Danyal Education (Contact: 9855 9224)
“A commitment to teach and nurture”
Experimental Chemistry: Experimental Design and Methods of Purification and Analysis
Candidates should be able to:
Methods of Purification and Analysis:
al
Ed
uc
ati
on
a) describe methods of separations and purification for the components of mixtures, to include:
i.
use of a suitable solvent, filtration and crystallisation or evaporation
ii.
sublimation (*) (#)
iii. distillation and fractional distillation
iv.
use of a separating funnel (*) (#)
v.
paper chromatography
b) suggest suitable separation and purification methods, given information about the substances involved
in the following types of mixtures:
i.
solid-solid
ii.
solid-liquid
iii. liquid-liquid (miscible and immiscible)
c) interpret paper chromatograms, including comparison with ‘known’ samples and the use of Rf values
(Rf values * #)
d) explain the need to use locating agents in the chromatography of colourless compounds
(knowledge of specific locating agent is not required) (*) (#)
e) deduce from the given melting point and boiling point the identities of substances and their purity
ny
f) explain that the measurement of purity in substances used in everyday life, e.g. foodstuffs and drugs, is
important (*) (#)
Da

* not in combined Science syllabus
# not in N level Science syllabus
O Level Chemistry – Methods of Purification and Analysis
1
Danyal Education (Contact: 9855 9224)
“A commitment to teach and nurture”

