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NCERT Solutions for Class 8 Science chapter 3 – Fibers and Plastics

Back Exercise

Question 1:
Explain why some fibres are called synthetic.

Answers
Some fibers which are not natural and are prepared by man are called synthetic fibers. These are called synthetic because they are made by chemical processing of petrochemicals in factories. These fibers are made by joining smaller units called monomers into long chain called polymers. eg-nylon, Rayon

Question 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.

Answers
(b) it is obtained from wood pulp.

Question 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 __________ .

Answers
(a) Artificial and man made.
(b) Petrochemicals.
(c) Polymer.

Question 4:
Give examples which indicate that nylon fibres are very strong.

Answers
Following are the examples that indicate nylon fibres are very strong
⚬ Parachutes and ropes of rock climbing are made of nylon
⚬ Nylon is used in making seat belts, fishing nets and tyre cords.
⚬ Nylon is used in making sports accessories like rackets.
⚬ They are used for making parachutes.

Question 5:
Explain why plastic containers are favoured for storing food.

Answers
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.

Question 6:
Explain the difference between thermoplastic and thermosetting plastics.

Answers

Question 7:
Explain why the following are made of thermosetting plastics.
(a) Saucepan handles
(b) Electric plugs/switches/plug boards

Answers
(a) Saucepan handles are made of (Melamine) thermosetting plastics because these plastics do not get softened on heating. Also, thermosetting plastics such as bakelite are poor conductors of heat.
(b) Thermosetting plastics such as bakelite are poor conductors of heat and electricity. Therefore, they are used for making electric plugs, switches, plug boards, etc.

Question 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.

Answers
Cannot be recycled
Telephone instruments
Cooker handles
Electrical switches

can be recycled
Plastic toys
Plastic chairs
Carry bags
Ball point pens
Plastic bowls
Plastic covering on electrical wires

Question 9:
Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.

Answers
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, whereas synthetic shirts don't absorbs sweat so, they are uncomfortable to wear in summers.

Question 10:
Give examples to show that plastics are noncorrosive in nature.

Answers
Plastics are not corroded even if they come in contact with strong chemicals. This is because of their non-reactive nature with most materials. 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.

Question 11:
Should the handle and bristles of a tooth brush be made of the same material? Explain your answer.

Answers
The bristles of a brush should be made from a soft material so that it does not harm the gums and the handle should be made from a hard material so that it can give a firm grip and last a long. Therefore, the bristles are made up of nylon fiber which are strong, elastic, lustrous, light-weight and easy to handle. While the handle is made of hard plastic as it is stronge and durable.

Question 12:
‘Avoid plastics as far as possible’. Comment on this advice.

Answers
Plastics must be avoided as far as possible. The materials made of plastics are non-biodegradable. The use of plastics has a bad effect on the environment. When the plastics are burnt, it releases a lot of poisonous fumes into the atmosphere causing air pollution. These plastic materials when eat up by the animals (like cows), choke their respiratory system. This can cause death of these animals. The waste plastic articles thrown here and there carelessly get into dirty water drains and sewers, and block them. In a nutshell, plastics can be considered a threat to our environment.

Question 13:
Match the terms of column A correctly with the phrases given in column B.

Answers

Question 14:
'Manufacturing synthetic fibres is actually helping conservation of forests'. Comment.

Answers
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.

Question 15:
Describe an activity to show that thermoplastic is a poor conductor of electricity.

Answers
We will design a circuit to see that thermoplastics are poor conductors of electricity. Materials needed for activity: bulb, some wires, a battery, a piece of metal, and a plastic pipe. Procedure: 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.