Synthetic Fibres and Plastics - Class 8 : NCERT Exercise Questions



Q.1 Explain why some fibres are called synthetic.
Sol. The fibres which are not natural and made by human beings are called synthetic fibres as they are manufactured by chemical processing of petrochemicals. These are made up of large units known as polymers. Examples of some synthetic fibres are rayon, nylon, etc.

Q.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
Sol. (b) it is obtained from wood pulp

Q.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 __________.
Sol. (a) Synthetic fibres are also called man-made or artificial fibres.
(b) Synthetic fibres are synthesised from raw material called petrochemicals.
(c) Like synthetic fibres, plastic is also a polymer.

Q.4 Give examples which indicate that nylon fibres are very strong.
Sol. Nylon is a very strong fibre which is used for making parachutes, rock climbing ropes, fishing nets, etc. Moreover, it is used to make many articles such as socks, ropes, tents, toothbrushes, bags, curtains, etc. A nylon thread is elastic and light but actually stronger than a steel wire.

Q.5 Explain why plastic containers are favoured for storing food.
Sol. Plastic containers are favoured for storing food because of following reasons:
(i) They are easy to handle because of their light weight.
(ii) They are cheaper when compared to other containers.
(iii) They are strong and durable.
(iv) They are non-reactive to moisture and air.
(v) They are corrosion free.

Q.6 Explain the difference between the thermoplastic and thermosetting plastics.
Sol. The differences between thermoplastic and thermosetting plastics are:

Thermoplastic

Thermosetting plastic

Plastic which gets deformed easily on heating and can be bent easily are known as thermoplastics.

Plastics which once moulded and cannot be softened on heating are known as thermosetting plastics.

Example: Polythene, PVC, etc.         

Example: Bakelite, melamine, etc.

Applications: Used for manufacturing toys, combs, different containers, etc.

Applications: Bakelite is used for making electrical switches, handles of utensils, etc. Melamine is used for floor tiles, kitchenware, fabrics, etc.

 Q.7 Explain why the following are made of thermosetting plastics.
(a) Saucepan handles
(b) Electric plugs/switches/plug boards
Sol. (a) Saucepan handles are made up of melamine (a thermosetting plastic) because it resists fire and can tolerate heat better than other plastics.
(b) Electric plugs/switches/plug boards are made up of Bakelite (a thermosetting plastic) because it is a poor conductor of heat and electricity.

Q.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.
Sol. Can be recycled products: Plastic toys, carry bags, ball point pens, plastic bowls, plastic covering on electrical wires and plastic chairs.
Cannot be recycled products: Telephone instruments, cooker handles and electrical switches.

Q.9 Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.
Sol. Rana is advised to buy cotton shirts for summers because a cotton shirt will absorb his sweat and cotton being a porous material will allow easy evaporation keeping his body cool. On the other hand, shirts made from synthetic material will not absorb his sweat, making him feel warmer in summers.    

Q.10 Give examples to show that plastics are noncorrosive in nature.
Sol. Following are some examples which show that plastics are noncorrosive in nature:
(i) Plastic containers are used for storing food, chemicals and other things as they do not react with water and air.
(ii) Teflon is used as coating on non-stick cookware.
(iii) Bakelite is used for making electric switches and handles of the utensils because it is a poor conductor of heat and electricity.
(iv) Melamine is used for making floor tiles and kitchenware because it is a good fire resistant and can tolerate heat better than other plastics.

Q.11 Should the handle and bristles of a tooth brush be made of the same material? Explain your answer.
Sol. No, the handle and bristles of a tooth brush should not be made of the same material. The bristles of a tooth brush should be made of soft material so that it does not harm the gums; while the handle should be made of hard material so that it can give a firm grip. Hence, bristles are usually made of nylon as they are strong, elastic and light in weight and the handle should be made of plastic which is strong and durable.

Q.12 ‘Avoid plastics as far as possible’. Comment on this advice.
Sol. These days’ plastics have become an integral part of our life. But, we must avoid plastics as far as possible because:
(i) Plastics are non-biodegradable and so take several years to get decomposed.
(ii) Plastics release lots of poisonous fumes into atmosphere on burning that cause air pollution.
(iii) The polybags carelessly thrown are responsible for clogging the drains.
(iv) Animals like cows might consume plastics which might result in choking of their respiratory system and can even cause death.
(v) Soil pollution is caused because of plastics.
(vi) Threat to the aquatic animals is caused due to the plastics thrown away into the water bodies.

Q.13 Match the terms of column I correctly with the phrases given in column II.
Column I                         Column II
(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
Sol. Column I              Column II
(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

Q.14 ‘Manufacturing synthetic fibres is actually helping conservation of forests’. Comment.
Sol. Synthetic fibres are man-made fibres made from petrochemicals. The manufacturing of synthetic fibres does not require the cutting of trees, while natural fibres do require cutting of trees and even killing animals. Hence, we can conserve forests by manufacturing synthetic fibres.

Q.15 Describe an activity to show that thermoplastic is a poor conductor of electricity.
Sol. Following is the activity which shows that thermoplastic is a poor conductor of electricity.
(i) Connect the circuit as shown in the figure below:(ii) Now, when switch is closed, it is observed that lamp does not glow. This means that the thermoplastic is a poor conductor.
(iii) On the other hand, if the thermoplastic is replaced by a conductor then it is observed that the bulb glows. Thus, we can conclude that thermoplastic is a poor conductor of electricity.



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