ch 4 separation techniques

8/12/2019 Ch 4 Separation Techniques

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Chapter 4: Separating Techniques

Name: ______________________( ) Class: ______ Date: ____________

Recommended reference:

Heyworth, R. M., & R, B. J. (2013). All About Chemistry.Malaysia: Pearson Education South Asia Pte Ltd.

Pages 13 - 29

4.1 A Pure Substance

Learning Outcomes:

Pupils are expected to

(a) deduce from the given melting point and boiling point the identities of substances and their purity

(b) explain that the measurement of purity in substances used in everyday life, e.g. foodstuffs and

drugs, is important

Comparing elements, compounds and mixtures

Diagram Element Compound Pure Mixture! !

! !

! !


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Diagram Element Compound Pure Mixture! !

! !

! !

! ! !

Pure Substances

A pure substance is a single substance not mixed withanything else.

For example, white sugar is a pure substance. The label

on a packet of white sugar says `pure refined cane sugar'.This means no other substance is present. A crystal is a

pure substance. White sugar is made up of crystals withthe same shape. This shows that it is made up of thesame single substance: it is pure.

In nature, very few substances are found pure. Mostsubstances are mixtures.

A mixture contains two or more substances. Sea water isa mixture. It contains water, salt and other dissolved gases.

Air is a mixture of oxygen, nitrogen and other gases.

Granite rock. You know it is a mixture because it is

made up of different black and white crystals.

The coin shows the relative sizes of the granite

crystals and the coin


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Some solid mixtures are made up of different crystals. Most rocks are mixtures. Granite iscommonly used for buildings and roads. Mixtures can easily be separated into puresubstances. The process is called purification. This is done by physical methods such asfiltration, crystallisationor distillation.

A pure substance has definite properties. It has a fixed melting point and boilingpoint.Pure water, for example, melts at 0C and boils at 100C. All samples of pure watermelt and boil at these temperatures.

A mixture does not have definite properties. It does not have fixed melting and boilingpoints.

An example of a mixture is coconut oil which is used for cooking. It melts over a range oftemperatures. It starts melting at 14C and completes melting at 22C. Petrol fuel formotorcars is also a mixture. It has a boiling point range of 35C to 75C.

Several methods of purification are discussed in greater detail. The separation of mixtures

into pure substances is important. Chemists need pure substances to study their properties.

Pure substances are used in industry to make useful products such as food and drugs.Impurities in food and drugs can be dangerous because they can poison people. An

example is a substance called tryptophan which some people consume as a health food.Some years ago, a cheap but impure quantity of tryptophan was made and sold by a

chemical company for health food. Unfortunately the impurities caused irreversibledamage to peoples' nervous systems and as a result several thousand healthy peoplebecame so sick that they were forced to live out the rest of their lives in wheelchairs.

Very pure silicon crystals are needed to make the `chips' used in calculators, computers,watches and compact disc players. The great accuracy of a quartz watch is due to a littlequartz crystal which vibrates at exactly 32 768 times per second. This controls the time.


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4.2 Separation TechniquesPupils are expected to

(c) describe methods of separations and purification for the components of the following types of

mixtures:

1. solid-solid

2. solid-liquid3. liquid-liquid (miscible and immiscible)

(d) explain how the following methods are used to separate constituents of a mixture based on the

properties of the constituents:

1. filtration

2. evaporation to dryness

3. separating funnel

4. crystallization

5. distillation & fractional distillation

6. chromatography

7. magnetic attraction

8. sublimation(e) describe paper chromatography and interpret chromatograms by comparison with 'known'

samples

(f) recognise chromatography as a technique for identification of substances and determining purity

(g) recognise that crystallisation is a means of obtaining pure substances or removing impurities

(h) recognise by that crystallisation often results from solutions which have exceeded their saturation

points

(i) state some useful applications of crystallisation e.g. production of silicon wafers for microchips

(j) describe the applications of the various separation techniques in everyday life and industries

(k) suggest suitable purification techniques, given information about the substances involved

Advanced Topics

(a) describe the techniques involved in obtaining pure water from sea water in desalination plants (e.g.

distillation and reverse osmosis)

(b) describe and explain the process reverse osmosis and how desalination plants work

(c) able to perform calculations of Rf value using a chromatogram

(d) explain the need to use locating agents in the chromatography of colourless compounds


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Comparing Separation Techniques

Techniques What do youseparate

Properties ofconstituents

involved

Principle of separation

filtration Insoluble solidfrom liquid Size of particles Pore size of filter paperprevents large particles

from passing through

evaporation to

dryness

Soluble solid fromliquid

Thermal stabilityof solid

Liquid evaporates away,leaving behind the solid.

