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					  2        SEPARATION OF MIXTURES




Rice is the staple diet of the people of   Take a teaspoon each of iron powder
Kerala. What is pictured above is a        and sulphur powder in a china dish.
common sight in the courtyards of          Mix them thoroughly. Look at the
farmers in the harvesting season. Why      mixture through a lens. Can you see
is the paddy winnowed after the            the black grains of iron and the yellow
reaped stalk bundles are threshed?         powder of sulphur?
When the bundles are threshed there
                                           Some of the properties of iron and
will be plenty of chaff mixed with
                                           sulphur are given in the table below
paddy. What happens when it is
                                           (table 2.1)
winnowed? Why?
                                                         Iron       Sulphur
When paddy is de-husked what we get        Density     More            Less
is rice mixed with bran. How are these
                                           Magnetism Magnetic Non-magnetic
separated?
                                           In water    Insoluble       Insoluble
Many of the substances which we use
                                                          Table. 2.1
in our daily life are found in the form
of mixtures.                               What might be the most practical
                                           method of separating this mixture?
What are the characteristic features of
mixtures? Let us conduct an                Find out on the basis of the properties
experiment to find out.                    of the components.



                                                              Physical Sciences 17
Can you suggest other methods?              Saline water is a mixture formed from
                                            a solid and a liquid. Can mixtures be
If the proportion of iron powder and
                                            formed from substances in any
sulphur powder in the mixture is
                                            physical state? Find more examples
changed, can the same methods be
                                            and add to the table.
used to separate the components?
Why is this possible?                       Type of mixture                       Examples
Now, with the help of the teacher,           Solid and solid                      Soil, ......, ......
conduct the following experiments
also, and record the observations.           Solid and liquid                     Saline water,
                                                                                  ......, ......
   Take a small portion of the iron and      Liquid and liquid                    ......, ......, ......
   sulphur mixture in a test tube and
                                             Gas and gas                          ......, ......, ......
   add dilute hydrochloric acid (HCl).
                                             ................................     .......................
   Which is the gas produced? What
   do you see at the bottom of the test                              Table. 2.2
   tube? Which component has
                                            Many mixtures like these are handled
   reacted with the acid? Write down
                                            by us in our everyday life. On many
   the equation.
                                            occasions we have to separate their
   Take another portion of this mixture     components.
   in a test tube and add a little carbon
                                            Consider some examples.
   disulphide. Shake the test tube
   gently. What do you observe?             • Removing the sediment from tea.
   Explain.                                 • Removing stones and paddy from
On the basis of the above experimental        rice.
observations discuss the following.
   What is the minimum number of
   substances necessary to form a
   mixture?
   What is the required proportionate
   quantity of each substance?
   Is there any change in the
   chemical/physical properties of the
   substances when they are mixed?
   Is the formation of a mixture a                                    Fig. 2.1
   chemical change or a physical change?
                                            List more instances after discussion.
Note down the conclusions in the            Tabulate the method of separation in
science diary.                              each instance and also the property on
Aren't the components of the mixture        the basis of which the separation is
used in the above experiment solids?        effected (table 2.3). Enter in the science
                                            diary.

18 Physical Sciences
          Instance                          Method                    Property basis
 Tea                                Filtration                    Difference in the size of
                                                                  the 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

Now it is clear that mixtures are                   Arrange the equipment as shown in the
separated into components based on                  illustration, and try to separate by
the properties of those components. Let             filtration clear water from water in
us familiarise ourselves with some                  which soil is mixed and water in which
other methods used to separate                      chalk powder is mixed.
mixtures.
                                                         Why was it possible to separate the
How to separate clear water                              water using filter paper?
from muddy water                                         Why does one component remain
   Is it possible to filter clear water                  in the filter paper after filtration?
   from muddy water by using the                    Think in terms of particle size. Write
   same type of strainer used for                   down your conclusion in the science
   filtering tea?                                   diary.
   If a filter paper is used instead of                  Can this method be adopted to
   the strainer?                                         separate water and salt from saline
                                                         water? Explain your answer.

