Chapter
2
Purification of Substances
Learning Outcomes
• Explain, with illustrative examples, why pure
substances are needed.
• Describe methods of separation and purification
for the components of solid-solid, solid-liquid
and liquid-liquid (miscible) mixtures.
• Describe techniques for separation and
purification including use of a suitable solvent,
filtration, crystallisation, simple distillation,
fractional distillation and paper
chromatography.
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Chapter
2
Purification of Substances
Learning Outcomes
• Describe paper chromatography and interpret
chromatograms.
• Deduce from the given melting point and boiling
point, the identities of substances and their
purity.
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Chapter
2
Purification of Substances
2.1 The Need for Pure Substances
• Many industries need pure substances to make
products.
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Chapter
2
Purification of Substances
2.1 The Need for Pure Substances
• A pure substance is a single substance not mixed
with anything else.
– E.g. white sugar, crystals of copper(II) sulphate.
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Chapter
2
Purification of Substances
2.1 The Need for Pure Substances
• A mixture contains two or
more substances.
– E.g. seawater, milk.
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Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Filtration
• Filtration is used to separate an insoluble solid
from a liquid.
• The solid collected in the filter paper is called
the residue.
• The liquid passing through the filter paper is
called the filtrate.
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Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Filtration
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Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Filtration
• Filtration is used to purify drinking water in the
home and at water purification plants.
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Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Crystallisation
• Crystallisation separates a dissolved solid from a
solution as well-formed crystals.
• Used in obtaining pure sugar.
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E X PE R I M E NTAL C H E M I STRY
Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Crystallisation
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Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Crystallisation
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E X PE R I M E NTAL C H E M I STRY
Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Simple Distillation
• Simple distillation separates a pure liquid from a
solution.
• Vapour condenses to a pure liquid which is called
a distillate.
• Used to obtain a pure solvent from a solution of a
solute.
• E.g. distillation of seawater to obtain pure water
(desalination).
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E X PE R I M E NTAL C H E M I STRY
Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Simple Distillation
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Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Fractional Distillation
• Fractional distillation separates mixtures of
miscible liquids with widely differing boiling
points.
• Miscible liquids are completely soluble in each
other.
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Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Fractional Distillation
• In the fractional
distillation of ethanol
and water:
– Ethanol, with a lower
boiling point of 78˚C, will
distill first.
– A complete separation of
the liquids is not possible.
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Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Fractional Distillation
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Chapter
2
Purification of Substances
2.2 Obtaining Pure Substances
Fractional Distillation
• Three uses of fractional distillation:
– To separate pure oxygen and pure nitrogen from
liquid air.
– To separate substances in petroleum into fractions.
– To produce alcoholic drinks.
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Chapter
2
Purification of Substances
• A pure substance is a single substance not mixed
with anything else.
• A mixture consist of two or more substances.
Mixtures can be separated by physical methods.
• Filtration is used to separate a solid from a
liquid.
• Crystallisation is used to separate a solid from a
solution.
• Simple distillation is used to separate a pure
solvent from a solution.
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Chapter
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Purification of Substances
• Fractional distillation is used to separate a
mixture of miscible liquids. For good separation,
the liquids must have widely different boiling
points.
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