Physical Sciences 17

Rice is the staple diet of the people ofKerala. What is pictured above is acommon sight in the courtyards offarmers in the harvesting season. Whyis the paddy winnowed after thereaped stalk bundles are threshed?When the bundles are threshed therewill be plenty of chaff mixed withpaddy. What happens when it iswinnowed? Why?

When paddy is de-husked what we getis rice mixed with bran. How are theseseparated?

Many of the substances which we usein our daily life are found in the formof mixtures.

What are the characteristic features ofmixtures? Let us conduct anexperiment to find out.

SEPARATION OF MIXTURES2

Table. 2.1

What might be the most practicalmethod of separating this mixture?

Find out on the basis of the propertiesof the components.

Take a teaspoon each of iron powderand sulphur powder in a china dish.Mix them thoroughly. Look at themixture through a lens. Can you seethe black grains of iron and the yellowpowder of sulphur?

Some of the properties of iron andsulphur are given in the table below(table 2.1)

Iron Sulphur

Density More Less

Magnetism Magnetic Non-magnetic

In water Insoluble Insoluble

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Can you suggest other methods?

If the proportion of iron powder andsulphur powder in the mixture ischanged, can the same methods beused to separate the components?Why is this possible?

Now, with the help of the teacher,conduct the following experimentsalso, and record the observations.

Take a small portion of the iron andsulphur mixture in a test tube andadd dilute hydrochloric acid (HCl).Which is the gas produced? Whatdo you see at the bottom of the testtube? Which component hasreacted with the acid? Write downthe equation.

Take another portion of this mixturein a test tube and add a little carbondisulphide. Shake the test tubegently. What do you observe?Explain.

On the basis of the above experimentalobservations discuss the following.

What is the minimum number ofsubstances necessary to form amixture?

What is the required proportionatequantity of each substance?

Is there any change in thechemical/physical properties of thesubstances when they are mixed?

Is the formation of a mixture achemical change or a physical change?

Note down the conclusions in thescience diary.

Aren't the components of the mixtureused in the above experiment solids?

Saline water is a mixture formed froma solid and a liquid. Can mixtures beformed from substances in anyphysical state? Find more examplesand add to the table.

List more instances after discussion.Tabulate the method of separation ineach instance and also the property onthe basis of which the separation iseffected (table 2.3). Enter in the sciencediary.

Fig. 2.1

Table. 2.2

Many mixtures like these are handledby us in our everyday life. On manyoccasions we have to separate theircomponents.

Consider some examples.

• Removing the sediment from tea.

• Removing stones and paddy fromrice.

Type of mixture Examples

Solid and solid Soil, ......, ......

Solid and liquid Saline water,......, ......

Liquid and liquid ......, ......, ......

Gas and gas ......, ......, ......

................................ .......................

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Now it is clear that mixtures areseparated into components based onthe properties of those components. Letus familiarise ourselves with someother methods used to separatemixtures.

How to separate clear waterfrom muddy water

Is it possible to filter clear waterfrom muddy water by using thesame type of strainer used forfiltering tea?

If a filter paper is used instead ofthe strainer?

Instance Method Property basis

Tea Filtration Difference in the size ofthe particles

Stones and paddy in rice Picking up by hand Colour, size

Saline water Vaporisation Tendency to evaporate

Muddy water - -

Iron + sand - -

Sugar + sand -, -, - -

Table. 2.3

Fig. 2.2

Arrange the equipment as shown in theillustration, and try to separate byfiltration clear water from water inwhich soil is mixed and water in whichchalk powder is mixed.

Why was it possible to separate thewater using filter paper?

Why does one component remainin the filter paper after filtration?

Think in terms of particle size. Writedown your conclusion in the sciencediary.

Can this method be adopted toseparate water and salt from salinewater? Explain your answer.

Separation of components ofsaline waterHow can the salt and water in salinewater be separated?

You know that common salt is obtainedby the vaporisation of sea water usingsolar heat.