                                                    Separation of components of
                           muddy water              saline water
glass
 rod                                                How can the salt and water in saline
                                                    water be separated?
                           filter                   You know that common salt is obtained
                           paper
                                                    by the vaporisation of sea water using
                                                    solar heat.
                                                         What happens to the water when
                                                         saline water is heated?
                                    clear                Why is the salt left behind? Is there
                                    water
                                                         any method for collecting the
                                                         evaporated water?
                Fig. 2.2

                                                                         Physical Sciences 19
See the illustration showing the           Separation of a mixture of
equipment for separating such              completely miscible liquids
mixtures (fig 2.3)                         Acetone and water are two liquids that
                                           mix with each other, in all proportions.
                thermo meter               The boiling point of water is 100°C and
                           water
                                           that of acetone is 56°C. Let us see a
                                           method of separating the components
                                           of this mixture based on the large
                             condenser     difference in their boiling points. See
                                           fig 2.4. Have you noticed the
     flask        saline                   thermometer fitted on the top of the
                  water
                             water         flask? When the mixture is heated
                                           which component boils and evaporates
                                           first? Why? How can this substance be
                               condensed   recovered from the vapour? Discuss
                                 water
                                           and note down the conclusions in the
                                           science diary.
                  Fig. 2.3

The mixture (saline water) is boiled in                    thermometer
a round bottomed flask
                                                                         water
   Which component evaporates, salt
   or water?                                                              condenser
When the vapour passes through the                          acetone
                                                               +
condenser, it condenses and flows into         flask
                                                             water
                                                            mixture
the collecting flask as a liquid.
   What is the substance that remains
   in the round bottomed flask?
This process is known as distillation.                                      acetone

Ordinary drinking water contains
many salts dissolved in it. These salts                     Fig. 2.4
are removed by distillation in order to    This method is possible only when
produce distilled water used for           there is a substantial difference
injecting medicines and in storage         between the boiling points of the two
batteries.                                 component liquids. Also they should
This process is used in separating         not decompose on boiling. This type
                                           of separation is also called distillation.
mixtures where one component is
volatile and the other is not. Find out    How can the component liquids be
other examples.                            separated if there is only a small
                                           difference in their boiling points?


20 Physical Sciences
Ethanol and methanol are two                          can also be separated on the basis of
completely miscible liquids with only                 the increasing order of their boiling
a small difference in their boiling                   points. This method is called fractional
points. The equipment shown in fig.                   distillation.
2.5 is used to separate this mixture.
                                                      Crude oil is a mixture of liquid hydro
                                                      carbons which have only small
     thermometer
                            water                     difference in their boiling points. The
                                                      components of this mixture such as
                                                      petrol, diesel, kerosene etc. are
                                condenser             separated by the above mentioned
       fractionating                                  method of fractional distillation.
       column
                                                      How to separate the different
                                                      components of air?
            methanol         water
               +                                      Ammonia is an important raw material
            ethanol                                   in the manufacture of chemical
            mixture
                                                      fertilizers. Ammonia is manufactured
                               methanol               by combining nitrogen and hydrogen.
                                                      Nitrogen is obtained by separating it
                                                      from air.

                Fig. 2.5                              Some details about the components of
When the vapours of the mixture pass                  air are given in table 2.4.
through the fractionating column, due                                Oxygen Argon Nitrogen
to the repeated condensation and                      Boiling point -1830C      -1860C    -1960C
evaporation the vapours of methanol,                  Quantity (%)    20.9          0.9    78.1
with a lower boiling point first passes
                                                                       Table. 2.4
out of the fractionating column,
condense and get collected in the                         What is the physical state of oxygen
conical flask. After this, ethanol which                  at - 1820C?
has a slightly higher boiling point can                   And at _1840C ?
be collected in similar fashion in
                                                          Which component has the highest
another flask. If there are more than
                                                          boiling point?
two components in the mixture, they

                                                                               Nitrogen_ 1960C
                                                 allowed to
                cooled at           Liquefied    evaporate
      Air                              air                                     Argon _ 1860C
              high pressure                      slowly in a
                 _1980C                         fractionating
                                                   column
                                                                               Oxygen_ 1830C