al
Ed
uc
ati
on
Separation of solid-solid mixtures:
 Sublimation:
o To separate a solid-solid mixture where one component can sublime upon heating
o Eg: to separate a mixture of iodine crystals and sand
(iodine will sublime while sand remains)
o Substances which sublimes: ammonium chloride, iodine crystals, dry ice (carbon dioxide in solid
state), naphthalene (moth balls)
Magnetic Attraction:
o To separate a solid-solid mixture where one component is magnetic
o Eg: to separate a mixture of iron and sulfur
(iron is magnetic while sulfur is not)
o Applications:
 in food processing to remove metal contaminants along the food processing line
 to salvage scrap iron and steel using industrial electromagnet
Da
ny
Separation of solid-liquid mixtures:
 Filtration
o To separate a solid-liquid mixture where one component is insoluble in the liquid
o Eg: to separate a mixture of sand and water
(sand is insoluble in water)
o Applications:
 In vacuum cleaners, the vacuum bag act like a filter allowing air to past through but
trapping dust.
 In coffee maker, a filter traps the ground coffee as residue while allowing the liquid
coffee to past through.
 In water treatment plant, the raw water will past through sand filters which will trap
suspended particles while allowing clear water to past through.
O Level Chemistry – Methods of Purification and Analysis
2
Danyal Education (Contact: 9855 9224)
“A commitment to teach and nurture”
al
Ed
uc
ati
on
Separation of solid-liquid mixtures:
 Evaporation
o To separate a solid-liquid mixture where the solid (solute) is soluble in the liquid (solvent), and
the solute does not decompose on strong heating.
o Eg: to get salt from salt solution
o Applications:
 Salt pans to get salt from sea water
Da
ny
Separation of solid-liquid mixtures:
 Crystallisation
o To separate a solid-liquid mixture where solid (solute) is soluble in liquid (solvent) and solute
decomposes on strong heating
o Eg: to get sugar crystals from sugar solution
o The solution is heated to saturation point instead of dryness, and the crystals obtained by filtration.
O Level Chemistry – Methods of Purification and Analysis
3
Danyal Education (Contact: 9855 9224)
al
Ed
uc
ati
on
Separation of solid-liquid mixtures:
“A commitment to teach and nurture”
 Distillation
o To separate a solid-liquid mixture where the solid (solute) is soluble in the liquid (solvent)
o Eg: to get pure water from salt solution
o Applications:
 To make perfumes, to extract fragrant oils from plant material
 Desalination plants, to obtain pure water from sea water
Da
ny
Separation of liquid-liquid mixtures:
 Fractional Distillation
o To separate a liquid-liquid mixture where the liquids are miscible
(miscible liquids refer to liquids which can dissolve into each other to form a single layer)
o It works on the principle that each component in the mixture has a different boiling point.
o Eg: separating crude oil into its components
o Applications:
 In oil refinery, to separate crude oil into its components
 To separate liquid air into its components
O Level Chemistry – Methods of Purification and Analysis
4
Danyal Education (Contact: 9855 9224)
“A commitment to teach and nurture”
al
Ed
uc
ati
on
Separation of liquid-liquid mixtures:
 Paper Chromatography
o To separate a liquid-liquid mixture of small amount
o It works on the principle that each component in the mixture has different degree of solubility in the
solvent, and hence will be carried for different distances in the solvent.
o The more soluble the component, the further distance it is carried.
o Eg: to separate dyes in food colouring and ink
o Applications:
 To find the dyes used in ink
Useful in forgery cases, to know if a written document has been tampered by analyzing the
ink used
 To find the dyes used in food
Useful to ensure that no harmful food colourings are used in the food products for
consumption
Da
ny
Separation of liquid-liquid mixtures:
 Separating funnel
o To separate a liquid-liquid mixture where the liquids are immiscible
o Eg: separating a mixture of oil and water
O Level Chemistry – Methods of Purification and Analysis
5
Danyal Education (Contact: 9855 9224)
“A commitment to teach and nurture”
How does paper chromatography work?
Chromatography is the technique of using a solvent to separate a mixture into its components.
A chromatogram is the chromatography paper with the separated components.
Chromatography works on the principle that each component in the mixture has different degree of solubility
in the solvent, and hence will be carried for different distances in the solvent.
on
Explain the need to use locating agents in the chromatography of colourless compounds.
al
Ed
uc
ati
Some substances, like amino acids, are colourless.
In order to locate the spots on the chromatogram for these colourless substances, a locating agent is sprayed
on the chromatogram after it has dried.
The locating agent will react with the components on the chromatogram to form coloured spots.
How do you make use of the Retardation Factor (Rf ) value to identify an unknown component in
chromatography?
Rf value is the ratio between the distance travelled by the substance to the distance travelled by the solvent.
The unknown component can be identified by comparing its Rf value with the Rf values from a data book.
Rf =
Dist. Travelled by the substance
Dist. Travelled by the solvent
Da
ny
Describe the process of paper chromatography
O Level Chemistry – Methods of Purification and Analysis
6
Danyal Education (Contact: 9855 9224)
“A commitment to teach and nurture”
Da
ny
al
Ed
uc
ati
on
What are the precautions to take when carrying out paper chromatography?
O Level Chemistry – Methods of Purification and Analysis
7
Danyal Education (Contact: 9855 9224)
“A commitment to teach and nurture”
Briefly describe 3 methods to determine the purity of a substance.

Check the boiling point of the substance.
If the substance boils at a fixed temperature, then it is a pure substance.
If the substance boils over a range of temperature, then it is impure.
(A substance with small amount of impurities will boil at a higher boiling point and over a range of
temperature. Eg, salt solution will boil at over 100 deg C.)
Check the melting point of the substance.
on


al
Ed
uc
ati
If the substance melts at a fixed temperature, then it is a pure substance.
If the substance melts over a range of temperature, then it is impure.
(A substance with small amount of impurities will melt at a lower melting point and over a range of
temperature. Eg, ice with salt will melt at below 0 deg C.)
Perform a chromatography on the substance.
If the substance is pure, it will separate out into only one spot on the chromatography paper.
(Take note that if the sample never moves from the start line, it shows that the substance is not soluble in
the solvent. It does not show that it is a pure substance.)
Explain why the measurement of the purity of substances that we use in foodstuffs and drugs is so important.
Da
ny
For substances used in preparing food and drugs, it is important to ensure that only permitted substances are
being used.
If a substance is impure, and laced with some other substances, these unapproved impurities might not be safe
for human consumption, and lead to undesirable effects on human health.
O Level Chemistry – Methods of Purification and Analysis
8