methods of purification (a)describe methods of separation for the components of the following types...

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

(b)

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

residuemixture 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.


The filter material acts as a sieve which

allows the smaller particles to pass

through and keeps the bigger particles

behind.


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 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.

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


Basic Steps :

Dissolution

Filtration

Evaporation to dryness

Crystallisation

OR


(2) Sublimationused when one of the

solid sublimesEg.

• mixture of iodine + copper;

• mixture of ammonium chloride + sodium chloride


(3) Other methods : make use of the special

properties of the substanceEg. 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

flasksea 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

flasksea water

thermometer

condenserBoiling chips

Water out

Water in

To maintain even boiling, with not too much bumping

Liebig Condenser

Vapour entersCondensed 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 entersCondensed 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 .

waterwaterether

Simple Distillation

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

flasksea 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.

flasksea 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.


flask

thermometer

mixture of ethanol and water

Water out

Water in

condenser

distillate

Boiling chips

Fractionating column


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.


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.


Sketch a graph of temperature versus time to show the changes in temperature readings throughout the distillation.

temperature

time

78°C

100°C


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 bead

s


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 bead

s

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(Solid INSOLUBLE in (Solid INSOLUBLE in liquid)liquid)

liquid + liquid


Mixtures

solid + solid

Residue

(solid/ solute)

Distillate

(liquid/ solvent)

Simple Distillation

solid + liquid(Solid SOLUBLE (Solid SOLUBLE

in liquid)in liquid)

liquid + liquid


Mixtures

solid + solid

Residue(insoluble

solid)

Filtrate(solution)

Filtration

solid + solution

Difference in

solubility?

YES

Add suitable solvent

Crystallisation

Crystals(soluble solid)

liquid + liquid


Mixtures

solid + solid

Filtration

solid + solution

Difference in

solubility?

YES

Add suitable solvent NO

Other physical difference?

Sublimation

Using magnet

Egs.

Crystallisation

liquid + liquid


Mixtures

miscible liquids

(different boiling pts.)

immiscible liquids

liquid + liquid

Separating funnel

Fractional distillation

methods of purification (a)describe methods of separation for the components of the following types...

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