A. Tick the correct answer:-
1.A 2.B 3.A 4.A 5.B
B. Differentiate between the following:-
1. A reversible change is a change that can be reversed.
An irreversible change is a change that cannot be reversed. example all chemical changes
2. A physical change is a change of property, structure or state that can be reversed. example melting of ice
A chemical change is one where new substances are formed and is irreversible example brusting of Diwali cracker
3. A desirable change is one which we wish to takes place example change of seasons
An undesirable change is one which we do not wish to take place example an earthquake
C. Give one word answer for each of the following:-
1. Destructive changes 2. Pasteurization
3. Digestion 4. Sublimation
D. Match column a with column b:-
1. Irreversible 2. Irreversible 3. Desirable change 4. Undesirable 5 Physical change
E. Give two example for each of the following:-
1 melting of ice, elongation of rubber
2 all chemical changes, rusting
3 seasons, photosynthesis
4 earthquake flood
F Give reason for each of the following:-
1. Because on cooling we returned back water, it is a case of distillation.
2. On filteration these two components of sand and water separate, which can be mixed again.
3. It is a chemical change because on mixing cement with water large number of hydrates are formed.
G. Write true or false:-
1. False 2. True 3. False 4. False
H. Circle the odd one out:-
1. Melting ice 2. Mixing water and cement 3. Stretching a rubber band 4. Landslides
I. Fill in the blanks:-
1. Change 2. Reason 3. Melting 4. Irreversible 5. Passing electricity
J. Short answer questions:-
1. A change that can be reversed is called a reversible change.e.g elongation of rubber
2. An irreversible change is one which cannot be reversed example (a)hydrolysis of lime(b)rusting of iron
3. (a) The decay of dead organic matter is desirable and desirable to spoilage of food by microorganisms
(b) Sunlight is a must for photosynthesis but it causes drought also
4. Examples of physical change are
(a) Stretching of rubber band
(b) Rotation of the earth
5. In a chemical change new substances is formed after the change.
K. Long answer questions:-
1.
2.
3. Melting is an example of a reversible change. Ice cubes melt on heating but can be changed back into solid ice cubes on cooling. Water changes into water vapour on heating but can be changed back into liquid water by cooling.
4. When baking soda (sodium bicarbonate NaHCO₃) is mixed with lemon juice citric acid bubbles are formed. This bubbles are formed due to evolution of carbon dioxide(CO₂) gas. Hair baking soda is a base which reacts with a weak organic acid. Hair acid base reaction takes place with the evolution of carbon dioxide gas(CO₂).
5. Ripening forms malic acid inside an apple which turns into a ester giving sweet smell and taste to an apple. Hence it is a desirable change and an apple is very nutritious fruit. Apples are rich in fibre, vitamin C and antioxidant which make them nutritious.
Rotting of Apple is is an undesirable change.
6. Burning paper is a chemical change which produce undesirable carbon monoxide and carbon dioxide gas depends upon supply of oxygen. After burning only black Charr residue is left behind. Where tearing of paper is a physical change which can be processed back to paper.
7. Burning of candle is both chemical and physical change. During burning hydrocarbon part of wax combines with atmospheric oxygen to produce carbon monoxide and carbon dioxide gas which is a chemical change.
On burning wax melts and trickle down whose composition remains intact. Hence it is a physical change also an irreversible one.
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