Class-6 Science, Chapter-6 Changes Around Us, Solutions and Additional Exercises in line with NCERT book / CBSE Syllabus


Chapter 6
Changes Around Us
=========================================================
Additional Exercises
Exercises from Textbook
Tables from Textbook
Additional Exercises

Choose the correct answer:

1-    Blooming of flowers is-
a.    Slow change (√)
b.    Fast change
c.    Reversible change
2-    Sharpening a pencil is a-
a.    Chemical change
b.    Irreversible change (√)
c.    Slow change
3-    Which of these changes can be reversed?
a.    Raw egg to boiled egg
b.    Batter to idli
c.    Cold milk to hot milk (√)
4-    Which of these changes is irreversible?
a.    Grain to flour (√)
b.    Woolen yarn to knitted sweater
c.    Stretched rubber band to its normal size
5-    Change in periods in your school is-
a.    Non-periodic
b.    Periodic (√)
c.    Reversible

Classify the following changes as reversible and irreversible changes-
·         Falling of fruit from tree
·         Maturing of age
·         Glowing of bulb when switched on and vanishing of light when switched off.
·         Melting of wax on heating and its solidification on cooling.
·         Burning of paper leaving behind ashes.
·         Combustion of fragrance sticks or candles.

Ans- Reversible changes-
·         Glowing of bulb when switched on and vanishing of light when switched off.
·         Melting of wax on heating and its solidification on cooling.
Irreversible Changes-
·         Falling of fruit from tree
·         Maturing of age
·         Burning of paper leaving behind ashes.
·         Combustion of fragrance sticks or candles.

Distinguish between-

(a)  Reversible and Irreversible Change
(b)  Slow and Fast Change
(c)  Periodic and Non-periodic Change
(d)  Physical and Chemical Change

Reversible change
Irreversible change
The changes which are possible in reverse direction are called reversible changes
 Changes which are not possible in reverse direction are called Irreversible changes.
Melting of Ice into water is a reversible change.
 Falling of fruits from a tree is a reversible change.


Slow change
Fast change
Changes which may take hours, day- night, months or years are said to be slow changes.
 Changes which are instant and occur speedily are said to be fast changes.
Germination of a seed is a slow change.
 lighting a matchstick is a fast change.


Periodic change
Non- periodic change
Changes which occur after a definite time interval are called periodic changes.
Changes which do not reoccur after a definite time interval are called non-periodic changes.
Periodic changes can be predicted.
 Non-periodic changes cannot be predicted.
Occurrence of days and nights, change in season and phases of moon are periodic changes
 Accident, illness, changes due to earthquake are non periodic changes.

Physical change
chemical change
A temporary change in which only the state, shape or size of a substance changes and no new substance is formed is termed as physical change.
 A permanent change which results in formation of new substance which differs in properties from the original substances are called chemical changes.
Melting of wax and grinding of sugar are physical changes.
 Rusting of iron and formation of curd from milk are chemical changes


Fill in the blanks-

(a)  Rusting of iron is irreversible chemical change. (reversible / irreversible)
(b)  Change in Seasons is a slow and periodic change. (slow_periodic / fast_ non-periodic)
(c)  Ignition of a matchstick is a fast change. (slow/fast)
(d)  Melting of ice is a physical and reversible change.(physical_reversible / chemical_irreversible)
(e)  The human heart - beat is a periodic change. (periodic / non-periodic)
(f)   Glowing of an electric bulb is a fast change. (slow / fast)

Give reasons for the following-

Metal rim is made slightly smaller than wooden wheel?

Ans- Metal rim is made slightly smaller than the wooden wheel. On heating, the rim expands and fits onto the wheel. Cold water is then poured over the rim, which contracts and fits tightly onto the wheel.



Why is the ring in iron tools smaller in size than the wooden handle?
Ans- The ring in iron tools is slightly smaller in size than the wooden handle. To fix the handle, the ring is heated and it becomes slightly larger in size (expands). Now the handle easily fits into the ring. When the ring cools down it contracts and fits tightly onto the handle.


Exercises from Textbook
Q. To walk through a waterlogged area, you usually shorten the length of your dress by folding it.  Can this change be reversed?
Ans- Yes, this change can be reversed.

Q. You accidentally dropped your favorite toy and broke it.  This is a change you did not want.  Can this stage be reversed?
Ans- No, this change can not be reversed.

Some changes are listed in the following table.  For each change, write in the blank column, whether the change can be reversed or not.

S.No.
 Change
 can be reversed (Yes\No)
1
 The sawing of a piece of wood
No
2
 The melting of ice candy
Yes
3
 dissolving sugar in water
Yes
4
 The cooking of food
No
5
 The ripening of mango
No
6
 Souring of milk
No

Q. A drawing sheet changes when you draw a picture on it. Can you reverse this change?
Ans- No, the change cannot be reversed after drawing on a sheet. However, if drawing is done with a pencil, we can rub it.

Q. Give examples to explain the difference between changes that can or can not be reversed.
Ans- Following examples explain the difference between reversible and irreversible change:
Reversible change
Irreversible change
Blow a balloon and see the change in its shape and size àThis change can be reversed by escaping air from balloon.
Blow a balloon and see the change in its shape and size à This change can not be reversed if we burst the balloon by pricking it.
Take a sheet of paper and make an aeroplane. à This change can be reversed by unfolding paper.
On the same sheet of paper if we draw and cut the outline of aeroplane. à This change can not be reversed.
Roll out a chapatti from a ball of dough à we can change it into a ball of dough again.
If we bake a roti on tawa à we can not make the ball of dough again.

Q. A thick coating of a paste of Plaster of Paris (POP) is applied over the bandage on a fractured bone. It becomes hard on drying to keep the fractured bone immobilized. Can the change in POP be reversed.
Ans- No, the change in POP can not be reversed as it becomes hard on drying.

Q. A bag of cement lying in the open gets wet due to rain during the night. The next day the sun shines brightly. Do you think the changes, which have occurred in the cement, could be reversed?
Ans- No, this change can not be reversed as the cement hardens on drying after being wet.




Tables from Textbook

Table 6.1 some common changes
Change
can be reversed
Raw egg boiled egg
 no
Batter to Idli
 no
Wet clothes to dry clothes
 yes
Woolen yarn to knitted sweater
 yes
Grain to its flour
 no
Cold milk to hot milk
 yes
Straight string to a coiled string
 yes
Bud to flower
 no
Milk to paneer
 no
Cow dung to biogas
 no
Stretched rubber band to its normal size
 yes
Ice cream to molten ice cream
 yes

Activity / Project

1.    Take a lemon, a paintbrush and a piece of paper. Cut the lemon and squeeze out its juice in a cup. Dip the brush in the lemon juice and write a message on the paper. Let the paper dry and you find that the letters of your message become invisible. Now, press the paper with hot iron or vomit by holding it above the flame of a candle (take care that it does not catch fire). As paper gets warm, invisible letters change into dark brown colour. Identify the changes that can be reversed.
2.    Observe preparation of dishes at your home. Identify two changes that can be reversed.
3.    Record for one year of the seasonal changes in vegetables, clothing, nature and events around you. Identify the changes that can or cannot be reversed.




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