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Methods of Purification


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1. What is a pure substance?

A pure substance is a single substance not mixed with anything else.

2. What is a mixture?

A mixture contains two or more substances not chemically combined together.

3. What is purification?

t is the process of separating mixtures into pure substances.

4. State one characteristic of a pure solid.

Fixed melting point

5. How do impurities affect a solid’s melting point?
  • Impurities lower the melting point.
  • Impurities cause the solid to melt over a range of temperature.
6. State one characteristic of a pure liquid.

Fixed boiling point

7. How do impurities affect a liquid’s boiling point?
  • Impurities raise the boiling point.
  • Impurities cause the liquid to boil over a range of temperature.
8. What is the relationship between the amount of impurities in a substance and the
substance’s melting / boiling point?

  • The greater the amount of impurities, the lower the melting point.
  • The greater the amount of impurities, the higher the boiling point.
9. State three methods that can be used to determine whether a substance is pure.
  • Melting point comparison
  • Boiling point comparison
  • Chromatography
10. You are given a substance labelled X. Describe an experiment how you can determine
the purity of substance X, given that the melting point of this element is 80°C
  1. Heat the substance
  2. The temperature of the substance is recorded at regular time intervals.
  3. During the heating, the thermometer will show a constant temperature for a few minutes. This is the melting point of the solid.
  4. If the melting point obtained is 80°C, then the substance is pure.
11. In addition to testing the purity of a substance, what can the methods of melting / boiling
point comparison and chromatography be used for?

To determine the identity of a substance.

12. State seven methods of separating mixtures.

  1. Filtration
  2. Crystallisation
  3. Evaporation
  4. Distillation
  5. Sublimation
  6. Use of a separating funnel
  7. Chromatography
13. When is filtration used?
  • To separate an insoluble solid from a solution.
  • To separate two solutes in which one can dissolve in a solvent.
14. The solid which remains on the filter paper when a suspension is filtered is called the


15. The liquid or solution that passes through the filter paper is called the __________.


16. When is crystallisation used?

To separate a dissolved solid from a solution.

17. Give an example of a dissolved solid in a solution.

Copper(II) sulphate powder dissolved in water to form copper(II) sulphate solution.

18. State whether the following salts are soluble or insoluble:
- Sodium salts
- Potassium salts
- Ammonium salts
- Nitrates
- Chlorides
- Sulphates
- Carbonates

19. Crystallisation will result in the formation of pure __________ from a __________

crystals; saturated

20. What is a saturated solution?

It is one that contains the maximum amount of dissolved solute at a given temperature.

21. When is evaporation used?

To separate a dissolved solid from a solution.

22. Some salts are obtained through crystallisation from a saturated solution while others
are heated and evaporated to dryness. Describe one factor that determines the choice of

Decomposition of the solute when heated to dryness.
  • If the solute decomposes when heated to dryness, crystallisation will be used as this method prevents the solute from being excessively heated and becoming decomposed.
  • If the solute does not decompose when heated to dryness, evaporation will be used.
23. Distillation is essentially the process of __________ a liquid and __________ the vapour.

boiling;  condensing

24. The apparatus used for distillation includes a distillation flask and a condenser. Describe what happens in the distillation flask and condenser during distillation.
  • In the distillation flask, the solution boils. The liquid vaporises and changes into a vapour, which rises and enters the condenser.
  • In the condenser, the vapour condenses and changes back into a liquid (pure).
25. Explain why a distillation flask should not be filled by more than two thirds of its
volume with the mixture.

This ensures sufficient space above the surface of the mixture. Thus, no mixture is spilled / propelled into the condenser to compromise the purity of the distillate.

26. The __________ is the pure liquid that is obtained after its vapour condenses in the


27. The diagram below shows a condenser used in distillation experiments. Indicate, on the diagram, the flow of water in and out of the condenser. Give two reasons for your answer.

  • The flow of water is such that the coldest part of the condenser is at the end just before the vapour escapes from the condenser. This ensures the most effective cooling of the vapour.
  • Having water entering from the bottom and leaving from the top of the condenser ensures that the entire interior of the condenser will be filled with water.
28. Give two reasons why it is preferable to add boiling chips / boiling stones into the
mixture in the distillation flask during distillation.

  • To prevent overheating of the liquid being distilled.
  • To ensure a smooth boiling process.
29. When is a water bath used during distillation?

When the liquid to be heated is flammable.

30. When is simple distillation used?

To separate a solvent from a solution.

31. Give an example of a solvent in a solution.

Fresh water in sea water.

32. When is fractional distillation used?

To separate a mixture of miscible liquids with different boiling points.

33. Give an example of a mixture of miscible liquids with different boiling points.

A mixture of ethanol and water.

34. What are miscible liquids?

Those that mix completely to form one liquid.

35. How can we conclude that oil and water are immiscible?

They do not mix well, and will form two separate layers when mixed.

36. How can we conclude that ethanol and water are miscible?

They mix together completely to form a single solution.

