Purification of Substances 1 1222944911769062 8

8/8/2019 Purification of Substances 1 1222944911769062 8

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Methods of Purification:a.Filtration

a.Crystallization

The need for pure substanceCriteria for purity

Objectives:

Know their principles,

proceduresandapplication



The NEED for pure substanceImportance of purity of substancesused in everyday life:

Chemist need pure substances tostudy their properties.

Pure substances are used inindustry to make useful productssuch as food and drugs .



In pharmaceutical industry, medicinesmust be tested for purity before they aresold. Impurities in drugs and food mayproduce undesirable side effects.

The NEED for pure substance



In highly precision engineering e.g.Production of silicon chips , even smallamount of impurities can greatly reducethe effectiveness of a component in anelectronic device.




In food and beverage industry, the typesof chemicals which can be added into our food are controlled by certaingovernmental guidelines to ensure thatour food and drinks contain chemicals

that are safe for consumption.




Criteria for purity

A pure substance is made up of only

one substance and not mixed withanything else.

In nature, many substances are notpure. Most are found impure asmixtures .



A mixture is a substance thatcontains two or more substances.

The need for pure substance

They can be easily separated intopure substances by purificationtechniques.

The substances do not react withone another chemically .

(Physical method)



Criteria for purity

MixturesPureSubstances

Properties

BoilingPoint

Melting

Point

Exact andFixed

Not Fixed(Higher)

Not Fixed

(Lower)

DefiniteNot

Definite

Exact and

Fixed



Impurities affect the melting andboiling point in the following ways:1

) Decrease the melting point;E.g. Frozen sea water melts below 0°Cat -2.5°C

2) Increase the boiling pointE.g. Seawater boils at about 1 02°C

Criteria for purity



3 ) Increase the range at which meltingand boiling occurs;

E.g. Coconut oil melts over a rangeof temperature. Starts melting at

14 °C and complete melting at 22°C.

Petrol fuel for motorcars boils over a

range of 3

5°C to 75°C.

Criteria for purity



A pure substance contains only one type ofsubstance (that is, one type of molecule oratom)

Melting point of pure water = 0 ºC

Boiling point of pure water = 100 ºC

So, do you think the temperature for seawater is also 100 ºC?



Pure solid/liquid will melt/ boil at onlyONE temperature.

E.g. Water melts at 0°C and boils at1 00°C.

Criteria for purity

To find out if the substance is pure:Melt at its truemelting point

Boil at its true

boiling point

Measure its melting point

Measure its boiling point



Pure Water melts at 0°C and boils at1 00°C.

Criteria for purity

When impure: Melting pointdecreases further, Boiling pointincreases

0oC 1 00 oC-2 oC 1 02 oC

LiquidSolid Gas



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Some important terms

Some terms that you should know:

A solute is a dissolved substance (e.g.salt in salt solution).

A solvent is a substance which dissolve

a solute (e.g. water in salt solution.Miscible liquids are liquids that arecompletely soluble in each other.



MixturesSolutions and suspensions

What is a solution ?A solution is a mixture that appears tobe made of one substance .copper (II) sulphate solution, fizzy drinks

What is a suspension ?A suspension is a mixture in which twoor more parts can be seen with our eyesoil + water, muddy water, chalk water



How do we obtain Puresubstances from a mixture?

We have to first consider what are

characteristics of the substancesinvolved before we decide on thesuitable method of purification.



Possible scenarios of mixtures!

To separate a soluble and insoluble solid(E.g. Salt and sand)/ To separateinsoluble solids from a solution (E.g.

Sand from water)

To separate pure solid from a solution

(E.g sugar from sugar solution)



To separate or identify colors,dyes and pigment.

To obtain a pure liquid from asolution (E.g. water from saltsolution).

To separate miscible liquids withdifferent boiling points.

Possible scenarios of mixtures!



Types of separationtechniques

Suitable method of purification:

Filtration

A im: To separate a soluble andinsoluble solid (E.g. Salt and sand)/ To

separate insoluble solids from asolution (E.g. Sand from water)



Filtration

1. Glass rod

2. Retort stand

3. Mixture

4. Filter paper 5. Filter funnel

6. Beaker

7. Filtrate

Filtration is

usual method of separating solidfrom a liquid.

Solid-Solid mixture

Solid-Liquid mixture



A Glimpse of our First miniPractical on FILTR A TION !!



D o you remember how to do filtration?



Filtration ± How it works

Mixture poured through filter paper with tiny

holes (´poresµ) .