What happens to the water whensaline water is heated?

Why is the salt left behind? Is thereany method for collecting theevaporated water?

clearwater

filterpaper

glassrod

muddy water

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Fig. 2.4

See the illustration showing theequipment for separating suchmixtures (fig 2.3)

Separation of a mixture ofcompletely miscible liquidsAcetone and water are two liquids thatmix with each other, in all proportions.The boiling point of water is 100°C andthat of acetone is 56°C. Let us see amethod of separating the componentsof this mixture based on the largedifference in their boiling points. Seefig 2.4. Have you noticed thethermometer fitted on the top of theflask? When the mixture is heatedwhich component boils and evaporatesfirst? Why? How can this substance berecovered from the vapour? Discussand note down the conclusions in thescience diary.

The mixture (saline water) is boiled ina round bottomed flask

Which component evaporates, saltor water?

When the vapour passes through thecondenser, it condenses and flows intothe collecting flask as a liquid.

What is the substance that remainsin the round bottomed flask?

This process is known as distillation.

Ordinary drinking water containsmany salts dissolved in it. These saltsare removed by distillation in order toproduce distilled water used forinjecting medicines and in storagebatteries.

This process is used in separatingmixtures where one component isvolatile and the other is not. Find outother examples.

Fig. 2.3

thermo meter

condenser

condensedwater

salinewaterflask

water

This method is possible only whenthere is a substantial differencebetween the boiling points of the twocomponent liquids. Also they shouldnot decompose on boiling. This typeof separation is also called distillation.How can the component liquids beseparated if there is only a smalldifference in their boiling points?

water

thermometer

water

condenseracetone

+water

mixtureflask

acetone

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Ethanol and methanol are twocompletely miscible liquids with onlya small difference in their boilingpoints. The equipment shown in fig.2.5 is used to separate this mixture.

can also be separated on the basis ofthe increasing order of their boilingpoints. This method is called fractionaldistillation.

Crude oil is a mixture of liquid hydrocarbons which have only smalldifference in their boiling points. Thecomponents of this mixture such aspetrol, diesel, kerosene etc. areseparated by the above mentionedmethod of fractional distillation.

How to separate the differentcomponents of air?Ammonia is an important raw materialin the manufacture of chemicalfertilizers. Ammonia is manufacturedby combining nitrogen and hydrogen.Nitrogen is obtained by separating itfrom air.

Some details about the components ofair are given in table 2.4.

Oxygen Argon Nitrogen

Boiling point -1830C -1860C -1960C

Quantity (%) 20.9 0.9 78.1

Table. 2.4

What is the physical state of oxygenat - 1820C?

And at _1840C ?

Which component has the highestboiling point?

When the vapours of the mixture passthrough the fractionating column, dueto the repeated condensation andevaporation the vapours of methanol,with a lower boiling point first passesout of the fractionating column,condense and get collected in theconical flask. After this, ethanol whichhas a slightly higher boiling point canbe collected in similar fashion inanother flask. If there are more thantwo components in the mixture, they

Aircooled at

high pressure_1980C

allowed toevaporate

slowly in afractionating

column

Liquefiedair

Nitrogen_ 1960C

Argon _ 1860C

Oxygen_ 1830C

Flow Diagram 2.1

Fig. 2.5

thermometerwater

water

condenser

fractionatingcolumn

methanol

methanol+

ethanolmixture

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Can the components of blackink be separated?The ink we use is a dye whichdissolves in water.

Carry out the following experiment.

Take some black ink in a china dishand heat it. (It is enough to keep thedish on hot sand bath.) Can you seethe vapours? Once the evaporationstops, remove the dish from the heatsource.

Which could be the substance thathas evaporated?

Is there anything left behind in thedish?

Is ink a single substance? Or is it amixture? What is your conclusion?

