1. Mixtures and Separations
Chapter 8
Mixtures and Separations
Learning Outcomes
Identify the different types of solutions
Investigate experimentally the effect of structure and temperature on solubility of solids in water
Distinguish among solutions, suspensions and colloids Identify suitable separation techniques based on difference in properties of the components of mixtures
Describe the extraction of sucrose from sugar cane

2. Pure substances Mixtures and Separations Chapter 8
A pure substance is a single substance not mixed with anything else.
Sugar and table salt are examples of pure substances.
Sugar crystals
Pure substances usually exist in crystal form.
For example, table salt, sugar and copper(II)
sulphate all exist as crystals.
A pure substance can be an element or a compound.
Copper(II) sulphate 2

3. Pure and impure substances
Chapter 8
Mixtures and Separations
Pure and impure substances
A pure substance has a fixed melting point and boiling point. For example, pure water boils at 100 oC, and melts at 0 oC.
b.p. 100 oC
m.p. 0 oC 3

4. Impure substances or mixtures
Chapter 8
Mixtures and Separations
Impure substances or mixtures
A mixture on the other hand, is an impure substance. It does not have a fixed melting point and boiling point. It melts or boils over a range of temperatures.
For example, sea water boils at about 102 oC and freezes at around – 2.5oC.
In general, an impurity lowers the melting point and increases the boiling point of a substance. 4

5. Impure substances or mixtures
Chapter 8
Mixtures and Separations
Impure substances or mixtures
In nature, most substances are impure. They consist of two or more substances mixed together.
Such substances are called mixtures.
Examples of mixtures are air, sea water, petroleum and granite rock.
Mixtures can be purified or separated by physical methods. 5

6. A Mixture Mixtures and Separations Chapter 8
Air is made up of different gases: nitrogen, oxygen, carbon dioxide and noble gases such as argon, neon, and helium.
The gases in air can be easily separated by liquefaction followed by fractional distillation.
Air is an example of a mixture.
A mixture is made up of two or more substances not chemically combined together.

7. Composition of a mixture
Chapter 8
Mixtures and Separations
Composition of a mixture
A mixture can be made up of two or more elements. E.g. Brass is a mixture of copper and zinc.
Copper atom
Zinc atom
Brass
A mixture can be made up of two or more elements or compounds. E.g. Air is a mixture which contains both elements and compounds.
Element
Compound
Air contains both elements and compounds.

8. Mixtures and Separations
Chapter 8
Mixtures and Separations
Experiment
3. Hold a magnet over the mixture of iron filings and sulphur.
The iron filings are attracted by the magnet and can be separated from the sulphur.
4. Heat the mixture of sulphur and iron filings strongly in an evaporating dish. Examine the substance formed carefully.
Mixture of iron and sulphur
A black solid is formed. It does not look like iron filings or sulphur.
Compound of iron and sulphur

9. Mixtures and Separations
Chapter 8
Mixtures and Separations
Experiment
5. Hold a magnet over the black solid. What happens?
The black solid is not attracted by the magnet.
The iron can no longer be separated from the sulphur in the black solid.
When iron filings and sulphur are heated, a compound called iron(II) sulphide is formed.
The equation for the reaction is: Fe(s) + S(s)  FeS(s)

10. Homogeneous mixtures Mixtures and Separations
Chapter 8
Mixtures and Separations
Homogeneous mixtures
A solution is a homogeneous mixture.
Characteristics of solutions:
When left to stand, the solute and solvent do not separate.
Solutes are not visible (particle size ≤ 1 nm).
All parts of the solution have the same chemical composition, chemical and physical properties. 10

11. Homogeneous mixtures Mixtures and Separations
Chapter 8
Mixtures and Separations
Homogeneous mixtures
Solubility of a solution refers to the amount of solute that is able to dissolve in a given quantity of solvent at a fixed temperature and pressure.
The rate of dissolving of a solute depends on:
Temperature (e.g. Solubility increases with temperature.)
Surface area of solute (e.g. Fine sugar dissolves faster than rock sugar.)
Rate of agitation (e.g. Stirring the solution increases the rate of dissolving.) 11

