
    anhydrous copper(II) sulfate powders, CuSO4
  • For such substances, evaporation to dryness is
    not a good method of purification, any soluble
    impurity present will be deposited together with
    the required solid.

38
Heating of hydrated copper(II) sulfate, CuSO4.5H2O
39
Separating soluble solid from liquid
Crystallisation
When a substance decomposes on heating or can
lose its water of crystallisation, we do not
evaporate it to dryness. Instead, we
crystallise it.
sugar decomposes on heating
40
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 formed.
  • If the resulting solution is allowed to cool to
    room temperature, the dissolved solid will be
    formed as pure crystals.

41
How do we test for a saturated solution?
  • A saturated solution is a solution that contains
    the maximum amount of solute at a given
    temperature.
  • If more solid is added to the saturated solution,
    the solid will not dissolve.
  • 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 its saturation point or
    crystallisation point.

42
Unsaturated Solution VS Saturated Solution
43
Purification and Separation Techniques
  • Separating Solids

44
Separating Solids e.g. Salt and Sand
  • Method By adding a suitable solvent followed
    by filtration

45
Procedure
1. Pour some distilled water into the mixture of
common salt and sand. Stir and warm the
mixture. 2. Filter the mixture.
46
Procedure
3. Wash the residue with a little distilled water
to remove all the salt solution from it. The
residue is sand.
47
Procedure
4. Evaporate the filtrate to dryness. Salt is
recovered.
48
How do we use a magnet to separate solids?
  • Some metals are magnetic.
  • We can use this property to separate these metals
    (e.g. iron, nickel, cobalt, steel) from mixtures.

49
How do we use sublimation to separate solids?
Video 1
  • Some substances, such as ammonium chloride and
    iodine, sublime.
  • We can make use of this property to separate a
    substance that sublimes from one that does not
  • e.g. salt and iodine.

Video 2
Mixture of salt and iodine
50
Purification and Separation Techniques
  • Separating Liquids from Solution (Distillation)

51
Separating a Liquid from a Solution
  • When a solid dissolves in a solvent, a solution
    is formed.
  • Examples of solutions are salt solution and sugar
    solution.
  • Salt solution
  • Solute _____________
  • Solvent ______________

52
Separating a Liquid from a Solution
  • A pure solvent can be separated from a solution
    by simple distillation.
  • Distillation is the process of boiling a liquid
    and condensing the vapour.

53
Separating a Liquid from a Solution
  • For example, pure water (solvent) can be obtained
    from a salt solution by simple distillation.
  • To collect the solute (salt) from the salt
    solution, we evaporate the solvent to dryness.

54
How does simple distillation work?
55
What steps are taken during distillation?
Thermometer
The thermometer should be placed beside the side
arm of the distillation flask.
56
What steps are taken during distillation?
Thermometer
It should not dip into the solution. This
ensures that the thermometer measures the boiling
point of the substance that is being distilled.
57
What steps are taken during distillation?
If the distillate is volatile, the receiver can
be put in a large container filled with ice.
This helps to keep the temperature of the
distillate low so that it remains in the liquid
state.
58
What steps are taken during distillation?
Condenser
  • The condenser consists of two tubes
  • an inner tube and
  • an outer water jacket.
  • Cold running water
  • enters from the bottom of the condenser and
  • leaves from the top of the condenser

59
Purification and Separation Techniques
  • Separating Immiscible Liquids

60
Separating Immiscible Liquids
  • Liquids that do not dissolve in each other are
    described as immiscible.
  • Oil and water are immiscible in each other.
  • To separate immiscible liquids, we use a
    separating funnel.

separating funnel
oil
water
61
Using a separating funnel to separate immiscible
liquids
1. Pour the mixture of oil and water into the
separating funnel. (Make sure the tap is
closed.)
62
Using a separating funnel to separate immiscible
liquids
2. Support the separating funnel using a retort
stand.
Then place a clean beaker below the separating
funnel.
63
Using a separating funnel to separate immiscible
liquids
3. Allow the liquids to separate completely. This
may take some time. The denser liquid (water)
will be the bottom layer.
64
Using a separating funnel to separate immiscible
liquids
4. Open the tap of the funnel to allow the bottom
layer to drain into the beaker. Close the tap
before the top layer of liquid runs out.
65
Using a separating funnel to separate immiscible
liquids
5. Place another beaker below the funnel. Open
the tap to allow a little of the top layer of
liquid into the beaker. Dispose of the liquid
collected.
Now, the separating funnel contains only oil
while the beaker from step 4 contains only water.
66
Purification and Separation Techniques
  • Separating Miscible Liquids

67
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 a technique called fractional
    distillation.

68
Compare simple distillation with fractional
distillation
69
Compare simple distillation with fractional
distillation
Many glass beads in the fractionating column
provide a large surface area for vapour to
condense on.
70
Compare simple distillation with fractional
distillation
Other than glass beads, a fractionating column
may be filled with plates or a spiral.
71
Application of Fractional Distillation
  • Use in oil refineries to extract the different
    fractions in crude oil.
  • Use to separate different gases in air.

72
An example of a mixture of 2 liquids
  • Ethanol and water are soluble in each other.
  • To separate these 2 miscible liquids, we use
    fractional distillation.

73
An example of a mixture of 2 liquids
  • The boiling point of ethanol is 78 oC while the
    boiling point of water is 100 oC.
  • In fractional distillation, the liquid with lower
    boiling point (ethanol) will be collected first.

74
How does fractional distillation work?
75
How does the temperature change as a solution of
ethanol and water undergoes fractional
distillation?
1. The temperature of the mixture increases as it
is heated.
76
How does the temperature change as a solution of
ethanol and water undergoes fractional
distillation?
2. At 78 C, ethanol distils over. The
temperature remains constant until all the
ethanol has distilled out of the round-bottomed
flask.
1. The temperature of the mixture increases as it
is heated.
77
How does the temperature change as a solution of
ethanol and water undergoes fractional
distillation?
2. At 78 C, ethanol distils over. The
temperature remains constant until all the
ethanol has distilled out of the round-bottomed
flask.
3. The temperature then increases until 100 C.
At 100 C, water distils over. The
temperature remains unchanged as water is being
distilled.
1. The temperature of the mixture increases as it
is heated.
78
What are the industrial applications of
fractional distillation?
  • 1. To obtain nitrogen, argon and oxygen from air.

79
What are the industrial applications of
fractional distillation?
  • 2. To obtain the different fractions of crude
    oil.

80
What are the industrial applications of
fractional distillation?
  • 3. To separate ethanol from glucose solution
  • Ethanol is formed when glucose solution undergoes
    fermentation in the presence of yeast.
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