NCERT Solutions For Class 8 Science Chapter 3 Exercise
Chapter 3 – Synthetic Fibres and Plastics
These are the NCERT Solutions for Class 8 Science. They cover textbook questions extra questions, example problems, worksheets multiple-choice questions, and short answer questions. Answering these questions helps you organize your notes better and gain a complete grasp of the subject.
Questions :
1) Explain why some fibres are called synthetic.
Answer:
Instead of occurring naturally, people create synthetic fibers using chemicals. They’re made up of tiny pieces that link to make long chains. Some standard synthetic fibres include nylon, rayon, acrylic, and polyester.
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 natural fibres.
Answer:
The answer is (b) it is obtained from wood.
3) Fill in the blanks with appropriate words.
(a) Synthetic fibres are also called __________ or __________ fibres.
(b) Synthetic fibres are synthesised from the raw material 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 a raw material called petrochemicals.
(c) Like synthetic fibres, plastic is also a polymer.
4) Give examples that indicate that nylon fibres are solid.
Answer:
Here are a few examples showing that nylon fibres are solid:
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Parachutes and rock climbing ropes are made of nylon.
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Sports gear, like rackets, is also made from nylon.
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Nylon is used for seat belts, fishing nets, and tire cords.
5) Explain why plastic containers are preferred for storing food.
Answer:
Plastic containers are popular for storing food because they are non-reactive. This means plastic does not react with the ingredients in the food, keeping it safe and fresh. This quality makes plastic an ideal material for food storage, preventing any chemical changes or contamination.
6) Explain the difference between thermoplastic and thermosetting plastics.
Answer:
Thermosetting plastics harden permanently when heated, while thermoplastics can be melted and reshaped multiple times. The table below shows the further differences between them:
Thermosetting plastics |
Thermoplastics |
Thermosetting plastics break if you try to bend them |
Thermoplastics can be bent easily. |
Heating thermosetting plastics does not soften them; they cannot be reshaped after moulding. |
Heating thermoplastics softens them, allowing them to be moulded and reshaped easily. |
7) Explain why the following are made of thermosetting plastics:.
(a) Saucepan handles
(b) Electric plugs/switches/plug boards
Answer:
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a) Thermosetting plastics are used for saucepan handles because they don’t conduct heat or soften when heated.
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b) Bakelite is a thermosetting plastic that doesn’t conduct heat or electricity, so it’s used for making electric plugs, switches, and plug boards.8. Categorise the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’.
8) Telephone instruments, plastic toys, cooker handles, carry bags, ballpoint pens, plastic bowls, plastic covering on electrical wires, plastic chairs, electrical switches
Answer:
The table below describes the materials that can be recycled and those that cannot be recycled.
Can be Recycled |
Cannot be Recycled |
Plastic chairs |
Cooker handles |
Plastic bowls |
Electrical switches |
Plastic toys |
Telephone instruments |
Plastic covering on wires |
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 a cotton shirt for summer instead of a synthetic one. Cotton absorbs water well, so it soaks up sweat and helps it evaporate, cooling the body.
10) Give examples to show that plastics are non-corrosive.
Answer:
Plastics don’t corrode and don’t react with solid chemicals. That’s why cleaning chemicals are stored in plastic bottles instead of metal containers.
11) Should the handle and bristles of a toothbrush be made of the same material? Explain your answer.
Answer:
The handle and bristles of a toothbrush should be made of different materials because the handle needs to be hard and strong, while the bristles should be soft and flexible.
12) ‘Avoid plastics as far as possible’. Comment on this advice.
Answer:
We should not use plastics because they don’t break down naturally, causing pollution when thrown away. Burning plastics releases harmful gases, and animals can eat plastic bags, making them sick or even causing death.
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 |
Answers:
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 helping conserve forests’. Comment.
Answer:
Making synthetic fibres helps protect forests because using natural fibres means cutting down many trees for their raw materials. So, using synthetic fibres helps save trees and conserve forests.
15) Describe an activity to show that thermoplastic is a poor conductor of electricity
Answer:
To demonstrate that thermoplastic is a poor conductor of electricity, we need a bulb, wires, a battery, metal, and a plastic pipe.
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In the experiment, the bulb glows when the circuit is set with the metal conductor because metal is a good conductor.
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However, when the plastic pipe (thermoplastic) replaces the metal, the bulb does not glow, showing that thermoplastic is a poor conductor of electricity.