NCERT Book Class 7 Science Chapter 5 Physical and Chemical Changes

5 Physical and
Chemical Changes

E
very day you come across many 5.1 PHYSICAL CHANGES
changes in your surroundings.
These changes may involve one Activity 5.1
or more substances. For example, your Cut a piece of paper in four square
mother may ask you to dissolve sugar pieces. Cut each square piece further
in water to make a cold drink. Making a into four square pieces. Lay these pieces
sugar solution is a change. Similarly, on the floor or a table so that the pieces
setting curd from milk is a change. acquire the shape of the original piece
Sometimes milk becomes sour. Souring of paper (Fig. 5.1).
of milk is a change. Stretched rubber Obviously, you cannot join the pieces
band also represents a change. back to make the original piece, but is
Make a list of ten changes you have there a change in the property of the
noticed around you. paper?
In this chapter we shall perform some
activities and study the nature of these
Activity 5.2
changes. Broadly, these changes are of Collect the chalk dust lying on the floor
two kinds, physical and chemical. near the chalkboard in your classroom.
Or, crush a small piece of chalk into
dust. Add a little water to the dust to
make a paste. Roll it into the shape of a
piece of chalk. Let it dry.
Did you recover chalk from the
dust?

Activity 5.3
Take some ice in a glass or plastic
tumbler. Melt a small portion of ice by
placing the tumbler in the sun. You have
now a mixture of ice and water. Now
place the tumbler in a freezing mixture
(ice plus common salt).
Does the water become solid ice once
Fig. 5.1 Paper pieces again?