12. Heterogeneous mixtures
Chapter 8
Mixtures and Separations
Heterogeneous mixtures
Heterogeneous mixtures have non-uniform compositions which can be mechanically separated.
Examples of heterogeneous mixtures are colloids and suspensions.
Colloids include fog, smoke, shaving cream, milk, blood, styrofoam, gelatin, and cheese.
Suspensions include muddy water, paint and chalk powder suspended in water. 12

13. Quick check 1 Mixtures and Separations Chapter 8
Explain why air is a mixture but water is a compound.
(a), State two reasons why a mixture of iron filings and sulphur before heating is a mixture. (b) When the mixture of iron filings and sulphur is heated strongly, state two reasons why the solid formed is a compound.
State whether the following diagrams on the right represent elements, mixtures or compounds.
Solution

14. Solution to Quick check 1
Chapter 8
Mixtures and Separations
Solution to Quick check 1
Air is a mixture because it is made up of many gases such as oxygen, nitrogen, etc. not chemically combined together. The gases in air can be separated by physical means. Water is a compound because it is made up of hydrogen and oxygen chemically joined together. We cannot separate the gases in water by physical means.
(a) It is a mixture because the iron and sulphur can be separated by physical means e.g. by using a magnet. Also, no heat or light is given out when the iron filings and sulphur are mixed together. (b) When the mixture of iron filings and sulphur is heated strongly, a compound is formed because a chemical reaction occurs e.g. heat and light are given off. Also, the sulphur and iron in the compound can no longer be separated by physical means such as by using a magnet.
(a) compound, (b) compound, (c) mixture, (d) mixture
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15. Methods of separating Mixtures
Chapter 8
Mixtures and Separations
Methods of separating Mixtures
Filtration
Evaporation
Crystallisation
Sublimation
Simple distillation
Fractional distillation
Use of separating funnel
Chromatography

16. Filtration Mixtures and Separations Chapter 8
Filtration is the method used to separate an insoluble solid from a liquid.
An example would be to separate a mixture of sand and water.
The solid (sand) left on the filter paper is called residue.
The liquid (water) that has passed through the filter paper is called filtrate.

17. Evaporation Mixtures and Separations Chapter 8
Evaporation is the method used to separate a solute from a solution.
It can only be used for solids which do not decompose under heat e.g. table salt (sodium chloride).
It is done by heating the solution in an evaporating dish until it is completely dry.
Crystals of salt remain after the solution is evaporated to dryness.

18. Crystallisation Mixtures and Separations Chapter 8
Crystallisation is the method used to obtain pure crystals from a solution.
It is done by heating the solution in an evaporating dish until it is saturated.
The hot solution is then allowed to cool.
Crystals will be formed on cooling. They are then dried between sheets of filter paper.

19. Sublimation Mixtures and Separations Chapter 8
Sublimation is the method used to separate a substance which sublimes from a mixture.
Examples of substances which sublime are: iodine, ammonium salts, naphthalene and dry ice.
The mixture is heated in an evaporating dish covered with a filter funnel.
The substance which sublimes will be changed into a vapour and then formed back on the inside of the funnel.
To separate ammonium chloride from sodium chloride, this method can be employed.

20. Simple distillation Mixtures and Separations Chapter 8
Distillation is the method used to obtain a pure solvent from a solution. E.g. pure water from seawater.
It is done by heating the solution in a distillation flask and collecting the vapour that boils off.
A condenser is used to condense the hot vapour and change it to a liquid.
The condenser is cooled by the flow of cold water (in the condenser) from the tap.
The pure liquid collected from distillation is called distillate.

