6. Lesson Objectives
At the end of the lesson, students
should be able to:
Name the 3 methods in which we can
test for pure substances
Identify the effect of impurities on a
substance.
Choose suitable separation
techniques to separate a given
mixture.
Explain the working principles behind
7. To test for Purity of a
Substance
Methods of Testing Observation
Melting point
If a substance is pure, it will
melt at a fixed temperature.
Boiling point
If a substance is pure, it will
boils at a fixed temperature.
Use of
chromatography
If a substance is pure, it will
show only one spot on the
chromatogram.
8. Effect of impurities in a
substance
Attributes Effect of impurities
Melting point
A substance melts below its
melting point.
Boiling point
A substance boils higher
than its boiling point.
Use of
chromatography
Gives rise to more than one
spot on the chromatogram.
9. Physical Methods of
Separation
Only separate the different substances
in a mixture.
No new substance is formed.
The choice of separation
technique depends on the
nature of the mixture.
10. Common physical methods to separate
a mixture
Filter
Crystallization
Distillation
Magnet
Decant
Evaporation
Centrifuge
Chromatography
14. Filtration
Solid-liquid mixture (insoluble solids) E.g.
sand in water
A filter paper is used because it contains very
tiny pores.
Mixture of solid
and liquid
17. Crystallisation
To separate dissolved pure solid
from a solution.
1. Heat the solution
until saturated.
3. Pure Crystals formed.
Evaporation of solution
2. Saturated solution
Maximum amount
of solid dissolved
in a given solvent.
18. How to test for saturated
solution?
Dip a glass rod into the solution and
removed.
If crystals are formed on the glass rod,
it means that the solution is saturated.
This is the saturation point or
crystallisation point.
19. Think!!!
Why can’t we just evaporate the
solution to dryness to obtain the
crystals?
For some substances, they will
decompose when heated. charring
When water is removed, any soluble
impurities will be left on the crystal
not pure.
20. Mixture of Sand and Sugar
How do I get sand and sugar back?
Filtration Sand is the residue,
filtrate is the sugar solution
Crystallisation pure sugar crystals
Is the filtrate always a pure liquid?
22. Distillation
To separate a liquid from a solution.
E.g. salt solution, sugar solution
Involves two physical state
changes.
23. Set-up for distillation
Liquid is heated until
its boiling point and
changes to vapour.
1
Mixture
Vapour is cooled
and changes to
liquid (distillate).
2
Main concept
Boiling point of the liquid
To ensure smooth
boiling.
Other examples:
1. Marble chips
2. Porcelain chips
Bulb of thermometer placed beside
the side arm of the distillation flask to
ensure accurate measurement of
boiling point.
24. Boiling chips
To ensure smooth boiling
During boiling of water, big air bubbles
can be seen.
These air bubbles causes ‘bumping’.
Boiling chips can reduce this
‘bumping’ effect.
28. Fractional distillation
For liquids that are miscible and
having different boiling points.
Miscible – mix together completely to
form a solution. E.g. water and ethanol
Liquid with lower boiling point distill
over first.
Liquids with higher boiling point will
return back into the round-bottomed
flask.
29. Set-up of fractional distillation
Round-bottomed flask
Fractionating
column
Thermometer
Liquid-liquid mixture
Boiling chips
Condenser
Distillate
Water inlet
Water outlet
Main concept
Boiling point of the liquids
where the liquid with the
lowest boiling point
will distill over first.
30. During fractional distillation,
• The liquid with lowest boiling point
will distill over to the condenser first.
• The vapour of liquids with higher
boiling point condenses along the
fractionating column and re-enter the
round-bottomed flask.
31. Fractionating Column
Packed with glass beads
Provide large surface area for
evaporation and condensation of
liquids with different boiling points
for effectively separation.
32. Temperature Profile
First liquid distill over
upon reaching its
boiling point
Liquid with lowest boiling
point will distill over first
Second liquid distill
over.
34. Think!!!
What is the difference between
distillation and fractional distillation?
Simple distillation Fractional
Distillation
No fractionating
column
Has a fractionating
column
Separate liquids
whose boiling points
differs more than
20°C
Able to separate
liquids whose boiling
point differs by less
than 20°C
35. Think!!
Is it possible to obtain salt from
seawater?
Yes
However, industrially, we use reverse
osmosis to obtain salt (Desalination).
What is the reason?
Distillation exhaust a lot of energy,
resulting in high production cost.
36. Centrifugation
Spin sample very rapidly:
denser materials go to
bottom (outside)
Separate blood into serum
and plasma
◦ Serum (clear)
◦ Plasma (contains red blood
cells ‘RBCs’)
Check for anemia (lack of iron)
Blood
RBC’s
Serum
A B C
AFTER
Before
39. Separation by Chromatography
sample
mixture
a chromatographic column
stationary phase
selectively absorbs
components
mobile phase
sweeps sample
down column
detector
http://antoine.frostburg.edu/chem/senese/101/matter/slides/sld006.htm
40. Separation by Chromatography
sample
mixture
a chromatographic column
stationary phase
selectively absorbs
components
mobile phase
sweeps sample
down column
detector
http://antoine.frostburg.edu/chem/senese/101/matter/slides/sld006.htm
41. Ion chromatogram of orange
juice
time (minutes)
detector
response
0 5 10 15 20 25
Na+
K+
Mg2+ Fe3+
Ca2+
42. TYPES of MIXTURES
MIXTURE SUBSTANCES TO
OBTAIN
METHODS
Salt mixed with
broken glass pieces
Salt crystals Evaporation
Sea Water Pure Salt Desalination/
Distillation
Copper (II) Sulphate
solution
Copper (II) Sulphate
Crystals
Crystallisation
Ink Pure Water Paper
Chromatography
Wine Ethanol Fractional
Distillation
43. Check your Understanding
Which process is used to separate
Three water-soluble dyes?
Two miscible liquids with boiling points of
78°C and 100°C?
Water containing an insoluble solid?
Water containing a dissolved solid?
Chromatography
Fractional distillation
Filtration
Crystallisation
44. Check your Understanding
A mixture contains the following three liquids that are
completely miscible:
The liquid can be separated by fractional distillation.
State, with a reason, which liquid will distill over first.
Propanone. It has the lowest boiling point among the 3
liquids.
Name an industrial process that involves fractional
distillation.
Fractional distillation of crude oil
Liquid Boiling point (°C
)
propanone 48
Ethanol 78
Water 100
45. What did you learnt today?
3 methods to test for purity
Effect of impurities in a substance
3 separation techniques and their
working principles
46. Summary
Simple distillation
Fractional distillation
To separate miscible
liquids
To separate solvent
from solution
Crystallisation
Filtration
Separation
technique
To separate solids
(only one is
soluble in a
solvent)
To separate
solid from a
liquid
To separate
solid from a
solution