37. State the difference in the apparatus used between simple and fractional distillation.

In fractional distillation, an additional apparatus – the fractionating column – is attached to the distillation flask and condenser.

38. The fractionating column contains many __________ which provide a large surface
area for the condensation of vapour.

glass beads

39. State the purpose of the glass beads in a fractionating column.

The glass beads provide greater surface area for repeated condensation of vapour and re-boiling of liquids. This ensures a more complete separation of the mixture.

40. State the factor that determines which liquid distills over first during fractional distillation.

The liquids’ boiling point.

41. Which liquid in a mixture will distill over first during fractional distillation?

The liquid with the lowest boiling point.

42. State five substances that are obtained industrially via fractional distillation.

  • Nitrogen
  • Argon
  • Oxygen
  • Petroleum fractions from crude oil
  • Alcoholic beverages
43. What type of mixture is a separating funnel used to separate?

A mixture of two immiscible liquids of different densities.

44. Give an example of a mixture of two immiscible liquids of different densities.

Crude oil and water

45. How can a separating funnel be used to separate a mixture of two immiscible liquids?

The less dense liquid floats on the denser liquid and the two liquids are collected separately by running them into separate containers.

46. What type of mixture is sublimation used to separate?

A mixture of solids where only one of the solids sublimes.

47. Give two examples of mixtures of solids where only one of the solids sublimes.
  • Mixture of ammonium chloride and sodium chloride; only ammonium chloride sublimes.
  • Mixture of iodine and sodium chloride; only iodine sublimes.
48. Describe how you can separate ammonium chloride from a mixture of ammonium chloride and sodium chloride.
  • The mixture of ammonium chloride and sodium chloride is placed on an evaporating dish.
  • The evaporating dish is heated gently.
  • Strips of filter paper soaked in water are placed on the filter funnel when it becomes hot in order to cool it.
  • Ammonium chloride sublimes after some time and is found on the filter funnel.
  • The ammonium chloride is scrapped off from the filter funnel. Sodium chloride remains on the evaporating dish.
49. State three uses of chromatography.
  • To determine if a substance is pure.
  • To separate coloured components in mixtures.
  • To identify complicated substances such as dyes and drugs.
50. State two advantages of chromatography.
  • Only tiny amounts of the substance are required.
  • Results can be quickly obtained.
51. Give three examples of mixtures that can be separated by chromatography.
  • Pigments from plants
  • Dyes from ink
  • Amino acids from proteins
52. Why should the dyes in food be identified?

Some artificial dyes are poisonous and harmful to human beings if consumed.

53. A ___________ shows the results of the separated components in a chromatography experiment.


54. How can we determine if a substance is pure by using chromatography?

A pure substance will leave only one spot on its chromatogram.

55. What type of substances is paper chromatography used to separate?

The coloured components in dyes or inks.

56. What is necessary to separate the components of a coloured mixture via paper chromatography?

A solvent.

57. State one factor that will determine how far a dye travels on a chromatogram.

The solubility of the dye in the solvent – the more soluble the dye is in the solvent, the farther it will travel.

58. Because of their difference in __________ in a solvent, each coloured component will travel to different __________ on a chromatogram.

solubility; locations

59. What are the two methods of paper chromatography?
  1. Ascending method
    • the solvent travels up the chromatogram.
  2. Descending method
    • The solvent travels down the chromatogram.
60. State one advantage and one disadvantage of the descending method of paper chromatography.

: The separation between the spots on the chromatogram will be greater.

Disadvantage: Separation may not be complete as the solvent will travel a longer distance due to the gravitational pull.

61. Describe how coloured spots will be left on different parts of a chromatogram in a paper
chromatography experiment.
  • As the solvent travels across the chromatogram, it dissolves the dyes.
  • A dye that is strongly absorbed onto the paper and not very soluble in the solvent will be left behind.
  • A dye that is weakly absorbed by the paper and is very soluble in the solvent will be carried furthest away from the starting line.
62. How can colourless substances on a chromatogram be identified?

The chromatogram is sprayed with a locating agent to show where the substances are on the paper.

63. What is a locating agent?

A locating agent is a substance that reacts with colourless substances on a chromatogram to produce coloured products.

64. Explain how the dyes left on a chromatogram can be identified.

By comparing the dye’s position in the chromatogram with that of a known dye.

65. State the purpose of the Rf value.

Used for the identification of substances on a chromatogram.

66. Write down the formula for determining Rf value.

Rf value = Distance moved by substance / Distance moved by solvent

67. Explain how the Rf value can be used to identify a dye.
  • The Rf value is first calculated using the formula: Rf value = Distance moved by substance / Distance moved by solvent
  • The calculated Rf value is then compared to known values of Rf values.

