DAV Class 8 Science Chapter 13 Question Answers – Synthetic Fibres and Plastics
Look around you. Your school bag, water bottle, toothbrush, raincoat and even some of your clothes are likely made from something that did not grow on a plant or come from an animal. They are made in factories from chemicals. Chapter 13 of the DAV Class 8 Science book is about these man-made materials: synthetic fibres and plastics. This post gives easy answers to every question in the chapter, with a short reason behind each one, so you can understand the answer and write it in your own words.
The chapter in one minute
A fibre is a thin, thread-like material that can be spun into yarn and woven into cloth. Natural fibres, such as cotton, wool and silk, come from plants and animals. Synthetic fibres are made by people using chemical processes. All these fibres are made of polymers. A polymer is a very long chain made by joining many small, identical units together, and each small unit is called a monomer.
Synthetic fibres come in two kinds. Regenerated fibres, like rayon, are made by chemically treating a natural material, such as wood pulp. True synthetic fibres, like nylon, are made entirely from chemicals, without any natural starting material.
Plastics are also polymers, and they come in two groups. Thermoplastics soften when heated and can be melted and shaped again and again, for example polythene and PVC. Thermosetting plastics set hard when moulded once and cannot be softened again by heating, for example bakelite and melamine.
A. Fill in the blanks
1. Cotton and wool are examples of natural fibres.
2. The simplest unit of a polymer is called a monomer.
3. Rayon is also known as a regenerated fibre.
4. Nylon is a synthetic fibre.
5. Thermosetting plastics cannot be remoulded by heating them again.
B. True or false
| Statement | Answer | Why |
|---|---|---|
| 1. Natural fibres strongly resist chemicals like acids and alkalis. | False | Natural fibres are easily damaged by chemicals. Synthetic fibres resist them better. |
| 2. Synthetic fibres absorb very little water. | True | That is why they dry quickly. |
| 3. Thermoplastics can be easily remoulded on heating. | True | They soften on heating and harden again on cooling. |
| 4. The plastic used to insulate electrical wires is Teflon. | True (as per the book) | Teflon is a poor conductor of electricity. PVC is also widely used for wire insulation. |
| 5. All plastics are biodegradable. | False | Most plastics do not break down naturally and stay in the environment for years. |
C. Choose the correct option
1. Thermoplastic: PVC. Thermosetting plastic: bakelite.
2. The “wonder polymer” is nylon.
3. A very strong synthetic fibre used for parachutes, climbing ropes and fishing nets is nylon.
4. The one that is NOT a characteristic of synthetic fibres is being biodegradable. Synthetic fibres do not decay naturally.
5. Melamine is a thermosetting plastic, and for that reason we can say its molecules are arranged in a cross-linked pattern. The cross-links hold the structure firmly, so heat cannot soften it again.
6. A correct statement about synthetic plastics is that they have quite high tensile strength, which means they can resist being pulled apart.
D. Short answer questions
1. What are synthetic fibres?
Synthetic fibres are man-made fibres, produced in factories using chemical processes.
2. Write two uses of rayon.
Rayon is used to make clothes such as dresses, jackets, suits and linings. It is also used in industry, for example in surgical products and in tyre cords.
3. Name two or three things made from nylon.
Carpets, clothes and toothbrushes. Ropes, fishing nets and parachutes are also made from nylon.
4. Why are clothes made of synthetic fibres uncomfortable in summer?
Synthetic fibres absorb very little water, so they do not soak up sweat. The sweat stays on the skin, and we feel hot and sticky.
5. Give two examples each of thermoplastics and thermosetting plastics.
Thermoplastics: polythene and PVC. Thermosetting plastics: bakelite and melamine.
E. Long answer questions
1. How is a regenerated fibre different from a true synthetic fibre? Give one example of each.
A regenerated fibre is made by chemically treating a natural material, so its raw material comes from nature. Rayon, which is made from the cellulose in wood pulp, is an example. A true synthetic fibre is made completely from chemicals and does not use any natural fibre as a starting point. Nylon is an example.
2. Why do clothes made of nylon last longer?
Nylon is very strong, elastic and does not wear out easily. It can be stretched without breaking and resists rubbing and chemicals, so nylon clothes stay in good shape for a long time.
3. Write two differences between thermosetting plastics and thermoplastics.
| Thermoplastics | Thermosetting plastics |
|---|---|
| They soften when heated and can be remoulded again and again. | They harden permanently once moulded and cannot be softened by heating again. |
| Their molecules are in long chains that are not strongly linked to each other. | Their molecules are joined by cross-links, forming a rigid structure. |
| Examples: polythene, PVC. | Examples: bakelite, melamine. |
4. State any three advantages of synthetic fibres.
Synthetic fibres last longer than natural fibres. They are easy to wash and dry quickly. They keep their shine for a long time and are usually cheaper than natural fibres.
5. “Excessive use of plastics is a threat to the environment.” Give your views.
I agree with this statement. Most plastics are not biodegradable, so they do not rot, and they stay in the soil and water for many years. Many kinds of plastic cannot be recycled easily. Plastic bags thrown away carelessly clog drains and can be eaten by animals, which can harm or kill them. Burning plastic releases poisonous gases that pollute the air. We can help by using cloth or paper bags, reusing plastic items, and recycling whatever we can.
6. Why is bakelite a good choice for electrical switches and handles of cooking utensils?
Bakelite is a poor conductor of both heat and electricity. In switches, it protects us from electric shocks. In cooking utensils, the handle stays cool even when the pan is hot. Also, being a thermosetting plastic, it does not melt or soften with heat.
Mistakes to avoid in the exam
Students often call rayon a true synthetic fibre. Rayon is a regenerated fibre, because it starts from natural wood pulp. Nylon is the true synthetic fibre.
Another common slip is mixing up the two plastics. Remember: “thermo” means heat, so thermoplastics soften with heat and thermosetting plastics “set” and stay hard.
Do not say that all plastics can be recycled or that plastics rot. Most plastics are non-biodegradable.
Finally, in the long answers, always give an example along with each point, such as polythene for thermoplastics and bakelite for thermosetting plastics.
Frequently asked questions
What is a polymer?
A polymer is a very large molecule made by joining many small, repeating units called monomers in a long chain.
Why is nylon called a wonder polymer?
Because it is strong, light, elastic, durable and easy to wash, and it can be used to make many different things.
Why do synthetic clothes dry faster than cotton clothes?
Synthetic fibres absorb very little water, so there is little water in them to dry out.
Why should we reduce the use of plastic?
Because most plastic does not decay, so it piles up and pollutes land, water and air, and harms animals.
Written for students following the DAV Science book for Class 8. Please cross-check answers with your own textbook and teacher.