Class 8 Science Chapter 3 |
Synthetic Fibres and Plastics NCERT Solutions
1. Explain why some fibres are called synthetic.
Answer:
Those fibres which are prepared by man using chemicals are called synthetic fibres. They are formed by combination of large number of small molecules joined together.
For ex: Rayon, nylon, polyester, acrylic etc.
2. Mark (√ ) the correct answer.
Rayon is different from synthetic fibres because
(a) it has a silk-like appearance.
(b) it is obtained from wood pulp.
(c) its fibres can also be woven like those of natural fibres.
Answer: (b) it is obtained from wood pulp.
3. Fill in the blanks with appropriate words.
(a) Synthetic fibres are also called __________ or __________ fibres.
(b) Synthetic fibres are synthesised from raw material called __________ .
(c) Like synthetic fibres, plastic is also a __________ .
Answer:
(a) artificial and man-made
(b) petrochemicals
(C) polymer
4. Give examples which indicate that nylon fibres are very strong.
Answer:
Nylon is synthetic fibre made by man without using any natural raw material. It is made up of the repeating units of a chemical called an amide. Nylon fibres are very strong because they are used for making parachutes , ropes for rock climbing, car seat-belt.
5. Explain why plastic containers are favoured for storing food.
Answer:
Plastic are favoured for storing food because:
Plastics do not react with air and water.Due to this, plastics are resistant to corrosion. They are also often unaffected by various chemicals.
2)Plastics are quite cheaper and easily made.
3)Plastics are light weight , strong and durable.
6. Explain the difference between thermoplastic and thermosetting plastics.
Answer:
| Thermoplastic | Thermosetting plastic |
| 1) A plastic which can be softened repeatedly by heating and can be moulded into different shapes again and again is called thermoplastic. | A plastic which once set, does not become soft on heating and cannot be moulded a second time, is called thermosetting plastic. |
| 2) They are flexible. | They are not flexible |
| 3) They are soluble in organic solvents | They are insoluble in organic solvents |
| 4) For Example : Polythene, Polyvinylchloride | For Example : Bakelite and melamine |
| 5) They are used for making insulations of electric wires and cables, various types of plastic containers, combs, toys, raincoats, seat cover, bristles of brushes, chains. | They are used for making handles of cooking utensils ,plates, cups, floor tiles, electrical fitting, ballpoint pens, telephones. |
| 6) The polymer chains are held together by weak Van der waal forces. | The Polymer chains are linked by covalent bonds. |
7. Explain why the following are made of thermosetting plastics.
(a) Saucepan handles
(b) Electric plugs/switches/plug boards
Answer:
a) The handles of saucepan are made of thermosetting plastic because:
1)they do not become soft on getting heated
2)they are poor conductors of heat.
b) Thermosetting plastics are used for making electric switches/plugs/ plug boards because
1)they do not become soft on getting heated
2)they are poor conductors of electricity i.e. it does not allow electric current to pass through it.
8. Categorise the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’. Telephone instruments, plastic toys, cooker handles, carry bags, ball point pens, plastic bowls, plastic covering on electrical wires, plastic chairs, electrical switches.
Answer:
| Cannot be recycled | Can be recycled |
| Telephone instruments | Plastic toys |
| Cooker handles | Carry bags |
| Electrical Switches | Ball point pens |
| plastic bowls | |
| plastic covering on electrical wires | |
| plastic chairs |
9. Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.
Answer:
Rana is advised to buy a cotton shirt for summers because cotton absorbs the sweat and the pores in cotton allows easy evaporation and keeps our body cool, whereas synthetic shirts don’t absorb sweat so, they are very uncomfortable to wear in summers.
10. Give examples to show that plastics are noncorrosive in nature.
Answer:
Plastics are noncorrosive because they do not react with air, water, and minerals.
For Ex: 1) Bakelite is a very hard and tough plastic. It is a poor conductor of heat and electricity so it is used for making electric switches and handles of the utensils.
2) Melamine is a plastic which can tolerate heat better than other plastics and resist fire. It is used for making floor tiles, unbreakable kitchenware ,ashtray and fire resistant fabric. Special plastic cookware made of melamine is used in microwave ovens for cooking food
3) Teflon is a special plastic on which oil and water does not stick because it has slippery surface. It can withstand high temperature.It is used for making soles of electric irons, giving non-stick coating on cookwares.
material? Explain your answer.
Answer:
No, the handle and bristles of the tooth brush should be made of different material. The handle should be made of hard material so that it can give a firm grip. The bristles of a tooth brush should be made of soft material so that it does not harm the gums.
12. ‘Avoid plastics as far as possible’. Comment on this advice.
Answers:
We should avoid them as far as possible due to following reasons:
1) Plastic is non-biodegradable. So the articles made of plastics do not rot when they are thrown away after use. The waste plastic articles keeps on accumulating in the surrounding and pollute the environment.
2) The waste plastic articles thrown here and there carelessly get into dirty water drains and sewers, and clog them. This makes the dirty drain water to flow over the streets and roads causing unhygienic conditions.
3) The animals eat up the used polythene bags or plastic wrappers along with the left over food and vegetables wastes thrown on garbage dumps. The plastic wastes can choke the respiratory system of these animals or form a plastic lining in their stomach.
4)When plastic bags are burnt they produce poisonous gases which pollute the air.
13. Match the terms of column A correctly with the phrases given in column B.
Column A Column B
(i) Polyester (a) Prepared by using wood pulp
(ii) Teflon (b) Used for making parachutes and stockings
(iii) Rayon (c) Used to make non-stick cookwares
(iv) Nylon (d) Fabrics do not wrinkle easily
Answer:
Column A Column B
(i) Polyester (d) Fabrics do not wrinkle easily
(ii) Teflon (c) Used to make non-stick cookwares
(iii) Rayon (a) Prepared by using wood pulp
(iv) Nylon (b) Used for making parachutes and stockings
14. ‘Manufacturing synthetic fibres is actually helping conservation of forests’. Comment.
Answer:
Manufacturing of synthetic fibres involves chemical processing of petrochemicals. For manufacturing synthetic fibres cutting of trees or killing of animals is not required.In this way manufacturing synthetic fibres actually helps in conservation of trees.
15. Describe an activity to show that thermoplastic is a poor conductor of
electricity.
Answer:
The circuit consist of bulb, key, connecting wires , battery, a metal piece and a thermoplastic. When we connect a metal piece in the circuit, current passes through it and the bulb glows. But if we connect thermoplastic in the circuit, no current passes and the bulb does not glow. This shows that thermoplastic is a poor conductor of electricity.
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