EVENTS
CONVENT HIGH SCHOOL
19/08/2021 CLASS-7 SESSION2021-22(SLOT-1)
SCIENCE
Chapter-6
PHYSICAL AND CHEMICAL CHANGE
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Q.1.Classify the changes involved in the following
processes as physical or chemical changes:
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminium foil
(f) Digestion of food
Answers .(a) Chemical
change (b) Physical change
(c) Chemical change
(d) Physical change
(e) Physical change
(/) Chemical change
Q.2.Write True or False:-
(a) Cutting a log of
wood into pieces is a chemical change. ( False)
(b) Formation of
manure from leaves is a physical change. (False)
(c)Iron pipes coated
with zinc do not get rusted easily. (True)
(d)Iron and rust are the
same substances. (True)
(e)Condensation of
steam is not a chemical change. (False)
Q.3.Fill in the blanks in the following statements:
(a) When carbon
dioxide is passed through lime water, it turns milky due to the formation of calcium
carbonate
(b) The chemical name of baking soda is sodium hydrogen carbonate
(c) Two methods by
which rusting of iron can be prevented are painting or greasing and galvanisation
(d) Changes in which
only physical
properties of a
substance change are called physical changes.
(e) Changes in which
new substances are formed are called chemical
changes..
Q.4. When baking soda is mixed with lemon juice, bubbles
are formed with the evolution of a gas. What type of change is it? Explain.
Ans. The reaction
between baking soda and lemon juice can be given as below:
Lemon juice + Baking
soda ————-> C02 (bubbles) + Other substances
(Citric acid) (Sodium hydrogen carbonate) (Carbon dioxide)
It is a chemical
change
Q.5. When a candle burns, both physical and chemical
changes take place. Identify these changes. Give another example of a familiar
process in which both the chemical and physical changes take place.
Ans. When a candle
burns, both physical and chemical changes occur:
(i) Physical change:
melting of wax, vapourisation of melted wax.
(ii) Chemical change:
Burning of vapours of wax to give carbon dioxide, heat and light.
LPG is another example
in which physical change occurs when LPG comes out of cylinder and is converted
from liquid to gaseous state and a chemical change occurs when gas burns in
air.
Q.6. How would you show that setting of a curd is a
chemical change?
Ans. We can say that
setting of curd is a chemical change because we can not get the original
substance, i.e., milk back and a new substance is formed with different taste, smell
and other chemical properties
Q.7. Explain why burning of wood and cutting it into small
pieces are considered as two different types of changes. ~
Ans. Burning of wood
is a chemical change because in burning new substances are formed as
Wood + Oxygen ———–>
Charcoal + Carbon dioxide + Heat + Light
But cutting it into
small pieces is physical change because no new substance is formed. We can only
reduce the size of wood.
Q.8. Describe how crystals of copper sulphate are prepared.
Ans. Take a cupful of
water in a beaker and add a few drops of dilute sulphuric acid. Heat the water.
When it starts boiling, add copper sulphate powder slowly. Continue to add
copper sulphate powder till no more powder can be dissolved. .During this process
continuously stir the solution. Filter the solution. Leave it for cooling. Look
it after some time, you can see the crystals of copper sulphate
Q.9. Explain how painting of an iron gate prevents it from
rusting?
Ans. It is known that
for rusting the presence of oxygen and moisture is essential. Painting prevents
the iron gate from coming in contact with oxygen and moisture.
Q.10. Explain why rusting of iron objects is faster in coastal
areas than in deserts.
Ans. As content of
moisture in the air in coastal areas is higher than in the air in deserts. So,
the process of rusting is faster in coastal areas.