separating funnel Immiscible liquid Density Denser fluid sinks, lessdense fluid floats. TheDenser fluid can be drained

first.

crystallization Soluble solid from

liquid

Solubility

changes withtemperature

When solubility of the salt

drops as the mixture cools,the salt that cannot bedissolved will crystalliseout.

distillation &

fractional distillation

Miscible liquids Different boilingpoints

The liquid with the lowerboiling point will be distilledout earlier.

chromatography Small quantities ofsolids dissolved insolvent

Different solubilityof constituent insolvent

The more solubleconstituent will travelfurther than the less solubleconstituent on the

chromatography paper

magnetic attraction Ferromagneticsolids from solids

Ferromagnetism The ferromagnetic materialwill be attracted to the

magnet

sublimation Solids from solid Solid thatundergoessublimationinstead of melting

Some solids undergosublimation instead ofmelting, hence it can beseparated as gas when

heated.


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Filtration

Filtration is the usual method of separating a solid from a liquid. A simple way of doingthis is shown below.

The mixture is poured through a filter which isusually made of paper. The filter paper has tinyholes through which the particles of the liquid areable to pass. The particles of the solid are large.They cannot pass through the holes and aretrapped by the filter paper.

The solid collected in the filter paper is called theresidue. The liquid passing through the filter iscalled the filtrate.

When a mixture of sand and water is poured intothe filter paper, the solid sand is trapped in thefilter paper (the residue) and the water passesthrough the paper (the filtrate).

The filtrate is often a solution of, for example, asolid dissolved in water. This solution can beseparated by crystallisation.

Filtration is one of the steps in making drinking water. All solid impurities are firstremoved by f iltration. The water is then treated to kill any harmful micro-organisms.

Do you know?Some special filters can filter biological cells. The antibiotic penicillin is made from a yeast. Thepenicillin is separated from the yeast by filtration. While the small penicillin molecules passthrough the filter the big yeast cells are trap.


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CrystallisationA solid dissolves in a liquid to produce a solution . The solid that dissolves is called asolute and the liquid is called a solvent .

When the solution is heated, most of the solvent is evaporated off. The hot solution is then

allowed to cool. The solution is now said to be saturated and as it cools, the dissolved solid(solute) appears as pure crystals. This process is called crystal lisation . The impurities re-main dissolved in the solution.

The main steps in purifying a solid by crystallisation are shown in the diagrams.

Impure solids often contain impurities which are insoluble in the solvent. These insolubleimpurities are removed by filtering the mixture before evaporating the solvent.

Crystallisation vs. Evaporation to dryness

In evaporation to dryness, any soluble impurities will be deposited along with the requiredsolid, hence it cannotseparate a pure substance from its impure form.

e.g. when we attempt to obtain pure copper(II) sulfate salt from an impure mixturecontaining copper(II) sulfate and trace amount of another soluble salt, both salts will bedeposited upon evaporation to dryness.

Furthermore, evaporation to dryness is not suitable for all solutes. For example, sugardecomposes to give water and carbon while hydrated copper(II) sulfate crystals (blue)decompose to form anhydrous copper(II) sulfate powder (white)

Crystallisation is similar to evaporation to dryness, except that we do not heat until allthesolvent is evaporated. Instead, we stop heating when a saturated solution is obtained.

Applications of crystallisation

Crystallisation from solution is the most common method used by chemists to purify solids.Pure sugar is obtained this way. Some solids can be purified by melting them. The hot

liquid is then cooled slowly. Crystals form when the liquid freezes. The first crystals to formare very pure. These are collected before the rest of the substance freezes completely.This is crystallisation without a solvent. This method is used to make the very pure


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silicon used in computer chips. A pure silicon crystal is obtained by freezing moltensilicon at 1410oC.

Magnetic Attraction

Magnetic attraction is used to separatemagnetic substances from non-magnetic ones in a mixture (e.g. mixtureof iron and sulfur).

Electromagnets are used to remove scrap

steel and iron at the junkyard.

In hospitals, magnets are often used toremove iron splinters from a patients eyes.

Distillation

Distillation is used to purify liquids. In thisprocess, the liquid is changed into a gas by

boiling. The gas is pure as other substances areleft behind. This gas is then cooled. It

condenses to a pure liquid which is called thedistillate.

Distillation can be used to obtain a puresolvent from a solution of a solute. Anexample of a simple distillation is the distillation ofsea water to obtain pure water. Below shows theexperimental set-up of obtaining pure waterfrom sea water by distillation.

The seawater is boiled in the flask and thesteam produced is cooled by the condenser.The pure liquid is then collected in the beakerand the salt remains in the flask.

The thermometer shows a temperature of 100Cduring the distillation. This is the boiling point of

water. Sea water is a solution of solid salts inwater. The salts, like other solids dissolved inwater, cannot be distilled and remain in the flask.


