1. Separation Techniques
CHAPTER 3
Separation Techniques
© 2013 Marshall Cavendish International (Singapore) Private Limited

2. Learning Outcomes At the end of this chapter, you should be able to:
describe methods of separation and purification by:
the use of a suitable solvent
filtration
crystallisation
evaporation
sublimation
distillation and fractional distillation (with particular references to the fractional distillation of crude oil, liquid air and fermented liquor).
use of separating funnel
paper chromatography.
describe paper chromatography and interpret chromatograms including comparison with “known” samples and the use of Rf values.
explain the need to use locating agents in the chromatography of colourless compounds.
suggest suitable methods of purification, given information about the substances involved to separate the following types of mixtures: i) solid-solid, ii) solid-liquid, iii) liquid-liquid (miscible and immiscible).
deduce from the given melting point and boiling point the identities of substances and their purity, as well as to describe heating and cooling graphs/curves.
explain that the measurement of purity in substances used in everyday life, e.g. foodstuff and drugs, is important.
describe methods associated with the drying and collection of gases. 2

3. Separation Techniques
Chapter 3
3.1 Obtaining Pure Substances from Mixtures
3.2 Separating a Solid from a Liquid
3.3 Separating Solids
3.4 Separating a Liquid from a Solution
3.5 Separating Liquids
3.6 Chromatography
3.7 Determining Purity
3.8 Methods for Collecting Gases
3.9 Methods for Drying Gases

4. 3.1 Obtaining Pure Substances from Mixtures
A pure substance is made up of one single element or compound. It is not mixed with any other substance.
Pure white diamond contains only one type of element – Carbon.
Diamond

5. 3.1 Obtaining Pure Substances from Mixtures
A mixture is made up of two or more substances that are not chemically combined together.
Orange juice contains many different types of compounds.
Air is a mixture of different gases that are not chemically combined together.
Air
Orange juice

6. 3.2 Separating a Solid from a Liquid
Filtration
Filtration is a process of separating an insoluble solid from a liquid.
A filter funnel and a filter paper are usually used.
Examples of mixtures:
sand and water
chalk powder and water (suspension)
clay and water
a mixture where solid particles are found throughout the liquid 6

7. 3.2 Separating a Solid from a Liquid
Large insoluble solid particles are trapped by the filter paper in the filter funnel.
Filtration
mixture of insoluble solid in a liquid
Clicking on the URL icon will link you to a simple animation that illustrates the process filtration. Video duration 0:47 minute.
small particles of liquid pass through
URL 7

8. Separation of Sand and Water
3.2 Separating a Solid from a Liquid
Separation of Sand and Water
Upon filtration, the solid that remains on the filter paper is called the residue.
(sand)
The liquid or solution that passes through the filter paper is called the filtrate.
With reference to the examples in this slide, students can be asked what, in each example, are the residue and filtrate.
(water) 8

9. Evaporation to Dryness
3.2 Separating a Solid from a Liquid
Evaporation to Dryness
This is a process of obtaining a soluble solid from a solution by heating the solution until all the water has boiled off.
Example:
salt (sodium chloride) and water
Students should know that a solution is made up of a solute and a solvent. If not, use the example above to illustrate that salt (the soluble solid) is the solute and water is the solvent. Solute + Solvent → Solution. 9

10. Separation of Salt from Water
3.2 Separating a Solid from a Liquid
Separation of Salt from Water
URL
Water in the solution is lost to the atmosphere.
evaporating dish
salt solution
This process can be easily carried out in the laboratory as a teacher demonstration. If not, it would be good if a video on this process/picture of salt remaining in evaporating dish is shown.
Note: Water is not recovered – lost to atmosphere.
To point out to students that the residue may contain other impurities. Thus, the solid obtained by this method may not be always pure, any soluble impurities will be left together with the solid. Also point out that safety goggles must be worn as very hot concentrated liquid can sputter from the dish.
Clicking on the URL icon will link to a simple experiment done by students for a Science project on Evaporation to Dryness. There is a slight difference between the video and the slide. In the slide, we make use of heating with a Bunsen burner while the video portrays evaporation under the Sun. But the concept behind evaporation to dryness is similar. Video duration: 1.54 minutes.
Salt remains as a white residue in the evaporating dish. 10

11. Evaporation to Dryness
3.2 Separating a Solid from a Liquid
Evaporation to Dryness
Not all soluble substances can be obtained by evaporation to dryness.
Example: Sugar decomposes on heating
The solid obtained by evaporation to dryness is not always pure. Any soluble impurities will be left together with the solid after heating.
Point out to students that not all soluble substances can be obtained by this method.
For compounds/substances with low thermal stability, we have to use crystallisation method instead. 11