MCQ Questions

1. Potassium nitrate crystals can be separated from sand using the four processes shown below. Which of the following shows the processes in the correct order?

           first --------------------------------------------------------------> last
 a filter dissolve evaporate crystallise
 b dissolve evaporate crystallise filter
 c dissolve evaporate filter crystallise
 d dissolve filter evaporate crystallise

2. Trichloroethane is a solvent used to remove grease from clothing. How, after use, is the solvent separated from the grease?
a. chromatography
b. crystallisation
c. distillation
d. filtration

3. In which pair of substances is each substance a mixture?
a. air; water
b. limewater; water
c. quicklime; limewater
d. sea water; air

4. Which statement explains why oxygen can be separated from nitrogen by the fractional distillation of liquid air?
a. oxygen is more dense than nitrogen
b. oxygen is more reactive than nitrogen
c. the two elements have different boiling points
d. the two gases are in different groups of the Periodic Table

5. From which mixture can the underlined substance be obtained by adding water, stirring and filtering?
a. calcium carbonate and sodium chloride
b. copper(II) sulphate and sodium chloride
c. ethanoic acid and ethanol
d. iron and magnesium

6. A separating funnel is most useful to separate
a. a residue from a solution
b. two immiscible liquids
c. a solute from a solvent
d. two powdered solids

7. Separation of different compounds in paper chromatography depends on the
a. difference in colour
b. size of container used
c. type of filter paper used
d. relative solubilities of the compounds

8. Which of the following is most likely to be a pure compound?
a. a blue precipitate
b. colourless crystals which melt at 54ºC
c. a liquid that separates into two liquids when distilled
d. blue crystals which melt over the range of 45ºC to 49ºC

9. What is the best method to remove mercury and alcohol when a mercury thermometer and an alcohol thermometer are broken in a beaker?
a. filtering
b. distilling
c. using a separating funnel
d. crystallising

10. Which separation technique is used to obtain potassium nitrate from its aqueous solution?
a. electrolysis
b. distillation
c. chromatography
d. crystallisation

11. What property must a mixture of miscible liquids have in order for it to be separated by fractional distillation?
a. the liquids in the mixture must be coloured
b. the liquids in the mixture must have the same densities
c. the liquids in the mixture must have the same boiling points
d. the liquids in the mixture must have different boiling points

12. Which of the following is a pure compound?
a. an orange dye that gives red and yellow spots in a chromatogram
b. a colourless solution that boils over the range of 80ºC to 98ºC
c. a yellowish brown solution
d. a brown liquid that boils at 130ºC

13. Which is the best method to separate a mixture of lead sulphate and sodium nitrate?
a. filtration
b. distillation
c chromatography
d. crystaliisation

14. During an experiment, it was discovered that water boiled at 108ºC. What could you infer from the experiment?
a. the experiment was done in the basement
b. pure water boils at 108ºC
c. the water was boiled for a long time
d. there were impurities present in the water

15. What is the best method to separate liquid air into its constituents?
a. crystallisation
b. distillation
c. fractional distillation
d. chromatography


1. d
2. c
3. d
4. c
5. a
6. b
7. d
8. b
9. b
10. d
11. c
12. d
13. b
14. d
15. c

Structured Question Worked Solutions

1a. Aqueous sodium chloride can be prepared by titrating aqueous sodium hydroxide with dilute hydrochloric acid. The equation for this reaction is shown below.

NaOH (aq) + HCl (aq) --> NaCl (aq) + H2O (l)

i. name the two pieces of apparatus used to measure accurately the volumes of the solutions in this titration
ii. name a suitable indicator for this titration. give the expected color change of this indicator.
iii. explain the meaning of the symbols (aq) and (l) in this equation


1ai. pipette and burette

1aii. phenolphthalein. from pink to colorless

1aiii. (aq) : aqueous solution
(l) : liquid state

2. Instructions for preparing hydrated crystals of magnesium sulphate are given below.

Add one spatula full of magnesium carbonate to 50 cm3 of dilute sulphuric acid. When it has reacted, add further amounts until no more will dissolve. Then filter the mixture. Evaporate the filtrate to about half its volume. Allow the filtrate to cool. Filter off the crystals. Dry them on filter paper. Do not heat the crystals.

a. explain the importance of the five instructions which are underlined

b. because it has a larger surface area, powdered magnesium carbonate reacts more rapidly then lumps of magnesium carbonate. Explain why the larger surface area leads to a greater rate of reaction.

c. name the gas evolved in this reaction and give a test for the gas

d. name three substances, other than magnesium carbonate, which react with dilute sulphuric acid to form magnesium sulphate


2ai. to make sure that all the acid has reacted

2aii. to remove the excess solid magnesium carbonate

2aiii. to concentrate the solution

2aiv. to allow crystallisation

2av. to avoid loss of water of crystallisation

2b. Reaction takes place only at the surface of the solid. With a greater surface area, more particles can react at the same time. Hence rate is faster.

2c. carbon dioxide. Pass the gas into limewater. A white precipitate will be formed

2d. magnesium, magnesium oxide, magnesium hydroxide

3. Name a process that is used to separate a mixture of 
a. three water-soluble dyes
b. insoluble solid and water
c. two liquids (ethanol and water)
d. water and a dissolved salt


3a. paper chromatography
3b. filtration
3c. fractional distillation
3d. crystallisation