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Fractional Distillation

Miscible liquids are completely soluble in each other. This means two liquids mix to formone liquid. Fractional distillation can be used toseparate a mixture of miscible liquids.

This is done using a fractionating column.

A fractionating column separates liquids in order of boiling points. The liquid with the lowestboiling point is distilled first. The liquid with the highest boiling point is distilled last.

Ethanol is an important solvent. It can be made by fermenting cane sugar. A mixture of

ethanol and water is obtained from the fermentation. Ethanol is then obtained from thismixture by fractional distillation. The experiment below shows how this can be done in the

laboratory.

The boiling points of ethanol and water are 78Cand 100C respectively. In the experimentalset-up (top right) the vapour from the boiling mixture of ethanol and water in the flaskcontains a larger percentage of the lower boiling point ethanol. As the vapour moves upthe fractionating column, it repeatedly condenses and boils inside the column. Each time themixture boils, the percentage of the lower boiling point ethanol increases. By the time

the vapour reaches the top of the fractionating column, it has become almost pureethanol.

This vapour then passes into the condenser where it is cooled, and condenses into liquid

ethanol. The thermometer shows a constant temperature of 78Cwhen the ethanol isbeing distilled. The water, which has a higher boiling point, remains in the flask untilalmost all the ethanol has distilled. (See graph on top left)


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In this experiment, a complete separation is not effected. The distil late contains a very largepercentage of ethanol and a little water.

Fractional distillation is used to obtain pure oxygen andpure nitrogen from air. The air is first cooled to about -200C.At this temperature, air is a l iquid.

The liquid air is then distilled by allowing the liquid to warmup. Nitrogen has a lower boiling point than oxygen, so itdistils first. Then the oxygen (with a higher boiling point)distils. This produces nitrogen and oxygen separately.

Fractional distillation is also used in the petroleumindustry. Petroleum is a mixture of liquids calledhydrocarbons. Fractional distillation is used to separatethe hydrocarbons into, for example, lower boiling pointpetrol and higher boiling point kerosene.

Large amounts of liquid chemicals are manufactured in the petroleum industry, andfractional distillation is commonly used to separate the chemicals into pure liquids.

Separating Funnel

Many liquids do not mix together. An exampleis petrol and water. In a mixture of petroland water, the lighter liquid (petrol) formsa separate layer on top of the heavierliquid (water).

A separating funnel can be used toseparate such mixtures that containliquids which are not miscible. The tapis opened so that the lower water layer runsout first. The tap is closed as the last dropof water leaves the funnel. The tap is thenopened again to run the petrol into

another beaker.

Separating petrol and water with a separating funnel


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Chromatography

Chromatography is a method of separating and identifying mixtures. There are severaltypes of chromatography, one of which is paper chromatography.An example of the useof paper chromatography is in the separation of the dyes in black ink. This is shown in theexperimental set-up below.

Paper chromatography of black ink

A im: To separate and identify the dyesin black ink

1 Draw a pencil line on a piece ofchromatography paper (this paper islike filter paper).

2 Place a drop of black ink on the pencil

line. Also place drops of coloured dyes(red, orange, blue, green, etc.) on thepencil line. These dyes are the ones youthink might be in the black ink.

3 Roll the paper into a cylinder andsecure it. Then place the paper in a

beaker with a suitable solvent.

4 The solvent will travel up the paper.When the solvent reaches the top of thepaper, take the paper out of thebeaker and allow it to dry.

A suitable solvent for this separation is a mixture of butanol (an alcohol), ethanoic acid anda little water. The solvent chosen depends on the type of substances being separated.

In the above experiment, the solvent travels up the paper. The dyes on the pencil line dissolve inthe solvent and travel up the paper at different speeds. Hence, the dyes are separated. Theresult is called a chromatogram. A typical example is shown on the next page.

.

Identical dyes travel the same distance up the paper. The unknown dyes in the black ink

can be ident ified by comparing them with the known dyes. The result of the experimentshows that - black ink is made up of four different dyes; - three of the dyes are the known

red, blue and green dyes; - there is no orange dye in the black ink; - black ink contains onedye that is different from the four known dyes.

Chromatogram of dyes in black ink


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Advanced:

Chromatography can also be used to separate and identify colourless substances. Thechromatogram is sprayed with a locating agent to show where the substances are onthe paper. The locating agent is a chemical that reacts with the substances to produce a

coloured product.

An example of separating colourless substances is in identifying the sugars in a fruit juice.The same method is used as for the black ink. A drop of fruit juice is used in place of the

black ink. A different solvent is used. At the end, the chromatogram is sprayed with silvernitrate solution followed by dilute sodium hydroxide. These chemicals make the

sugars appear as brown spots. A typical result is shown.