12. 3.2 Separating a Solid from a Liquid
Crystallisation
Process of obtaining pure solid sample (soluble solid) from its solution.
Examples:
Compare with evaporation. Students should note that we are still on the topic of separating a soluble solid from a solution but crystallisation enables us to obtain a pure sample, that is free from impurities.
hydrated copper(II) sulfate crystals
sodium carbonate crystals

13. Preparation of Pure Copper(II) Sulfate Crystals by Crystallisation
3.2 Separating a Solid from a Liquid
Preparation of Pure Copper(II) Sulfate Crystals by Crystallisation
Step 1
impure copper(II) sulfate solution
The solution is heated to remove most of the solvent (water).
Heating is stopped when a saturated solution is formed.
Compare with evaporation. Students should note that we are still on the topic of separating a soluble solid from a solution but crystallisation enables us to obtain a pure sample, that is free from impurities.
Clicking on the url icon will link you to a simple experiment showing how crystallisation is carried out in order to obtain copper(II) sulfate crystals.
Video duration: 2.41 minutes.
URL 13

14. 3.2 Separating a Solid from a Liquid
Preparation of Pure Copper(II) Sulfate Crystals by Crystallisation
Step 2
a solution that contains as much dissolved solute as it can at a given temperature
The hot, saturated solution is allowed to cool.
The dissolved copper(II) sulfate appears as pure crystals.
Rapid cooling produces small crystals while slow cooling produces large crystals.
copper(II) sulfate crystals 14

15. 3.2 Separating a Solid from a Liquid
Preparation of Pure Copper(II) Sulfate Crystals by Crystallisation
Step 3
The cold solution is removed by filtration. The residue of pure crystals is washed with cold distilled water.
pure copper(II) sulfate crystals
filter paper
The crystals are then dried by pressing them between pieces of filter paper. 15

16. Using a Suitable Solvent
3.3 Separating Solids
Using a Suitable Solvent
To separate a mixture of two solids, we use a solvent in which one solid is soluble. (The other solid is insoluble in that solvent.)
Different solids dissolve in different solvents. Some common solvents are water and ethanol. 16

17. Mixture of sodium chloride and sand
3.3 Separating Solids
Example:
Separating sodium chloride and sand
Mixture of sodium chloride and sand
Add water to dissolve sodium chloride
sodium chloride solution + sand

18. 3.3 Separating Solids Example: Separating sodium chloride and sand
sodium chloride solution + sand
Filter the mixture
filtrate
residue
sodium chloride solution
sand +
traces of sodium chloride solution

19. 3.3 Separating Solids Example: Separating sodium chloride and sand
residue
filtrate
sand +
traces of sodium chloride solution
sodium chloride solution
Evaporate to dryness
Wash with distilled water
sodium chloride
sand

20. 3.3 Separating Solids Sublimation
Sublimation is used to separate a solid that sublimes from one that does not.
Some substances, such as ammonium chloride and iodine, sublime.

21. 3.3 Separating Solids Using Sublimation to Separate Two Solids
We can make use of this property to separate a substance that sublimes from one that does not
e.g. salt and iodine.
heat
iodine vapour
wet cloth to cool
mixture of iodine and sand
solidified iodine 21

22. 3.3 Separating Solids Using a Magnet
A magnet can be used to separate a magnetic substance from a non-magnetic substance.
Examples of magnetic materials:
Iron
Steel
Nickel
Cobalt

23. 3.3 Separating Solids Using Magnets to Separate Two Solids
Some metals are magnetic.
We can use this property to separate these metals (e.g. iron, nickel, cobalt, steel) from mixtures.

24. 3.4 Separating a Liquid from a Solution
Obtaining the Solvent from a Solution
SOLUTION
Evaporation
Crystallisation
Simple distillation
It is an introduction to simple distillation. While we have discussed the methods used to extract the solute from a solvent, how do we recover the other part of the mixture, that is the solvent?
SOLUTE
SOLVENT

25. 3.4 Separating a Liquid from a Solution
Simple Distillation
Simple distillation is used to separate a pure solvent (liquid) from a solution.
Examples:
Extraction of water from:
sea water
salt (sodium chloride) solution
sugar solution 25

26. 3.4 Separating a Liquid from a Solution
Setting Up the Distillation Apparatus
Water enters the condenser from the bottom and leaves from the top.
Condenser must be sloping downwards.
Thermometer bulb should be just beside the side arm leading to the condenser.
water out
Boiling chips are placed in the flask to ensure smooth boiling.
Discuss with students why the above steps are taken.
Why do we take that precaution with the thermometer?
Answer: It registers the boiling point of the liquid we want to collect. That way, we will know when it is distilling over.
2. What would the reading be if the liquid is water?
Answer: 100°C
3.Why do we take that precaution with the condenser?
Answer: This ensures that the condenser is completely filled with water to cool and condense the vapour more effectively. It is sloping downwards to ensure that the distillate flows downwards into the receiver.
water in
Volatile liquids can be kept in the liquid state by placing receiver on ice. 26

