3. Q. 1 Classify the changes involved in the
following processes as physical or chemical
changes:
Ans: a) Photosynthesis – Chemical change
b) Dissolving sugar in water –Physical change
c) Burning of coal – Chemical change
d) Melting of wax – Physical change
e) Beating aluminium to make aluminium foil –
Physical change
f) Digestion of food – Chemical change
4. Q. 2 State whether the following statements are true or false.
In case a statement is false, write the
corrected statement in your notebook.
(a) Cutting a log of wood into pieces is a chemical change.
(True/False)= False
(b) Formation of manure from leaves is a physical change.
(True/False)= False
(c) Iron pipes coated with zinc do not get rusted easily.
(True/False)= True
(d) Iron and rust are the same substances.
(True/False)= False
(e) Condensation of steam is not a chemical change.
(True/False)= True
5. FILL IN THE BLANKS:
(a) When carbon dioxide is passed through lime
water, it turns milky due to the formation of calcium
carbonate (CaCO3).
(b) The chemical name of baking soda is sodium
hydrogen carbonate (NaHCO3).
(c) Two methods by which rusting of iron can be
prevented are galvanizing and painting.
(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.
6. 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 formation of bubbles with evolution
of a gas indicates that a new substance is
formed in the
reaction. Thus, it is a chemical change.
Lemon juice + Baking soda Carbon dioxide +
Other substances.
7. 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, following physical and chemical changes
take place:
1. Physical change – melting of wax.
2. Chemical change – burning of wax produces carbon dioxide
gas, water vapour and heat.
Formation of soil from rocks is another familiar process in which
both, the chemical and physical
Formation of soil from rocks is another familiar process in which
both, the chemical
Physical change – Rocks broken down by weathering.
Chemical change – New product soil is formed from rocks. The change
is irreversible.
8. Q. 6 How would you show that setting of curd
is a chemical change?
Ans: Setting of curd is a chemical change due to
the following reasons:
1. In a chemical change, a new substance is
formed. Here, curd is the new substance
obtained from milk.
2. Both milk and curd have different properties.
3. Milk cannot be re-obtained from curd.
9. Q. 7 Explain why burning of wood and cutting it into
small pieces are considered as two different types of
changes.
Ans: 1) Chemical change – Burning of wood involves
the formation of a new substance .i.e. coal, with the
evolution of heat and unpleasant gases. Thus, it is a
chemical change.
2) Physical change – Cutting of wood into small pieces
does not form any new substance. Here, only
the shape and size of the wood is changed. Thus, it is
a physical change.
10. Q. 8 Describe how crystals of copper sulphate are prepared?
Ans: Crystals of copper sulphate are prepared by the method
of crystallisation. The procedure is as follows:
1. Take a cupful of water in a beaker.
2. Add few drops of dilute sulphuric acid, and heat the water.
3. Once the water starts boiling, add copper sulphate powder
slowly by stirring it.
4. Continue adding copper sulphate powder till no more can
powder be dissolved
5. Now filter this solution and allow it to cool without
disturbing it.
6. After sometime, crystals of copper sulphate are prepared.
11. Q. 9 Explain how painting of an iron gate
prevents it from rusting.
Ans: An iron object rusts, when its surface
is exposed to oxygen and water, or water
vapour . Painting an Iron gate forms a
protective coat on it. This coat prevents the
iron gate from coming in contact with
oxygen, water, or both. Thus, it prevents
rusting.
12. Q. 10 Explain why rusting of iron objects is
faster in coastal areas than in deserts.
Ans: An iron object rusts due to the
presence of oxygen and water, or water
vapour. The content of moisture is high in
coastal areas than in deserts. So, rusting of
iron objects is faster in coastal areas
than in deserts.
13.
14. 1. Broadly there are only two type of changes they
are chemical changes and physical changes.
2. Cutting of paper in pieces is physical change.
3. If you leave a piece of iron in the open for some
time ,
it acquires a film of brownish substance.
4. Heat, light or any other radiation (ultraviolet, for may
be example) given off or absorbed.
6. Ozone layer protect us from ultra violent radiation
which comes from the sun.
5. The ash is the new substance formed when
magnesium is burnt.
15. 6. A change in which one or more new substances are is
called a chemical change.
7. A chemical change is also called a chemical reaction.
8. All new substances are formed as a result of chemical
changes.
9. This process of depositing a layer of zinc on iron is called
galvanisation.
10. The iron pipes we use in our homes to carry water are
galvanised to prevent rusting.
11. ships suffer a lot of damage from rusting in spite of being
solutions. The process is called crystallisation.
It is an example of a physical change.
16. 12. salt can be obtained by the evaporation of sea
water.
13. Physical changes are changes in the physical
properties of substances. No new substances are
formed in these
changes. These changes may be reversible.
14 In fact, if the content of moisture in air is high, which
means if it is more humid, rusting becomes faster.
15. Ozone absorbs the radiation and breaks down to
oxygen.
16. Ozone acts as a natural shield against this
radiation.
17. What we have learnt
:
1Changes can be of two types, physical and chemical.
2 Physical changes are changes in the physical
properties of substances.
No new substances are formed in these changes.
These changes may
be reversible.
3 In chemical changes new substances are produced.
4Some substances can be obtained in pure state from
their solutions by
