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A. Fill in the blanks.
- All irreversible changes bring a chemical change in a substance.
- A change, in which the substance produced can get back to the original form, is called a reversible change.
- Bending of an iron rod is a reversible change.
- Burning of fuel is an irreversible chemical change, whereas rusting of iron is a slow chemical change.
- In chemical change, new substances with different properties are formed.
B. Write True or False for the following statements.
- Glowing of an electric bulb is a fast change. — True
- Deforestation is a reversible change. — False
- Burning of paper is a temporary change. — False
- Cutting of an apple is a chemical change. — False
- Making of fruit salad is a chemical change. — False
- Changing milk into curd is a physical change. — False
C. Tick (✓) the correct option.
- 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
- Evaporation of water is a—
- ✅ Physical change
- Fast change
- Irreversible change
- Chemical change
- 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
- Which one of these is an irreversible change?
- ✅ Growth of a child
- Stretching a rubber band
- Evaporation of water
- Formation of ice from water
- A piece of paper undergoes a chemical change, when it is—
- Cut
- Folded
- Reshaped into a paper toy
- ✅ Burned
D. Answer the following questions in brief.
1. Define the following terms.
(a) Reversible change
A reversible change is a change in which the original substance or object can be brought back to its previous form. In such changes, no new substance is formed. These changes are usually temporary in nature and help us understand how materials can change without losing their original identity. For example, melting of ice and inflating a balloon are reversible changes.
(b) Chemical change
A chemical change is a change in which one or more new substances with different properties are formed. Such changes are usually permanent and cannot be reversed easily. Chemical changes are important in our daily life because they help in cooking food, digestion, and many natural processes. Burning of paper and formation of curd from milk are examples of chemical changes.
(c) Physical change
A physical change is a change in which only the shape, size, colour, or state of a substance changes, but no new substance is formed. Physical changes are generally temporary and reversible. These changes help us use materials in different forms without changing their actual nature. Cutting paper and freezing water into ice are examples of physical changes.
2. Distinguish between fast and slow changes. Give two examples of each type.
| Fast Changes | Slow Changes |
|---|---|
| Fast changes take place in a short period of time. | Slow changes take a long time to complete. |
| These changes happen quickly and can be observed immediately. | These changes occur gradually and may take days, months, or years. |
| Example: Bursting of a cracker | Example: Growth of a plant |
| Example: Lighting an electric bulb | Example: Rusting of iron |
Fast and slow changes teach us that nature works at different speeds. Some events happen suddenly while others require patience and time.
3. Give two examples each of reversible and irreversible changes.
Reversible Changes
- Melting of ice into water.
- Stretching of a rubber band.
These changes can be reversed and the original form can be obtained again.
Irreversible Changes
- Burning of paper.
- Cooking of food.
These changes cannot be reversed because completely new substances are formed. We should understand irreversible changes carefully because many human activities such as cutting trees and polluting nature may cause permanent damage to the environment.
4. State some of the irreversible biological changes that take place in nature.
Some irreversible biological changes taking place in nature are:
- Growth of a child into an adult.
- Germination of seeds into plants.
- Formation of fruits from flowers.
- Aging in living organisms.
- Death of living organisms.
These natural changes remind us that life continuously moves forward and many processes in nature cannot be reversed. Therefore, human beings should respect nature and protect living organisms with care and responsibility.
5. Give one example of a physical change in which:
(i) Energy is given out
Freezing of water into ice is a physical change in which heat energy is given out.
(ii) Energy is absorbed
Melting of ice into water is a physical change in which heat energy is absorbed.
Energy changes are very important in nature. They help maintain balance in the environment and support many natural and human activities.
E. Answer the following questions.
1. Give two differences between reversible and irreversible changes.
| Reversible Changes | Irreversible Changes |
| The original substance can be obtained again. | The original substance cannot be obtained again easily. |
| No new substance is formed. | A new substance is usually formed. |
| These changes are generally temporary. | These changes are generally permanent. |
| Example: Melting of ice | Example: Burning of paper |
Reversible changes are useful in our daily life because they allow materials to be used again and again. Irreversible changes, on the other hand, remind us to use resources carefully because some changes cannot be undone.
2. ‘Formation of curd is an irreversible as well as a chemical change.’ Justify this statement.
Formation of curd from milk is both an irreversible change and a chemical change. It is called an irreversible change because once milk changes into curd, it cannot be changed back into milk. The original substance is permanently changed. It is also a chemical change because a new substance called curd is formed with properties different from milk. The taste, smell, texture, and composition of curd are different from those of milk. This process is caused by bacteria that convert milk into curd. It teaches us how useful microorganisms can help in preparing healthy food for humans. Such natural changes are important for human life and show the usefulness of science in everyday activities.
3. Water cycle is a natural change. Classify it as a:
(i) Physical or chemical change
The water cycle is a physical change because no new substance is formed during the process. Water only changes its state from liquid to vapour and then back to liquid.
(ii) Reversible or irreversible change
The water cycle is a reversible change because water can continuously change from one form to another and return to its original state.
Justification
In the water cycle, water from rivers, lakes, and oceans evaporates due to the heat of the Sun. The water vapour cools and condenses to form clouds, and later falls back to the Earth as rain. Since only the state of water changes and no new substance is produced, it is a physical change. Also, the process keeps repeating naturally, so it is a reversible change. The water cycle is very important for all living beings. It maintains the balance of nature and provides fresh water for plants, animals, and humans. We should therefore save water and avoid polluting natural water resources.
4. How can we say that burning of candle is a chemical change but the glowing of bulb is a physical change?
Burning of a candle is a chemical change because when the candle burns, the wax reacts with oxygen in the air and forms new substances such as carbon dioxide and water vapour. Heat and light are also produced during this process. Since new substances are formed and the change cannot be reversed completely, it is a chemical change. On the other hand, glowing of an electric bulb is a physical change. In this process, electricity heats the filament inside the bulb and it glows to produce light. No new substance is formed and the bulb can glow repeatedly whenever electricity passes through it. These examples help us understand the difference between physical and chemical changes in daily life. They also teach us to use electricity and fuels carefully to conserve energy and protect the environment.
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