27. 3.4 Separating a Liquid from a Solution
Simple distillation of salt solution
4. The salt solution becomes more concentrated as distillation continues. Salt will be collected as residue.
2. Water boils and becomes vapour. Thermometer measures temperature of the vapour.
3. Water vapour is cooled and condenses into pure liquid (distillate).
1. Boiling chips are added to ensure smooth boiling. Water vaporises, rises and enters the condenser.
This can be carried out in the laboratory as a demonstration using salt (sodium chloride) or coloured water. Students usually will be quite excited to see the distillate coming through as a colourless liquid. Some will volunteer to drink it!
5. Pure water is collected as distillate. 27

28. 3.5 Separating Liquids Using a Separating Funnel
This method can be used to separate immiscible liquids. Liquids that do not dissolve in each other are described as immiscible.
Example:
Oil and water
separating funnel
oil
water 28

29. 3.5 Separating Liquids
Using a Separating Funnel to Separate Immiscible Liquids
Step 1
Pour the mixture of oil and water into the separating funnel. (Make sure the tap is closed.)

30. 3.5 Separating Liquids
Using a Separating Funnel to Separate Immiscible Liquids
Step 2
Support the separating funnel using a retort stand.
Then, place a clean beaker below the separating funnel.

31. 3.5 Separating Liquids
Using a Separating Funnel to Separate Immiscible Liquids
Step 3
Allow the liquids to separate completely. This may take some time. The denser liquid (water) will be the bottom layer.

32. 3.5 Separating Liquids
Using a Separating Funnel to Separate Immiscible Liquids
Step 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.

33. 3.5 Separating Liquids
Using a Separating Funnel to Separate Immiscible Liquids
Step 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.

34. 3.5 Separating Liquids Fractional Distillation
This method can be used to separate a mixture of miscible liquids with different boiling points.
Liquids that mix together completely to form a solution are described as miscible.
Examples:
Ethanol and water
Purified air
Crude oil 34

35. Comparing Simple Distillation with Fractional Distillation
3.5 Separating Liquids
Comparing Simple Distillation with Fractional Distillation
A fractionating column is attached to the round-bottomed flask and the condenser for fractional distillation.
Glass beads/plates/spiral in the fractionating column provide a large surface area for vapour to condense on.
simple distillation
fractional distillation 35

36. 3.5 Separating Liquids Fractional Distillation
3. Ethanol, which has a lower boiling point than water, reaches the upper part of the column and is distilled over.
2. The water vapour condenses in the fractionating column and falls back into the flask.
1. Ethanol vapour and water vapour rise up the column as the solution is heated.
4. At this stage, the thermometer shows a constant temperature of 78oC, which is the boiling point of ethanol. 36

37. 3.5 Separating Liquids Fractional Distillation 5. In this condenser,
hot ethanol vapour condenses as running water cools it;
liquid ethanol flows down the inner tube of the condenser and into the receiver.
Fractional Distillation
7. When all the ethanol has distilled over, the temperature rises rapidly to 100oC, which is the boiling point of water. At this temperature, water distils over and can be collected separately.
6. Ethanol is collected as the distillate in the receiver. 37

38. 3.5 Separating Liquids Fractional Distillation
Graph showing how temperature changes as a solution of ethanol and water is fractionally distilled
The temperature of the mixture increases as it is heated up until 78°C. At 78°C, ethanol distils over. The temperature remains constant until all the ethanol has distilled out of the round-bottomed flask. The temperature then increases until 100°C. At this temperature, water distils over. The temperature remains unchanged as water is being distilled. 38

39. 3.5 Separating Liquids
Industrial Applications of Fractional Distillation
To separate:
the various components of crude oil such as petrol, kerosene, diesel in oil refineries.
nitrogen, oxygen and argon in liquid air.
ethanol from the fermentation mixture of water, glucose and malt in breweries. 39

40. 3.6 Chromatography
Chromatography is the method of separating two or more components that dissolve in the same solvent.
Examples:
Pigments in plants
Dyes in paints
Colouring in food

41. 3.6 Chromatography
The chromatography paper with the separated components is called a chromatogram.
lid
glass tank
chromatogram

42. 3.6 Chromatography Separating Dyes Found in Green Food Colouring
Place a spot of green food colouring on the paper.
Dip the paper into a solvent such as ethanol or water. Ensure that the coloured spot is above the solvent level.
The components will separate as the solvent travels up the paper.
Chromatography paper
Green food colouring
Pencil line
Ethanol or water as solvent

