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Is Matter Around Us Pure : NCERT Intext Questions


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Page 15

Q.1     What is meant by a pure substance? 
Sol.      A pure substance is the one that consists of a single type of particles, i.e., all constituent particles of the substance have the same chemical nature. Pure substances can be Classified as elements or compounds.


Q.2     List the points of differences between homogeneous and heterogeneous mixtures.
Sol.      
A homogeneous mixture is a mixture having a uniform composition throughout the mixture.
For 
example: salt in water, sugar in water, copper sulphate in water A heterogeneous mixture is a mixture having a non-uniform composition throughout the mixture. For example: sodium chloride and iron fillings, salt and sulphur, oil and water


Page 18

Q.1     Differentiate between homogeneous and heterogeneous mixtures with examples.
Sol.      
A homogeneous mixture is a mixture having a uniform composition throughout the mixture. For
example, mixtures of salt in water, sugar in water, copper sulphate in water, iodine in alcohol, alloy, and
air have uniform compositions throughout the mixtures. On the other hand, a heterogeneous mixture is a mixture having a non-uniform composition throughout the mixture. For example, composition of mixtures of sodium chloride and iron fillings, salt and sulphur, oil and water, chalk powder in water, wheat flour in water, milk and water are not uniform through out
the mixtures.


Q.2     How are sol, solution and suspension different from each other?
Sol.      
is a heterogeneous mixture. In this mixture, the solute particles are so small that they cannot be seen with the naked eye. Also, they seem to be spread uniformly throughout the mixture. The Tyndall effect is observed in this mixture.For example: milk of magnesia, mud Solution is a homogeneous mixture. In this mixture, the solute particles dissolve and spread uniformly throughout the mixture. The Tyndall effect is not observed in this mixture. For example: salt in water, sugar in water, iodine in
alcohol, alloy Suspensions are heterogeneous mixtures. In this mixture, the solute particles are visible to the naked eye, and remain suspended throughout the bulk of the medium. The Tyndall effect is observed in this mixture. For example: chalk powder and water, wheat flour and water


Q.3     To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration at this temperature.
Sol.     Mass of solute (sodium chloride) = 36 g (Given)

           Mass of solvent (water) = 100 g (Given)
           Then, mass of solution= Mass of solute + Mass of solvent
           = (36 + 100) g
           = 136 g
           Therefore, concentration (mass by mass percentage) of the solution
               = {{Mass\,\,of\,solute} \over {Mass\,of\,solvent}} \times 100\%
            = {{36} \over {136}} \times 100\%
           = 26.47 %


Page 24

Q.1     How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25°C), which are miscible with each other?
Sol.     A mixture of two miscible liquids having a difference in their boiling points more than 25°C can be separated by the method of distillation. Thus, kerosene and petrol can be separated by distillation.

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In this method, the mixture of kerosene and petrol is taken in a distillation flask with a thermometer
fitted in it. We also need a beaker, a water condenser, and a Bunsen burner. The apparatus is arranged as shown in the above figure. Then, the mixture is heated slowly. The thermometer should be watched
simultaneously. Kerosene will vaporize and condense in the water condenser. The condensed kerosene is
collected from the condenser outlet, whereas petrol is left behind in the distillation flask.


Q.2     Name the technique to separate
           (i) butter from curd
           (ii) salt from sea-water
           (iii) camphor from salt
Sol.     (i) Butter can be separated from curd by centrifugation.
           (ii) Salt can be separated from sea-water by evaporation.
           (iii) Camphor can be separated from salt by sublimation.


Q.3     What type of mixtures is separated by the technique of crystallization?
Sol.    
By the technique of crystallization, pure solids are separated from impurities. For example, salt obtained from sea is separated from impurities; crystals of alum (Phitkari) are separated fromimpure samples.


Q.1     Classify the following as chemical or physical changes:
           - Cutting of trees
          - Melting of butter in a pan
          - Rusting of almirah
          - Boiling of water to form steam
          - Passing of electric current through water, and water breaking down into hydrogen and oxygen gas
         - Dissolving common salt in water
         - Making a fruit salad with raw fruits
         - Burning of paper and wood
Sol.    
Cutting of trees→ Physical change
          Melting of butter in a pan → Physical change
          Rusting of almirah → Chemical change
          Boiling of water to form steam → Physical change
          Passing of electric current through water, and water breaking down into hydrogen and oxygen gas →
          Chemical change
          Dissolving common salt in water → Physical change
          Making a fruit salad with raw fruits → Physical change
          Burning of paper and wood → Chemical change


Q.2     Try segregating the things around you as pure substances or mixtures.
Sol.    
Pure substance: Water, salt, sugar Mixture: Salt water, soil, wood, air, cold drink, rubber, sponge, fog, milk, butter, clothes, food

 



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