Separation of Mixtures

1. Separating MIXTURES

2. Learning Objectives
By the end of the lesson, students sbat:
• Describe methods of separation and
purification for the components of the
following types of mixtures:
1. Solid-solid
2. Solid-liquid
3. Liquid-liquid (miscible and immiscible)

3. Learning Objectives
•
Techniques to be covered for separations and
purification include:
i.
Use of a suitable solvent, filtration and
crystallisation or evaporation
ii. Distillation and fractional distillation
iii. Paper chromatography
•
•
Describe paper chromatography and interpret
chromatograms
Deduce from the given melting and boiling
point, the identities of substances and their
purity

4. Purity of Substances
• Pure substances are made up of only one
substance.
• Mixtures are substances that contain two
or more substances that are not
chemically combined.

5. Purity of Substances
• Pure solids have a fixed (i.e. exact and constant)
boiling point and melting point.
• In the case of pure water, ice will melt at 0 oC
and water boils at 100 oC

6. Classification of Mixtures
• Mixtures can be classified as a solution or
a suspension.
• When the solid dissolves in a liquid, a
solution is obtained.
• The solid that dissolves is known as the
solute.
• The substance that dissolves the solid is
known as the solvent.

7. Sugar dissolves in the solvent
Therefore resulting mixture is a SOLUTION.

8. Classification of Mixtures
Example
•When salt dissolves in water, a salt solution
is obtained.
salt +
water
→ salt-water mixture
(solute) (solvent)
(solution)
•When the solid does not dissolve in the
liquid, a suspension is obtained.

9. Sand does NOT
dissolve in the
solvent
Therefore
resulting mixture is
a SUSPENSION

10. Methods of Purification

11. Magnetic Separation
• To separate a
magnetic substance
from a mixture.

12. Filtration
• To separate an insoluble
solid from a mixture.
• E.g. sand from seawater.
• Upon filtration the mixture
is separated into residue
and filtrate.
• Residue – the insoluble
solid that remains on the
filter paper.
• Filtrate – the liquid that
passes through the filter
paper.

13. Evaporation to Dryness
• To evaporate solvent
from the solution,
obtaining the soluble
salt from the mixture.
• E.g. to obtain salt from
seawater.
mixture
evaporating dish
wire gauze
tripod stand
Bunsen burner
Figure 3 Setup for evaporation

14. Crystallisation
• Steps include:
– Evaporate until a saturated solution is
obtained.
– Cool, crystals will form.
– Filter to obtain crystals.
– Rinse with cold distilled water.
– Dry by pressing between filter paper.

15. Wake Up Call 1
1. Which method is most suitable for obtaining a
pure, dry sample of sodium chloride from a
mixture of solid sodium chloride and sand?
A. Heat the mixture gently and collect the substance
which boils off.
B. Heat the mixture gently and collect the substance
which melts.
C. Shake the mixture with water and distil off the liquid.
D. Shake the mixture with water, filter and evaporate
the filtrate.

16. Wake Up Call 1
2.
A mixture contains two compounds, copper(II) sulfate crystals and
calcium carbonate. They have the following properties:
Name
Properties
Copper(II) sulfate
•Soluble in water
•Low melting point
Calcium carbonate
•Insoluble in water
•High melting point

17. Wake Up Call 2
List the steps that should be taken to separate
the mixture to obtain dry samples of copper(II)
sulfate and calcium carbonate.

18. Simple Distillation
• To obtain a solvent from a mixture

19. Simple Distillation
• How it works:
– Water in the distilling
flask boils.
– Water vapour rises,
enters the condenser.
– Water vapour is
condensed back to
water.
– Pure water (distillate)
is collected in a
receiver.
To separate a solvent from a
mixture.
water
out
thermometer
retort stand
condenser
distilling
flask
mixture
water in
boiling chips
distillate
tripod stand
heat
Figure 4 Setup for simple distillation
wooden
block

20. Things to note
Thermometer: bulb is placed at the opening of
the distilling flask (to measure the temperature
of the pure vapour)
Condenser: water enters the condenser from
the bottom and exits through the top.
Boiling chips: ensures smooth boiling

21. Paper Chromatography
• Uses a solvent to separate a mixture into
its components.
• Applications
– separate dyes in ink,
– pigments in plants,
– amino acids obtained from proteins,
– to identify poisons (eg. pesticides) or drugs,
and
– to detect traces of banned substances in food

22. Paper Chromatography
• Procedure for Ascending
Paper Chromatography
– Separating Dyes in a
Black Ink
1. Draw a pencil line on a
piece of chromatography
paper about 2 cm from the
end of the paper.
2. Put a tiny spot of black ink
on the pencil line. Allow
the spot to dry.
Concentrate the ink spot
by dipping the ink two or
three times on the same
spot.
3. Using a split cork, suspend
the paper inside a boiling tube
containing a small volume of
solvent. Ensure the ink spot is
about 1 cm above the level of
the solvent.
4. Remove the chromatography
paper when the solvent has
almost reached the top of the
paper.

23. Paper Chromatography
Things to note:
•
•
–
Paper chromatography works only if the sample is
soluble in the solvent.
The starting line should be drawn using a pencil and
not a pen.
Pencil lead is insoluble and does not contain any
dyes.
•The starting line should not be below the solvent level.
– This is so that the sample will not fully dissolve into
the solvent.
•The solvents used are usually water and ethanol.

24. Paper Chromatography
• How it works
– A spot of food colouring is applied to the
chromatography paper.
– Chromatography paper is dipped into a solvent
(usually ethanol or water).
– Solvent is soaked up by the paper, it continues to
travel up the paper, carrying the dyes along.
– A dye that is less soluble in the solvent will not be
carried far along the paper; a dye that is more soluble
in the solvent will be carried far along the paper.
– Coloured spots are left in different places on the
paper at the end of the experiment.

25. Paper Chromatography
Experimental setup
Chromatogram
pure
substance

26. Paper Chromatography
Experimental setup
Chromatogram

27. Paper Chromatography
Experimental setup
Chromatogram

28. Interpreting a Chromatogram
• The chromatogram
shows 2 coloured spots
above the starting line.
• This means that
– the sample is a mixture
– it is made up of 2 other
substances.

29. Interpreting a Chromatogram
• The chromatogram shows 1
spot above the starting line.
• This means that
– the sample is pure (it is either
an element or a compound)
– it cannot be separated into
other substances.

30. Interpreting a Chromatogram
• The chromatogram shows that no
spots above the starting line. This
means that
1. the sample is insoluble in the solvent
used.
2. the solvent needs to be changed.

31. Wake Up Call 4
1. For two substances to be separable by
paper chromatography, it is necessary
that
A
B
C
D
they are both liquids.
they are both soluble in the same solvent.
they have different densities.
they have different colours.

32. Wake Up Call 4
2. Dyes are used to colour some
sweets. A student placed a drop
of orange dye on a piece of filter
paper. The end of the filter paper
was then dipped into a beaker of
water. The results are shown
below.

33. Wake Up Call 4
(a) What is the name of the process shown
in the diagram?
(b) Is the orange dye a mixture or a
compound? Give a reason for you
answer.

34. Wake Up Call 4
The results were compared to another known
chromatogram of colours as follows:
yellow
orange
green
red
original drop of
orange dye
What colours did the dye contain?