43. Principle Behind Paper Chromatography
Separation of a substance into its different components depends on the relative solubility of the component.
The more soluble component travels faster and further up the paper than the less soluble components.
Identical dyes travel up the same distance and produce the same colour on the paper when the same solvent is used.
The chromatography paper with the separated components is called a chromatogram. 43

44. 3.6 Chromatography Interpretation of a Chromatogram
This food colouring is not pure.
It consists of 2 component dyes.
This is a pure substance. Why? 44

45. 3.6 Chromatography What is Rf value?
The positions of the solvent front (position reached by solvent) and spot on a chromatogram depend on how long the experiment was allowed to run.
Chromatogram after a period of time.
Chromatogram after a longer period of time.

46. 3.6 Chromatography What is Rf value?
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.

47. 3.6 Chromatography
Rf value of substance is the same regardless of distance travelled on chromatogram.
Rf =
= 0.67
Rf =
= 0.67
Chromatogram after a period of time.
Chromatogram after a longer period of time.

48. 3.6 Chromatography Rf Value
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.

49. 3.6 Chromatography Example: Comparison with ‘known’ samplesB 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.

50. 3.6 Chromatography Example: Comparison with ‘known’ samplesB C D
Conclusions that can be drawn from the chromatogram:
Identical dyes produce spots at the same height.
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.

51. 3.6 Chromatography Example: Comparison with ‘known’ samplesB C D
Other conclusions that can be drawn from the chromatogram:
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.

52. 3.6 Chromatography 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.
The locating agent reacts with the colourless substances to form coloured spots.

53. 3.6 Chromatography Uses of Chromatography
Given a sample, chromatography can be used to:
separate the components in a sample;
identify the components present in a sample;
identify substances;
determine its purity. 53

54. 3.7 Determining Purity Importance of Purity
Impurities in drugs must be detected as they may cause undesirable side effects.
Chemicals are often added to food and beverages. It is important to ensure that our food contain only chemicals that are safe for consumption.

55. 3.7 Determining Purity Determination of a Pure Substance
By doing one of the following:
Checking for exact and constant (or fixed) melting point of a solid
Checking the exact and constant (or fixed) boiling point of a liquid
Performing chromatography

56. 3.7 Determining Purity
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. 56

57. 3.7 Determining Purity Effect of Impurities on Melting Points
Impurities decrease the melting point of a solid.
The greater the amount of impurities, the lower the melting point of the substance.
• Impurities cause melting to take place over a range of temperatures.

58. 3.7 Determining Purity Effect of Impurities on Boiling Points
Impurities increase the boiling point of a liquid.
The greater the amount of impurities, the higher the boiling point of the substance.
• Impurities cause boiling to take place over a range of temperatures.

59. 3.7 Methods for Collecting Gases
The method of collection of gas depends on:
• solubility of the gas in water;
• density of the gas, compared to air. 59

60. 3.7 Methods for Collecting Gases
Downward displacement of water
For collecting gases that are insoluble or slightly soluble in water
E.g. carbon dioxide, hydrogen, oxygen
delivery tube
gas
gas jar
Clicking on the URL button will link you a video on the collection of a gas by displacement of water.
URL 60

61. 3.7 Methods for Collecting Gases
Downward delivery of gas
For collecting gases that are soluble in water and denser than air
delivery tube
gas jar
gas
E.g. chlorine, hydrogen chloride 61

62. 3.7 Methods for Collecting Gases
Upward delivery of gas
For collecting gases that are soluble in water and less dense than air
delivery tube
gas jar
gas
E.g. ammonia 62

63. 3.7 Methods for Collecting Gases
If we need to collect and measure the volume of a gas, we can also use a gas syringe.
barrel
plunger 63

64. 3.8 Methods for Drying Gases
Drying with Concentrated Sulfuric Acid
moist gas in
dry gas out
Note the positions of the delivery tubes
Concentrated sulfuric acid used to dry most gases except ammonia
Ammonia is an alkaline gas, thus it will react with concentrated sulfuric acid to form salt and water.
Gas inlet tube must be submerged in concentrated sulfuric acid. Students are required to draw the diagrams. 64

65. 3.8 Methods for Drying Gases
Drying with Quicklime (Calcium Oxide)
moist gas in
dry gas out
Quicklime used for drying ammonia
Students are required to draw the diagrams. 65

66. 3.8 Methods for Drying Gases
Drying with Fused Calcium Chloride
moist gas in
dry gas out
Fused calcium chloride used for drying most gases
Students are required to draw the diagrams. 66