Lesson-6physical and chemical changes

I SCIENCE

PART 1

Question and answers:

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 changec) Burning of coal – Chemical changed) Melting of wax – Physical change

e) Beating aluminium to make aluminium foil – Physical change

f) Digestion of food – Chemical change

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

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.

Q. 4 When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. Whattype of change is it? Explain. Ans: The formation of bubbles with evolution of a gas indicates that a new substance is formed in thereaction. Thus, it is a chemical change.

Lemon juice + Baking soda Carbon dioxide + Other substances.

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 physicalFormation 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.

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.

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 theevolution 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, onlythe shape and size of the wood is changed. Thus, it is

a physical change.

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 dissolved5. Now filter this solution and allow it to cool without

disturbing it.6. After sometime, crystals of copper sulphate are prepared.

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.

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

areasthan in deserts.

PART 2

Fact file

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.

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.

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.

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 bycrystallisation.

Made by :Shashank Panwar

TO :RAMKESH

SIR