You Must Know
1. We observe many changes occurring in our surroundings.
2. These changes can be classified as-
(i) slow or fast changes
(ii) reversible or irreversible changes.
(iii) physical or chemical changes
3. Slow change is a change that takes place in a longer duration of time. A fast change takes place in a much shorter duration of time.
4. Changes, which can be reversed, to get back the original substance, are called reversible changes.
5. Changes, which cannot be reversed, to get back the original substance, are called irreversible changes.
6. In a chemical change, new substances, with different properties, are formed.
7. Physical changes are those in which no new substance is formed.
8. Some changes, like the burning of fuels, in our sorroundings, involve release of energy.
9. Some other changes, like melting of ice, involve absorbtion of energy.
Something To Know
A. Fill in the blanks.
1. All irreversible changes bring a permanent change in a substance.
2. A change, in which the substance produced can get back to the original form, is called a reversible change.
3. Bending of an iron rod is a physical change.
4. Burning of fuel is a fast chemical change, whereas rusting of iron is a slow chemical change.
5. In chemical change, new substances with new/ different properties are formed.
B. Write True or False for the following statements.
1. Glowing of an electric bulb is a fast change. True
2. Deforestation is a reversible change. False
3. Burning of paper is a temporary change. False
4. Cutting of an apple is a chemical change. False
5. Making of fruit salad is a chemical change. False
6. Changing milk into curd is a physical change. False
C. Tick (✔) the correct option.
1. Which of the following can be considered as a fast change?
growth of a child
germination of seeds
bursting of a fire cracker (✔)
cooking of food
Answer – bursting of a fire cracker(✔)
2. Evaporation of water is a—
physical change (✔)
fast change
irreversible change
chemical change
Answer – physical change(✔)
3. Which one of these changes is a reversible change?
burning of a candle
inflating a balloon (✔)
baking of chappati
grinding of wheat grains into flour
Answer – inflating a balloon(✔)
4. Which one of these is an irreversible change?
growth of a child (✔)
stretching a rubber band
evaporation of water
formation of ice from water
Answer – growth of a child(✔)
5. A piece of paper undergoes a chemical change, when it is-
cut
folded
reshaped into a paper toy
burned (✔)
Answer – burned(✔)
D. Answer the following questions in brief.
1. Define the following terms.
(a) Reversible change
Answer – A change in which the substance produced can be brought back into its original form is called a reversible change.
(b) Chemical change
Answer – A change in which a new substance is formed is called a chemical change.
(c) Physical change
Answer – A change in which no new substance is formed is called a physical change.
2. Distinguish between fast and slow changes. Give two examples of each type.
Answer – Fast changes – Happens in short duration. Examples – burning of papers, burning of fire crackers.
Slow changes – Happens in long duration. Examples – movement of earth, growth of a plant.
3. Give two examples each of reversible and irreversible change.
Answer – Examples of reversible change are freezing of water and evaporation. Examples of Irreversible change: getting older and making curd from milk.
4. State some of the irreversible biological changes that take place in nature.
Answer – The Irreversible biological changes that take place in nature are the growth of a child, getting older, germination of seed, growth of a plant, curdling of milk, decomposition of dead plants and animals.
5. Give one example of a physical change in which (i) energy is given out (ii) energy is absorbed.
Answer – Physical change where energy is given out: freezing of water. Physical changes where energy is absorbed: melting of ice.
E. Answer the following questions.
1. Give two differences between reversible and irreversible changes.
Answer – Reversible changes – Can be reversed. Most of the reversible changes are physical changes. Examples – Inflating a balloon, melting of ice.
Irreversible changes – Cannot be reversed. Most of the reversible changes are chemical changes. Examples – Cooking of food, germination of seed.
2. ‘Formation of curd is an irreversible as well as a chemical change.’ Justify this statement.
Answer – Formation of curd is an irreversible change because once the curd is made from milk, it cannot be brought back into milk again. It is a chemical change also because a new substance is formed which is very much different from milk.
3. Water cycle is a natural change. Classify it as a
(i) physical or chemical change
Answer – Water cycle is a physical change because no new substance is formed, only the state of water is changing.
(ii) a reversible or irreversible change. Also, justify your answer.
Answer – It is a reversible change because when the liquid water gets changed into water vapour, water vapor can again be brought back to its original liquid form.
4. How can we say that burning of candle is a chemical change but the glowing of bulb is a physical change?
Answer – Burning of the candle is a chemical change because new substances are formed like carbon dioxide and water, but the glowing of the bulb is a physical change because no new substance is formed. When the switch is on, the filament gets heated up, and the bulb glows and when switched off, the bulb turns off immediately.
Value Based Question
Madam Emani, a very sincere and dedicated teacher, had a very honest, considerate and understanding approach towards the problems of her students. This helped her to make her students improve their performance and become better persons. Her school principal would often say, “The changes, brought about by Madam Emani, are quite similar to those that occur during a chemical change.”
1. State the values that helped Madam Emani become a popular teacher.
Answer – Values of madam Emani are sincere, dedicated, considerate, and understanding.
2. State with reason, whether you agree or disagree with the principal’s statement.
Answer – I agree with the principal’s statement because the changes that were brought by her in students are permanent. They have now become better persons and the values added are very much different than before.
3. Have a ‘group-discussion’ in which one student suggests a possible ‘change’ (like a change of the section of the class) and the other students think of the likely effects of that change.
Answer – Do it yourself
Something To Do
1. Solve the puzzle with the help of clues given below:
Across →
a. A change which cannot be reversed. IRREVERSIBLE
b. A metal which gets rusted. IRON
f. An example of a chemical change in which energy is released. RESPIRATION
Down↓
c. A change in which original condition can be re-attained. REVERSIBLE
d. The change which takes place in long duration of time. SLOW
e. A reversible change involving change of liquid into gas. EVAPORATION
2. Go to your neighbourhood and make a list of all the changes you observe, such as setting of sun, burning of leaves, etc. Also, classify these as slow/fast; reversible/ irreversible and controllable/uncontrollable. Record your observations in a tabular form.
Answer –
| Change | Slow/Fast | Reversible/ Irreversible | Controllable/ Uncontrollable |
| Setting of Sun | Slow | Irreversible | Uncontrollable |
| Burning of leaves | Fast | Irreversible | Controllable |
| Cooking of food | Slow | Irreversible | Controllable |
| Melting of ice | Slow | Reversible | Controllable |
| Inflating a balloons | Fast | Reversible | Controllable |
3. Take two test tubes, half filled with water. To the first test tube, add salt and to the second add quicklime. What do you observe? Is there a release of energy? Comment.
Answer – in the first test tube salt is dissolved in water. There is no change in temperature of test tube. In the second test tube, quick lime is dissolved in water. The test tube becomes hot. This shows that energy is released during this change.