                                            Flow Diagram 2.1

                                                                             Physical Sciences 21
   Which one has the lowest?
See the flow diagram showing the
process of separation of these gases.
(flow diagram 2.1)
   Which gas will condense first into                              separating
                                                                   funnel
   liquid state?
   From the liquid mixture which
   component will evaporate first?
   Which component is obtained first
   from the fractionating column?
   What will be its state - solid, liquid
   or gas?
                                                                       water
   Which will be the last component
   obtained? In which physical state?
                                                            Fig. 2.6
Note down your conclusions in the
                                            Can the components of black
science diary.
                                            ink be separated?
Thus we have become familiar with
                                            The ink we use is a dye which
some of the processes of industrial
                                            dissolves in water.
importance.
                                            Carry out the following experiment.
How to separate a mixture of                Take some black ink in a china dish
two immiscible liquids                      and heat it. (It is enough to keep the
Suppose kerosene gets mixed with            dish on hot sand bath.) Can you see
water. How can they be separated?           the vapours? Once the evaporation
Take some kerosene and some water           stops, remove the dish from the heat
in a bottle. Shake the bottle well. Keep    source.
the bottle motionless for a while. What
                                               Which could be the substance that
do you see? Why does this happen?
                                               has evaporated?
The arrangement for separation of the
                                               Is there anything left behind in the
components of such a mixture is shown
                                               dish?
in the figure. See fig 2.6.
                                               Is ink a single substance? Or is it a
   Which liquid gets collected at the
                                               mixture? What is your conclusion?
   bottom?
   Why does kerosene remain above           In black ink is there just one
   water?                                   colour or is it a combination
                                            of many colours?
   How can the water underneath be
   removed?                                 Take a filter paper strip 1 inch wide
                                            and 6 inches long. Put one drop of
What if these two liquids were
                                            black ink (from a sketch pen or a
miscible?
                                            fountain pen) at about 1 inch from one
22 Physical Sciences
end of the filter paper. Allow it to dry         greater extent whereas some other
for one minute. Take water in a jar.             components are not adsorbed to that
Immerse the filter paper (as shown in            extent.
the figure) into the water in the jar so
that the ink spot is just above the water       The dyes present in ink are usually a
level (fig 2.7).                                mixture of two or more coloured
Observe carefully as the water rises up         substances. The water rises through the
through the filter paper. Write down            filter paper by capillary action. The
your observations in the science diary.         weakly adsorbed component rises
                                    glass rod   faster through the paper. Those which
                                   paper clip   are strongly adsorbed rise slowly. In
                                   glass jar    this way the colours are separated. As
           filter                               this method was first used for
           paper                                separating coloured substances it was
                                   filter
                                   paper        termed chromatography. Chromato-
         ink drop
                                                graphy is the process used for
                                   water
                                                separation of solutes soluble in the
                    Fig. 2.7                    same solvent. See the uses of this
   Do different colours appear in the           process.
   filter paper?                                • separation of components from
   What could be the reason for the               dyes
   colours rising through the paper?            • separation and identification of
                                                  chemicals mixed in blood

             Adsorption                         Look at a situation in every day life
                                                where separation of mixtures happens.
 Some substances like cotton, sponge etc.
 absorb liquids into them. But some other       Butter from curd
 substances hold particles of certain other     You would have seen curd being
 substances on their surface alone. This        churned to obtain butter. During
 phenomenon is called adsorption. In this       churning, particles of butter are
 way, the carbon particles in gas masks         separated. Being less dense, these
 remove poisonous gases from the air and        particles float on top of the curd.
 the charcoal in water filters removes          An instrument called centrifuge (fig
 coloured and foul smelling substances          2.8-a) is used to separate components
 from water. So also, dust particles stick      from a mixture using the difference in
 to walls because of adsorption. The rate       densities of the components.
 at which adsorption occurs on a
                                                Examine some situations where this
 substance depends on the nature of the
                                                process is used.
 substance. During adsorption of mixtures
 some of the components are adsorbed to a

                                                                   Physical Sciences 23
                                      Can the constituent elements
                                      of a compound be separated?
                                      As already understood, formation of
                                      mixtures is a physical change. But the
                                      formation of compounds is a chemical
                                      change. Compounds are formed by
                                      chemical combination of elements in
                                      fixed proportions. Therefore, the
                                      elements in a compound can be
              Fig. 2.8 (a)
                                      separated only by suitable chemical
                                      processes.
                                      See the compounds and their formulae
                                      given below.
                                      Mercuric oxide - HgO
                                      Calcium oxide - CaO
                                      Both are metal oxides.
                                      If HgO is heated it decomposes into Hg
              Fig. 2.8 (b)            and O 2 . But if CaO is heated the
                                      elements do not separate.
                                      In order to separate the elements in
• Separation of cream from milk
                                      compounds different methods suited
• Separation of blood plasma in a     to the nature of the compounds are
  clinical lab                        adopted.
• Separation of precipitates in a     You can familiarise yourselves with
  laboratory                          these methods when you study more
You have now gone through some of     of chemical processes in higher classes.
the methods of separating mixtures.




24 Physical Sciences

				
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