The result of the experiment shows that

- fruit juice contains three different sugars;- two of the sugars are sucrose andglucose;

- the fruit juice does not contain maltose;- the fruit juice contains one other sugar different from the three known sugars.

Paper chromatography can also be carried

out with the solvent running down the

paper. The arrangement of the apparatus is

shown on the left. This descending method

(shown on the left) of paper

chromatography works better for longer

pieces of paper as the solvent does not

have to move against gravity, and thusflows more quickly. This means that the

solutes which are separated can travel

further and thus the separation between

the spots is greater.


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The chromatogram of the sugars present in fruit juice shows the presence of threedifferent sugars. One of the sugars is different from the three known sugars (sucrose,glucose and maltose) on the chromatogram. This sugar can be identified from its Rfvalue.

The Rfvalue for any substance on a chromatogram is equalto:

The distances are measured from the starting line. In the figureon the right, the Rfvalue of the substance in the `spot' equalsx/y.

The Rfvalue for the unknown sugar in the earlier

chromatogram can be calculated as follows (shown below).

The sugar is then identified from the R f valuesfor different sugars that have already been

calculated. The R fvalues depend on the solventand the temperature.

The R f values for some sugars in the solvent

used to produce the chromatogram are given in thetable (right). From the table, the unknown sugar is

fructose.

Chromatography can be used to separate and identify complicated substances such asdyes and drugs. It is used to identify artificial dyes in food. Only a few dyes are safe for food.Many are poisonous. Government laboratories use chromatography to check that onlyapproved dyes are used in food.

Pesticides are sprayed on green vegetables to kill insects while herbicides are used todestroy weeds. These chemicals can be harmful to people. That is why green vegetables must

be tested to check that they only contain tiny amounts of these chemicals when they are sold in

Distance moved by theDistance moved by the solvent


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markets. This testing is done by chromatography which can identify the chemicals present andtheir amounts.Paper chromatography is only one type of chromatography. Another important type is gaschromatography. In gas chromatography, a gas is used in place of a liquid to separate

the substances being analysed. Gas chromatography is used to find out if athletes areusing illegal drugs to improve their performance in competitions (as in the OlympicGames).

The big advantage of chromatography is that it can be used to identify very tiny amounts ofsubstances. It can detect less than 0.000 000 000 001 gram or 10-12 g of the substance.

Testing the Purity of Substances

Pure substances have fixed melting and boiling points. These temperatures are given in most

books. So we can easily find out the melting point or boiling point of a pure substance.

Melting and boiling points are changed by the presence of impurities. This fact can be used to

find out if a substance is pure.

Melting Point

Impure substances are mixtures. Mixtures do not have a fixed melting point. An example of amixture is coconut oil which is used for cooking. It melts over a range of temperature. It starts

melting at 14C and completes melting at 22C. This fact can be used to find out if asubstance is pure. If it is pure, then all of it should melt at the same temperature. An impure

substance melts over a range of temperature.

Also, impurities lower the melting point. Pure naphthalene melts at 80C. If the

naphthalene is impure, it melts below 80C. Impure naphthalene may melt at 78C or76C.The greater the percentage of impurity, the lower the melting point. Solids must be verypure to melt exactly at their true melting points. Hence we can find out i f a substance ispure by measuring its melting point.

Boiling PointWe can find out if a liquid is pure by distilling it. If it is pure, all of it distils at the same

temperature. This temperature is the boiling point.If a liquid is impure (that is, a mixture), it distils over a range of temperature. Forexample, petrol which is used as a fuel for motorcars is a mixture. It has a boiling range of35C to 75C. Similarly, impure ethanol containing water as an impurity, might startdistilling at about 78C (the boiling point of pure ethanol), but the distillation may not be

complete until the temperature reaches about 100C.The boiling range for an impure substance depends on the actual impurities present andtheir percentage.


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Reverse OsmosisIn an osmosis process, a selective membrane is used. There are very small holes in thismembrane (known as pores) which only allow very small molecules like water molecules topass through. Undesirable molecules, bacteria and virus cannot pass through the membrane

and is filtered out.

In a normal osmosis process, the solventnaturally moves from an area of low solute

concentration (higher water potential, througha membrane, to an area of high soluteconcentration (lower water potential).

In reverse osmosis, pressure is applied onone side of the membrane, forcing the watermolecules to pass through the membrane,

leaving behind the undesirable salt, virusand bacteria. Clean water is obtained fromthe other side of the membrane.

References

http://en.wikipedia.org/wiki/Desalination

http://www.pub.gov.sg/water/Pages/DesalinatedWater.aspx

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