Large particles of the solid getstrapped.

- Residue

Small particles of liquid passesthrough the filter.

- Filtrate

Filter paperwith small holes

Small particlesof liquid

Large particlesof solid

Solid-Solid mixture




A pplications:

Filtration ± How it works

Solid-Solid mixture




A little extra: Buchner funnel

Vacuum filtration is used primarily to collect adesired solid,Vacuum filtration is faster than gravity filtration,because the solvent or solution and air is forcedthrough the filter paper by the application of reduced pressure.

Note: D o not use vacuum filtration to filter a solidfrom a liquid if it is the liquid that you want and if the liquid is low boiling. A ny solvent which boilsat about 1 25 degrees or lower will boil off under the reduced pressure in the vacuum flask.



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Buchner funnel

V acuum filtration



W hat happens to the filtrate?

Can filtration be done toseparate solids dissolved in

solution?

FiltrationSolid-Solid mixture




NotesCan D yes be separated by filtration?Can salt be obtained from a mixture of saltsolution by filtration?D yes in ink go through the filter A s dye moleculesare smaller than the pores in the filter paper and gostraight through them.

Salt in sea water cannot be separated from water byfiltration, as the sodium chloride molecules aremuch smaller than the pores in the paper.



Possible Scenario

? ?

Evaporate

Crystallisation

Suitable method of purification:W hich should we use?



Evaporation

A im: To obtain a solute from its solution byvaporising A LL the solvent. (E.g. Toobtain salt from salt solution.)

How it works?When a solution is heated, only the

solvent boils away while the soluteremains. Evaporation is a processwhere A LL the liquid has been

vaporised .




When a solution isheated, the liquid(solvent)

evaporates, leavingbehind the solid(solute) as residue.

You will have only

the solidcomponent, as theliquid component islost as vapour tothe surrounding.

EvaporationSolid-Liquid mixture

salt solutionevaporating dish

Salt solution

heat



2 types of evaporation technique:


Evap oration

Boiling to dryness Evaporation using water bath (slower)



Conditions!!!!!

Evaporation

This method cannot be used for

substance which will decompose uponstrong heating.

E.g. Sugar, potassium nitrate etccannot be obtained from their solution by evaporation them todryness. They will decompose.

There isA lways

conditionsinvolved!!!




CrystallisationA im: To obtain PURE solids from a

solution (solid will decomposeupon strong heating).

E.g. To obtain sugar from sugar solution , Copper (II) Sulphate from

Copper (II) Sulphate solution .

Do youknow

others?


solution



CR Y STA LLIS ATION



GlassRod

SolventImpureSolid

1. 2.

SolutionEvaporatingdish

Crystals

3. Pure crystalsappear when thehot solution cools.

4.

Filter paper

Crystals

Crystals are thencollected and driedbetween filterpapers.

The solution must be

Often after filtration.

Crystallisation

saturated.

Heat the solution until MOST of the solvent evaporate off. A llowit to cool. (Steps to make

solution saturated )

D issolved solid appears aspure crystals as the solutioncools. Impurities remain

dissolved in the solution .




Crystals

S alt crystalsS ugar crystals

Copper(II)

sulphate crystals




Have you noticed the difference of evaporation from Crystallisation?

Evaporation Crystallisation

D oes solid decomposeupon strong heating?

No Yes

How much solventhave to be vaporised?

A ll Most

In a Solid-Liquid mixture , both Evaporation andCrystallisation is used to obtained solid from itssolution.



Summary:Evaporation & Crystallization

T o obtain aT o obtain a solublesolublesolidsolid from afrom a liquidliquid ininsolidsolid--liquid mixtureliquid mixture

EvaporationEvaporation CrystallizationCrystallizationIf solidIf solid does notdoes not

decomposedecompose ononheatingheating

If solidIf soliddecomposesdecomposes onon

heatingheating



1 . Which of these is a pure substance?Salt, Cooking oil, Tea leaves, Milk

A liquid is likely to be pure if A . it is colourless.

B. it boils at an exact temperature.C. it dissolves in water.D . it is neutral.

Q uestions



Q uestions1 . Which of these techniques would produce

the largest crystals from an aqueous solutionof copper ( II) sulphate? A ) A llowing the solution to evaporate

naturally.B) Boiling off the water from the aqueous

solution.

C) Boiling off the water until crystals juststart to appear.D ) Heating the solution strongly then

cooling it using ice.

Purification of Substances 1 1222944911769062 8

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