21. Fractional distillation
Chapter 8
Mixtures and Separations
Fractional distillation
Fractional distillation is used to separate two or more miscible liquids with different boiling points.
A fractionating column and a condenser are used.
Example: Mixture of ethanol and water
When the thermometer reaches a steady temperature of 78 oC, ethanol is collected.
Water is collected when the thermometer reaches a steady temperature of 100 oC.
Separation of ethanol-water mixture by fractional distillation

22. Use of separating funnel
Chapter 8
Mixtures and Separations
Use of separating funnel
This method is used to separate a mixture of immiscible liquids.
The mixture is placed into a separating funnel and allowed to settle into two layers.
Example: Mixture of oil and water
The tap is first opened to allow the water to drain out of the funnel.
After all the water has drained out, another beaker is placed below the funnel and oil is now drained out.
Separation of oil-water mixture by using a separating funnel

23. Chromatography Mixtures and Separations Chapter 8
Chromatography is a method used to separate and identify small quantities of substances.
There are different kinds of chromatography techniques, such as gas chromatography, liquid chromatography and paper chromatography.
It is based on the principle that different substances have different solubilities in the same solvent.
The more soluble substance will get carried along faster by the solvent and move further ahead than the less soluble substances.
Paper Chromatography
Chromatography can be used to detect and identify very small quantities of substances. It is used to detect banned dyes used in food colourings, and detect illegal drugs used by athletes during major competitions.

24. Setup for paper chromatography
Chapter 8
Mixtures and Separations
Setup for paper chromatography
-The starting line must be drawn in pencil, not ink. This is because unlike ink, pencil lead is insoluble in the solvent and will not interfere with the chromatogram.
-The spots of mixtures must be placed above the solvent level, so that they will not immediately dissolve in the solvent, and the solvent has time to slowly move up.
solvent front
glass cover
chromatography paper
starting line
large beaker
solvent
-The solvent front must be allowed to move as far up the paper as possible to
ensure that all the dyes are separated.

25. The Rf value of a substance
Chapter 8
Mixtures and Separations
The Rf value of a substance
(10 cm)
Rf value = distance moved by a substance distance moved by solvent front
(7 cm)
E.g. Rf value of red dye = 7 cm = cm
Sometimes the chromatogram needs to be sprayed with a locating agent to make the colourless spots in the chromatogram appear.

26. Paper chromatography Mixtures and Separations Chapter 8 Worked example
The chromatogram shows 3 single dyes red, green and blue, and also four unknown samples P, Q, R and S.
Identify the dyes present in each of the samples, P, Q, R, S.
Solution
Sample P contains green dye and one unknown dye.
Sample Q contains only blue dye.
Sample R contains green, blue and red dyes.
Sample S contains green and red dyes.

27. Mixtures and Separations
Chapter 8
Mixtures and Separations
Identify the various method separation methods used in the diagram below .

28. Quick check Mixtures and Separations Chapter 8
State the method you will use to separate the following substances. (a) calcium carbonate from table salt (b) iodine from sodium chloride (c) table salt from seawater (d) sugar from sugar solution (e) pure water from sewage water (f) ethanol from beer (g) yellow dye from durian ice cream
Explain the following in chromatography. (a) Why is the starting line not drawn with ink or a ball point pen? (b) The spots of samples on the start line should be small. (c) What is the biggest advantage of chromatography?
A sample of ink was analysed using paper chromatography (see diagram above). Identify the dyes present in the ink.
Solution

29. Solution to Quick check
Chapter 8
Mixtures and Separations
Solution to Quick check
State the method you will use to separate the following substances. (a) dissolution, followed by filtration (b) sublimation (c) evaporation (d) crystallisation (e) distillation (f ) fractional distillation (g) chromatography
Explain the following in chromatography. (a) Ink contains dyes which could dissolve in the solvent and interfere with the chromatogram. (b) So that they would not smudge the paper. (c) It can detect and identify very small amounts of substances.
Blue and yellow dyes
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30. Mixtures and Separations
Chapter 8
Mixtures and Separations
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