Mixtures and Separations

1
 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
Mixtures and Separations
Chapter 8
Learning Outcomes

Copper(II) sulphate
Pure substances
 A pure substance is a single substance not
mixed with anything else.
 Sugar and table salt are examples of pure
substances.
 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.
Sugar crystals
Chapter 8

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
Chapter 8

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.
Chapter 8

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.
Chapter 8

6
A Mixture
 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.
Chapter 8

7
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.
 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.
Copper
atom
Zinc
atom
Air contains both elements and
compounds.
Chapter 8
Brass
Element
Compound

8
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.
 A black solid is formed. It does not look
like iron filings or sulphur.
Compound of iron and sulphur
Mixture of iron and sulphur
Experiment
Chapter 8

9
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.
6. 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)
Experiment
Chapter 8

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.
Chapter 8

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.)
Chapter 8

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.
Chapter 8

13
Quick check 1
1. Explain why air is a mixture but water
is a compound.
2. (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.
3. State whether the following diagrams on the
right represent elements, mixtures or
compounds.
Solution
Chapter 8

14
Solution to Quick check 1
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.
2. (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.
3. (a) compound, (b) compound, (c) mixture, (d) mixture
Return
Chapter 8

Methods of separating Mixtures
 Filtration
 Evaporation
 Crystallisation
 Sublimation
 Simple distillation
 Fractional distillation
 Use of separating funnel
 Chromatography
Chapter 8

Filtration
 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 liquid (water) that
has passed through
the filter paper is called
filtrate.
The solid (sand) left on the
filter paper is called residue.
Chapter 8

Evaporation
 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.
Chapter 8

Crystallisation
 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.
Chapter 8

Sublimation
 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.
Chapter 8

Simple distillation
 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.
Chapter 8

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.
Chapter 8
Separation of ethanol-water mixture
by fractional distillation

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
Chapter 8

Chromatography
 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.
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.
Paper Chromatography
Chapter 8

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.
starting line
solvent
solvent front
chromatography
paper
large
beaker
glass cover
-The solvent front must be allowed to move as far up the paper as possible to
ensure that all the dyes are separated.
Chapter 8

The Rf value of a substance
Rf value = distance moved by a substance
distance moved by solvent front
(7 cm)
(10 cm)
E.g. Rf value of red dye = 7 cm = 0.7
10 cm
Sometimes the chromatogram needs to be sprayed with a locating
agent to make the colourless spots in the chromatogram appear.
Chapter 8

Paper chromatography
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.
Chapter 8

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

Quick check
1. 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
2. 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?
3. A sample of ink was analysed using paper chromatography (see
diagram above). Identify the dyes present in the ink.
Solution
Chapter 8

Solution to Quick check
1. 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
2. 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.
3. Blue and yellow dyes
Return
Chapter 8

1. http://orgchem.colorado.edu/hndbksupport/filt/filtration.html
2. http://en.wikipedia.org/wiki/Distillation
3. http://orgchem.colorado.edu/hndbksupport/dist/dist.html
4. http://en.wikipedia.org/wiki/Fractional_distillation
5. http://www.chemistrydaily.com/chemistry/Chromatography
To find out more about Methods of Purification and
Analysis, click on the links below!
Chapter 8

References
 Chemistry for CSEC Examinations by Mike
Taylor and Tania Chung
 Longman Chemistry for CSEC by Jim Clark
and Ray Oliver
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Mixtures and Separations

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