Methods of Purification

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Methods of Purification

• (a)Describe methods of separation for the
  components of the following types of mistures
• solid-solid
• solid-liquid
• liquid-liquid(miscible)
• Techniques to be covered for separations and
  purification include
• Use of a suitable solvent, filtration and
  crystallization or evaporation
• Distillation and fractional distillation
• Paper chromatography
• (b)

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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
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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

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Methods of Purification

• The particular method used for separating any
  given mixture depends on the nature (eg.
  solubility, physical state) of its constituents.

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Filtration

• Used to separate a solid from a liquid in which
  the solid is insoluble

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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.

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Principle of Filtration

• The filter material acts as a sieve which allows
  the smaller particles to pass through and keeps
  the bigger particles behind.

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Principle of Filtration

• Can a mixture of common salt and water be
  separated by filtration? Why?

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Filtration - large scale use

• Sand filters used in water treatment plants to
  remove solid impurities
• Dissolved substances and bacteria will not be
  removed

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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.

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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

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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 left 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.

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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.

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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 90C, 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?

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Separating a mixture of solids

• Method used depends on the nature of the solids
  -
• 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
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Separating a mixture of solids

• Basic Steps

Evaporation to dryness
Dissolution
Filtration
OR
Crystallisation
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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

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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.

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Separating the Solvent from the Solution
Crystallisation
Distillation
Evaporation to dryness
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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.

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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.

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Simple Distillation
3. the condenser is cold, so the vapour
condenses to liquid water.
2. .. vapourise. The vapour rises up the flask
thermometer
flask
sea water
condenser
Boiling chips
4. Pure water drips into the beaker. It is
distilled water.
1. Solution is heated, causing the solvent to
distillate
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Simple Distillation
Water out
To maintain even boiling, with not too much
bumping
Water in
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Liebig Condenser
Condensed vapour in liquid form (distillate)
leaves
Vapour enters
Direction of water flow
Cold water in
Water out
Water flows in anti-current to the flow of
vapour.
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Liebig Condenser
Condensed vapour in liquid form (distillate)
leaves
Vapour enters
Direction of water flow
Cold water in
Water out
This is to make sure the coldest part of the
condenser is just before the vapour escapes.
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Simple Distillation

• Simple distillation can be used to obtain
• from salt solution
• from copper(II) sulphate solution
• from a solution of sugar in ether

water
water
ether
Note The liquid that distils over is called
the distillate . The solid that remains in
the flask is called the residue .
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Simple Distillation

• Qns. Where is the thermometer placed? What is
  the reason for this?

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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.

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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.

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Separating miscible liquids - Fractional
distillation
thermometer
Fractionating column
Water out
condenser
flask
distillate
Water in
Boiling chips
mixture of ethanol and water
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Separating miscible liquids - Fractional
distillation
3. Eventually, the liquid with the lower boiling
point reaches the top and distils over.
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.
1. When heated, the liquid with the lower
boiling point will vaporize more readily.
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Separating miscible liquids - Fractional
distillation
4. The temperature stays constant at 78C. When
all the ethanol has distilled over, the
temperature reading rises above 78C. At 100C,
water starts to distil over.
5. The receiver is changed to collect each
distillate separately.
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Separating miscible liquids - Fractional
distillation
Sketch a graph of temperature versus time to show
the changes in temperature readings throughout
the distillation.
temperature
100C
78C
time
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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
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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
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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

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Fractional distillation of crude oil
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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

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The main points so far
Mixtures
Filtration
Residue (solid)
Filtrate (liquid)
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The main points so far
Mixtures
Simple Distillation
Residue (solid/ solute)
Distillate (liquid/ solvent)
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The main points so far
Mixtures
solid solid
Difference in solubility?
Add suitable solvent
Filtration
Crystals (soluble solid)
Residue (insoluble solid)
Filtrate (solution)
Crystallisation
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The main points so far
Mixtures
solid solid
YES
Difference in solubility?
solid solution
Add suitable solvent
Filtration
Sublimation
Crystallisation
Other physical difference?
Using magnet
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The main points so far
Mixtures
miscible liquids (different boiling pts.)
Fractional distillation
immiscible liquids
Separating funnel