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CBSE Board NCERT Solutions for Class 8th Science Chapter3:Synthetic Fibres and Plastics
NCERT Solutions for Class 8th Science Chapter 3 : Synthetic Fibres and Plastics
Page No: 41
Excercise
1. Explain why some fibres are called synthetic.
Answer:
Some fibres are called synthetic fibres because they are man-made. These fibres made by the combination of different types of chemicals in industries and their nature are also different in respect of natural fibre
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 materials called ________.
(c) Like synthetic fibres, plastic is also a _________.
Answer:
(a) Synthetic fibres are also called artificial or man-made fibres.
(b) Synthetic fibres are synthesised from raw materials calledpetrochemicals.
(c) Like synthetic fibres, plastic is also a polymer.
4. Give examples which indicate that nylon fibres are very strong.
Answer:
Nylon fibers are very strong thus they are used to prepare ropes for mountaineering. They are also used to prepare parachutes.
5. Explain why plastic containers are favoured for storing food.
Answer:
Plastic containers are favoured for storing food because:
→ The plastics do not react with the food stored in them.
→ The plastics are light weight and are strong.
→ They are easy to handle and safe.
→ They are cheap and ductile.
6. Explain the difference between thermoplastic and thermosetting plastics.
Answer:
| Thermosetting plastic | Thermoplastic |
| Thermosetting plastic cannot be bent easily. It may break when forced to bend. | Thermoplastic can be bent easily. |
| Thermosetting plastic cannot be softened by heating. Thus, it cannot be reshaped once moulded. | Thermoplastic can be softened easily by heating. Thus, it can be reshaped. |
7. Explain why the following are made of thermosetting plastics.
(a) Saucepan handles
(b) Electric plugs/switches/plug boards
Answer:
(a) The handles of saucepans are made of thermosetting plastics because it is a bad conductor of heat and so, do not get heated up while cooking. So it is easier to handle the utensil while cooking. Moreover it does not get softened due to heat.
(b) Electrical plugs, switches and plug-boards are made up of thermosetting plastics because it is a bad conductor of electricity. It does not allow the electric current to pass through it, thus safe in using in electric appliances.
8. Categorize 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 | Plastic chairs |
| Electrical switches | Carry bags |
| Plastic covering on electrical wires | |
| Ball point pens | |
| Plastic bowls |
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 should buy shirts made from cotton. This is because cotton is a good absorber of water. It can soak the sweat coming out of our body and expose it to the environment. Thus, it helps in evaporating the liquid (sweat), thereby cooling our body.
10. Give examples to show that plastics are non-corrosive in nature.
Answer:
Plastics are noncorrosive in nature because it is a nonreactive substance. It does not react with air, water, minerals and other chemicals. So in present days most of the containers, utensils and laboratory equipments are made with plastics to store food, chemicals etc.
11. Should the handle and bristles of a tooth brush be made of the same material? Explain your answer.
Answer:
No. The handle and bristles of a tooth brush should be made of different materials. The handle of a toothbrush should be hard and strong, while the bristles should be soft and flexible.
12. ‘Avoid plastics as far as possible’. Comment on this advice.
Answer:
Plastics are non-biodegradable materials. They take several years to decompose. It is not environment friendly and causes environment pollution. Also burning process of plastics is quite slow. In the process of burning it releases lots of poisonous gas into the atmosphere causing air pollution. Some plastics can’t be recycled and so, it is very difficult to finally dispose off such materials. Polythene bags, carelessly thrown in an eating garbage can cause death for the animals like – cows, dogs etc or sometimes even become a cause for clogging of the drains.
13. Match the terms of column A correctly with the phrases given in column B.
A 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 cookware
(iv) Nylon (d) Fabrics do not wrinkle easily
Answer:
A B
(i) Polyester (d) Fabrics do not wrinkle easily
(ii) Teflon (c) Used to make non-stick cookware
(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:
The natural fibers are obtained from plants and animals, whereas the synthetic fibers are obtained by chemical processing of petrochemicals. Unlike natural fibers, manufacturing of synthetic fibers do not require to cut trees or hunting of any animal. In this way manufacturing synthetic fibers is actually helping conservation forests.
15. Describe an activity to show that thermoplastic is a poor conductor of electricity.
Answer:
We will design a circuit to see that thermoplastics are poor conductors of electricity. We need a bulb, some wires, a battery, a piece of metal, and a plastic pipe. Set up the circuit first with the metal and then with the plastic pipe (as shown in the figure). After you switch on the current, you will observe that the bulb glows in the former case. In the latter case, the bulb does not glow. Hence, a plastic pipe (which is a thermoplastic) is shown to be a poor conductor of electricity.
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