Chapter 3 of NCERT class 8 science is about synthetic fibres and plastics. Here you can find answers for textbook questions from this chapter. Extra questions from this chapter can be found here.

1. Explain why some fibres are called synthetic.

Some fibres are called synthetic because they are made by humans through the chemical processing of petrochemicals.

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)

3. Fill in the blanks with appropriate words.

(a) Synthetic fibres are also called …………………. or ……………… fibres

Manmade or artificial fibres

(b) Synthetic fibres are synthesised from raw material called ……………………..

Petrochemicals

(c) Like synthetic fibres, plastic is also a ………………………

Polymer

4. Give examples which indicate that nylon fibres are strong.

Nylon fibres are very strong. Actually, they are stronger than a steel wire. For this reason, they are used for making car seat belts, parachutes and ropes for rock climbing.

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

Plastic is a versatile material. It is used for storing food because:

(a) it does not react with the food

(b) it is light weight and strong

(c) it is cheap and easy to handle

6. Explain the difference between thermoplastic and thermosetting plastic.

Plastic which can be bent easily and which gets deformed on heating is called thermoplastic. Examples are polythene and PVC.

Thermosetting plastic cannot be softened by heating once it is moulded into a shape. Examples are Bakelite and Melamine.

7. Explain why the following are made of thermosetting plastics.

(a) saucepan handles

(b) electric plugs / switches / plug boards

Thermosetting plastic is a poor conductor of heat and electricity. Hence, if saucepan handles are made of thermosetting plastic, they will not get hot while cooking.

Thermosetting plastic is also used to make electric plugs and switches because electricity does not pass through this kind of plastic.

8. Categorise the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’.

Can be recycled

Plastic toys, carry bags, plastic bowls, plastic chairs, plastic covering on electrical wires

Cannot be recycled

Telephone instruments, cooker handles, ball point pens, electrical switches

9. Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material?

He should buy cotton shirts because cotton has a greater capacity to absorb moisture than synthetic shirts. In summer, we sweat excessively. While cotton shirts can effectively absorb the moisture and keep us comfortable, synthetic shirts will cause us to soak in the sweat because of their poor ability to absorb moisture.

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

If articles made of iron get exposed to air or moisture, they rust. By contrast, plastic does not rust even after prolonged exposure.

Plastic is used to store food and even chemicals. This is because plastic is non-reactive.

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

No, the handle and bristles should not be made of the material. The bristles should be made of a soft material because our gums are soft whereas the handle should be made of a hard material so that we can have a firm grip on it.

12. Avoid plastic as far as possible. Comment on this advice.

Plastic is non-biodegradable. That means it does not decompose due to the action of microbes. As a result, plastic remains in the soil and water for millions of years. We cannot get rid of plastic waste by burning it because plastic does not burn properly. Also, it releases poisonous gases on burning. When animals eat garbage, they often end up eating the polythene bags and choke on them. Plastic bags carelessly thrown around may clog drains. For these reasons, we have to limit the use of plastic.