In black ink is there just onecolour or is it a combinationof many colours?Take a filter paper strip 1 inch wideand 6 inches long. Put one drop ofblack ink (from a sketch pen or afountain pen) at about 1 inch from one

Which one has the lowest?

See the flow diagram showing theprocess of separation of these gases.(flow diagram 2.1)

Which gas will condense first intoliquid state?

From the liquid mixture whichcomponent will evaporate first?

Which component is obtained firstfrom the fractionating column?What will be its state - solid, liquidor gas?

Which will be the last componentobtained? In which physical state?

Note down your conclusions in thescience diary.

Thus we have become familiar withsome of the processes of industrialimportance.

How to separate a mixture oftwo immiscible liquidsSuppose kerosene gets mixed withwater. How can they be separated?Take some kerosene and some waterin a bottle. Shake the bottle well. Keepthe bottle motionless for a while. Whatdo you see? Why does this happen?

The arrangement for separation of thecomponents of such a mixture is shownin the figure. See fig 2.6.

Which liquid gets collected at thebottom?

Why does kerosene remain abovewater?

How can the water underneath beremoved?

What if these two liquids weremiscible?

Fig. 2.6

separatingfunnel

water

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Fig. 2.7

end of the filter paper. Allow it to dryfor one minute. Take water in a jar.Immerse the filter paper (as shown inthe figure) into the water in the jar sothat the ink spot is just above the waterlevel (fig 2.7).

Observe carefully as the water rises upthrough the filter paper. Write downyour observations in the science diary.

Do different colours appear in thefilter paper?

What could be the reason for thecolours rising through the paper?

AdsorptionSome substances like cotton, sponge etc.absorb liquids into them. But some othersubstances hold particles of certain othersubstances on their surface alone. Thisphenomenon is called adsorption. In thisway, the carbon particles in gas masksremove poisonous gases from the air andthe charcoal in water filters removescoloured and foul smelling substancesfrom water. So also, dust particles stickto walls because of adsorption. The rateat which adsorption occurs on asubstance depends on the nature of thesubstance. During adsorption of mixturessome of the components are adsorbed to a

greater extent whereas some othercomponents are not adsorbed to thatextent.

The dyes present in ink are usually amixture of two or more colouredsubstances. The water rises through thefilter paper by capillary action. Theweakly adsorbed component risesfaster through the paper. Those whichare strongly adsorbed rise slowly. Inthis way the colours are separated. Asthis method was first used forseparating coloured substances it wastermed chromatography. Chromato-graphy is the process used forseparation of solutes soluble in thesame solvent. See the uses of thisprocess.

• separation of components fromdyes

• separation and identification ofchemicals mixed in blood

Look at a situation in every day lifewhere separation of mixtures happens.

Butter from curdYou would have seen curd beingchurned to obtain butter. Duringchurning, particles of butter areseparated. Being less dense, theseparticles float on top of the curd.

An instrument called centrifuge (fig2.8-a) is used to separate componentsfrom a mixture using the difference indensities of the components.

Examine some situations where thisprocess is used.

glass rod

glass jar

paper clip

filterpaper

water

ink drop

filterpaper

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Can the constituent elementsof a compound be separated?As already understood, formation ofmixtures is a physical change. But theformation of compounds is a chemicalchange. Compounds are formed bychemical combination of elements infixed proportions. Therefore, theelements in a compound can beseparated only by suitable chemicalprocesses.

See the compounds and their formulaegiven below.

Mercuric oxide - HgO

Calcium oxide - CaO

Both are metal oxides.

If HgO is heated it decomposes into Hgand O2. But if CaO is heated theelements do not separate.

In order to separate the elements incompounds different methods suitedto the nature of the compounds areadopted.

You can familiarise yourselves withthese methods when you study moreof chemical processes in higher classes.

Fig. 2.8 (b)

Fig. 2.8 (a)

• Separation of cream from milk

• Separation of blood plasma in aclinical lab

• Separation of precipitates in alaboratory

You have now gone through some ofthe methods of separating mixtures.