Page 57 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key
P. 57

v     The rate of rusting will be fast if the water is salty and the moisture content in the air is high.

          v     Rusting can be prevented by the following methods :

                (a)   Painting, greasing or oiling regularly prevents rusting of iron objects. These prevent
                     the surfaces of iron objects from coming in direct contact with the air & moisture.

                (b)   Galvanisation is the process of deposition of a thin layer of zinc on iron objects. Zinc

                     reacts with air and mosture to form a layer of zinc oxide. This layer prevents rusting.
                (c)   Rusting can be prevented by using alloys. An alloy is a substance made by mixing two

                     or more metals or a metal and a non-metal. This process is called alloying.

          v     Crystallisation is the process of obtaining crystals of a pure substance from its solution.
          v     In  crystallisation,  the  solid  is  allowed  to  dissolve  in  water  and  then  heated.  The  water

                evaporates after heating and a pure substance is left behind.

          v     Crystallisation is a physical change.
          v     Crystallisation is better than evaporation due to the following reasons :

                (a)  Due to heating, some solids decompose or get charred because of dryness.

                (b)   During  filtration,  some  of  the  impurities  remain  dissolved,  which  on  evaporation,

                     contaminate the solid.
          v     Crystallisation can be used in purification of salts and in the separation of crystals.

          v     French engineer, Stanislas Sorel, in 1836, patented the process of galvanising. He coined

                the term galvanising. In 1837, a British patent was issued for the same process to William
                Crawford.


                                                       Exercise

          A.  Tick (3) the correct answer.
                  1.  Which of the following is a chemical change?

                      (a) Twinkling of stars                       (b) Cooking of vegetables

                      (c) Cutting of fruits                        (d) Boiling of water
                Ans.    (b)

                  2.  Galvanisation  is  a  process  used  to  prevent  the  rusting  of  which  of  the  following

                       metals?

                      (a) Iron               (b) Zinc              (c) Aluminium          (d) Copper
                Ans.   (a)

                  3.  Physical changes are mostly ................... .

                      (a) Reversible         (b) Irreversible      (c) Permanent          (d) Undesirable
                Ans.   (a)


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