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
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 0°C (at atmospheric pressure).
When salt is added it lowers the m.p. to values between –5°C to –25°C, 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 100°C (at atmospheric pressure).
When salt is added, it increases the b.p. of water.

14. Purification and Separation Techniques
Chromatography

15. Determining Purity - Chromatography
Solvent e.g. ethanol
Chromatography paper
Mixture to be separated
Starting line (drawn by pencil)
Chromatography is a technique of using a solvent to separate a mixture into its components.

16. Chromatography - Procedure
Draw a pencil line on a piece of chromatography paper about 2 cm from the end of the paper.
Put a tiny spot of black ink on the pencil line. Allow the spot the dry.
Suspend the paper inside a glass tank containing a small volume of ethanol (solvent).
Ensure that the ink spot is 1 cm above the level of ethanol.
Observe the solvent front carefully as it travels up the paper.
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 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
= (3 s.f.)
Rf = 3.5 cm ÷ 5.2 cm
= (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?X A B C D
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.
salt solution
evaporating dish

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.
residue
filtrate

46. Procedure
3. Wash the residue with a little distilled water to remove all the salt solution from it. The residue is sand.
residue
(sand)

47. Procedure 4. Evaporate the filtrate to dryness. Salt is recovered.
(salt solution)

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
iodine
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?
2. In the condenser, water vapour condenses.
3. Pure water is collected as the distillate.
1. In the distillation flask, water boils and enters condenser.

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?
Boiling chips are used to smooth the boiling.
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:
cold water in
out
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
A fractionating column, is attached to the round-bottomed flask 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.

69. Compare simple distillation with fractional distillation
A fractionating column, is attached to the round-bottomed flask and the condenser for fractional distillation.
Other than glass beads, a fractionating column may be filled with plates or a spiral.
Many glass beads in the fractionating column provide a large surface area for vapour to condense on.

70. Compare simple distillation with fractional distillation
A fractionating column, is attached to the round-bottomed flask 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.

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?
3. Ethanol distils over. In the condenser, ethanol vapour condenses.
2. Vapour of the liquid with higher boiling point condense along the fractionating column and re-enter the round-bottomed flask.
4. Ethanol is collected as the distillate.
1. In the distillation flask, the mixture of ethanol and water boils and both liquids may distil together.

75. How does the temperature change as a solution of ethanol and water undergoes fractional distillation?
Temperature / oC
Time / s
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.
Temperature / oC
Time / s
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.
Temperature / oC
Time / s
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.