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Methods of Purification
Pure Substances & Mixtures
A pure substance contains only one type of substance, and is not mixed with any other substance.
Are the following pure substances or mixtures?
Clean tap water 100% orange juice Distilled water
mixture!
mixture!
Pure substance!
Methods of Purification
Some terms : • Solute - the solid that dissolves • Solvent - the liquid that does the dissolving • Solution - solid + solvent • Residue - the insoluble solid trapped in the
filter paper • Filtrate - the liquid that passes through the
filter paper • Sublimate - the condensed solid produced
during sublimation
Methods of Purification
The particular method used for separating any given mixture depends on the nature (eg. solubility, physical state) of its constituents.
Filtration
Used to separate a solid from a liquid in which the solid is insoluble
filter paper
filter funnel
residue mixture of solid and liquid
filtrate
Principle of Filtration
• The separation works only if the particles concerned are of different sizes, such as larger particles of an insoluble solid and smaller particles of a liquid or solution.
Principle of Filtration
The filter material acts as a sieve which allows the smaller particles to pass through and keeps the bigger particles behind.
Principle of Filtration
Can a mixture of common salt and water be separated by filtration? Why?
Filtration - large scale use
• Sand filters used in water treatment plants to remove solid impurities
• Dissolved substances and bacteria will not be removed
Decanting • Separates an insoluble solid from a liquid
• liquid is carefully poured away from the solid
which is usually heavier and settles at the
bottom of the container.
Evaporation (Heating )to Dryness
used to recover a soluble solid from its solution
Only for solids that will NOT decompose on heating.
Eg. sodium chloride (common salt) from salt solution
Crystallisation used to recover a soluble solid from its solution for solids that decompose on heating Eg. Copper(II) sulphate and most other salts
Steps : • The solution is heated (evaporated) to saturation
point OR ‘heated to remove most of the solvent’ • The saturated solution is le& to cool; crystals are
formed. • The crystals are removed by filtration. To purify
the crystals, they can then be washed with cold distilled water and dried between filter papers.
Crystallisation - the Principle behind
Substances are usually more soluble in hot water than cold water eg. more copper(II) sulphate will dissolve in water at 80ºC than at 30 ºC.
When the hot saturated solution is cooled, the cooled solution is unable to hold as much solute as when it was hot. The extra solute that cannot remain dissolved appears as crystals.
Solubility & Crystallisation Solubility data for NH4Cl in grams/100 mL H2O
• 30oC - 41.4 g • 50oC - 50.4 g • 70oC - 60.2 g • 90oC - 71.3 g
If the water was heated to 90°C, how much of the solute will be able to dissolve?! If this hot solution is then cooled to 30 °C, how much solute can the water contain now?! Hence what is the mass of crystals you would expect to obtain?
Separating a mixture of solids
Method used depends on the nature of the solids :- (1) If the mixture of solids behave differently in a
particular solvent , that is, one component is soluble in it while the other is insoluble…
carefully choose a solvent that will dissolve only one of the solids
Egs. common salt and sand; naphthalene and sand
Use water as solvent
Use methylated spirits as solvent
Separating a mixture of solids
Basic Steps :
Dissolution Filtration Evaporation to dryness
Crystallisation OR
Separating a mixture of solids
(2) Sublimation used when one of the
solid sublimes Eg.
• mixture of iodine + copper;
• mixture of ammonium chloride + sodium chloride
Separating a mixture of solids
(3) Other methods : make use of the special properties of
the substance Eg. mixture of iron filings and sulphur ….
…use a magnet • Bring a magnet to the mixture.!• The iron filings will be attracted to the
magnet while the sulphur will be left behind.
Separating the Solvent from the Solution
solute solvent
solution
Crystallisation!
Evaporation to dryness!
Distillation!
Simple Distillation
Some Terms Used : • Distillate - the liquid that distils
over • Miscible liquids - liquids that mix
completely to form a single layer • Immiscible liquids - liquids that do not
mix A solvent can be separated from a
solution and collected by simple distillation.
Principle of Distillation
A liquid boils and turns into vapour at its boiling point.
When the vapour is condensed, the (pure) liquid is obtained again.
Simple Distillation
flask sea water
thermometer
1. Solution is heated, causing the solvent to …
2. .. vapourise. The vapour rises up the flask
3. the condenser is cold, so the vapour condenses to liquid water.
condenser
distillate
Boiling chips 4. Pure water drips into the beaker. It is distilled water.
Simple Distillation
flask sea water
thermometer
condenser Boiling chips
Water out
Water in
To maintain even boiling, with not too much bumping
Liebig Condenser
Vapour enters Condensed vapour in liquid
form (distillate) leaves
Cold water in
Water out
Direction of water flow
Water flows in anti-current to the flow of vapour.
Liebig Condenser
Vapour enters Condensed vapour in liquid
form (distillate) leaves
Cold water in
Water out
Direction of water flow
This is to make sure the coldest part of the condenser is just before the vapour escapes.
Simple Distillation Simple distillation can be used to obtain
• from salt solution • from copper(II) sulphate solution • from a solution of sugar in ether
Note : The liquid that distils over is called the
distillate . The solid that remains in the flask is called the residue .
water water ether
Simple Distillation
Qns. : Where is the thermometer placed? What is the reason for this?!
flask sea water
thermometer
condenser
distillate
Boiling chips
Simple Distillation Thermometer placed at the side arm of the flask so
that it records the temperature of the vapour as it enters the condenser.!
flask sea water
thermometer
condenser
distillate
Boiling chips
Separating miscible liquids - Fractional distillation
• miscible liquids can only be separated by fractional distillation if they have different boiling points .
• Eg. mixture of ethanol and water.
Separating miscible liquids - Fractional distillation
flask
thermometer
mixture of ethanol and water
Water out
Water in
condenser
distillate
Boiling chips
Fractionating column
Separating miscible liquids - Fractional distillation
1. When heated, the liquid with the lower boiling point will vaporize more readily.
2. The fractionating column is packed with
glass beads to increase its surface area.
Vaporisation followed by condensation takes place
many times as the vapour is swept
upwards.
3. Eventually, the liquid with the lower boiling point reaches the top and distils over.
Separating miscible liquids - Fractional distillation
5. The receiver is changed to collect each distillate separately.
4. The temperature stays constant at
78°C. When all the ethanol has distilled
over, the temperature reading
rises above 78°C. At 100°C, water
starts to distil over.
Separating miscible liquids - Fractional distillation
Sketch a graph of temperature versus time to show the changes in temperature readings throughout the distillation.
temperature
time
78°C!
100°C!
Separating miscible liquids - Fractional distillation
Note : • The glass beads in the
fractionating column provides a large surface area so that condensation occurs more readily.
• The liquid with the lower boiling point distils over first, followed by the liquid with the next higher boiling point.
glass beads!
Separating miscible liquids - Fractional distillation
Note : • If the liquids in the mixture
have the same boiling point, fractional distillation is not possible.
• If the difference in boiling point is great, fractional distillation occurs readily.
glass beads!
Fractional distillation - applications
Fractional distillation can be used to separate :- • nitrogen and oxygen from liquid air • the components of crude oil • ethanol from fermented liquor
Fractional distillation of crude oil
Separating immiscible liquids
A separating funnel can be used to separate two immiscible liquids. • Eg. water and petrol.
This method can be used to separate : • a mixture of petrol
and water • engine oil and water
The main points so far… Mixtures!
solid + solid!
Residue!(solid)!
Filtrate!(liquid)!
Filtration!
solid + liquid!
liquid + liquid!
The main points so far… Mixtures!
solid + solid!
Residue!(solid/ solute)!
Distillate!(liquid/ solvent)!
Simple Distillation!
solid + liquid!
liquid + liquid!
The main points so far… Mixtures!
solid + solid!
Residue!(insoluble
solid)!
Filtrate!(solution)!
Filtration!
solid + solution! Difference in
solubility?!
YES!
Add suitable solvent!
Crystallisation!Crystals!(soluble solid)!
liquid + liquid!
The main points so far… Mixtures!
solid + solid!
Filtration!
solid + solution! Difference in
solubility?!
YES!
Add suitable solvent!
NO!
Other physical difference?!
Sublimation!
Using magnet!
Egs.!
Crystallisation!
liquid + liquid!
The main points so far… Mixtures!
miscible liquids
(different boiling pts.)!
immiscible liquids!
liquid + liquid!
Separating funnel!
Fractional distillation!
