Class 8 Science

Synthetic Fibres and Plastics

NCERT Solution

Question 1: Explain why some fibres are called synthetic.

Answer: Some fibres are made in factories or laboratories. Hence, they are called synthetic fibres.

Question 2: Rayon is different from synthetic fibres because

  1. It has a silk like appearance
  2. It is obtained from wood pulp
  3. Its fibres can also be woven like those of natural fibres

    Answer: (b) It is obtained from wood pulp

Question 3: Fill in the blanks with appropriate words:

  1. Synthetic fibres are also called ______ or _______ fibres.
  2. Synthetic fibres are synthesised from raw material called ________.
  3. Like synthetic fibres, plastic is also a __________ .

Answer: (a) man-made, artificial, (b) petrochemicals, (c) polymer

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

Answer: Nylon is used for making parachutes. A parachute needs to strong as it has to withstand high speed wind and huge amount of air pressure. Nylon ropes are used by mountaineers. A mountaineer’s life highly depends on the strength of ropes he is using. These examples show that nylon fibres are very strong.

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

Answer: Plastic does not react with most of the substances. Hence, food kept in plastic container remains fresh for longer duration. Due to this, plastic containers are preferred for storing food.

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

ThermoplasticThermosetting plastic
Can be remolded.Cannot be remolded.
It is not fire-resistant.It is fire-resistant.
Used for making toys, buckets, mugs, etc.Used for making switches, switchboards, utensils, etc.

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

(a) Saucepan handles

Answer: Thermosetting plastic is fire-resistant and can withstand very high temperature. Due to this, saucepan handles are made of thermosetting plastic. Such handles do not heat up during cooking.

(b) Electric plugs/switches/plug boards

Answer: Thermosetting plastic is bad conductor of electricity and heat. Hence, electric plugs, switches and plug boards are made of thermosetting plastic.

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.

Answer: Can be recycled: plastic toys, ball point pens, plastic bowls, plastic chairs

Cannot be recycled: Telephone instruments, cooker handles, carry bags, plastic covering on electrical wires, electrical switches

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.

Answer: A shirt made from synthetic materials does not allow air to pass through it. On the other hand, a shirt made from cotton allows air to pass through it. Due to this, a cotton shirt keeps the body temperature lower. Hence, a cotton shirt is more comfortable than a shirt made from synthetic material. This shows that Rana should buy a cotton shirt.

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

Answer: We know that acids and bases are corrosive in nature. Now-a-days, many acids and bases are stored in plastic containers. This is possible because of non-corrosive nature of plastic. A plastic chair does not get rusted; unlike an iron chair. This also happens because of non-corrosive nature of plastic.

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

Answer: Purpose of the handle of a toothbrush is different from the purpose of bristles of the toothbrush. The handle needs to be strong and less flexible. The bristles need to be highly flexible, soft yet durable. Hence, bristles need to be made up of different material than the material of toothbrush handle.

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

Answer: We know that plastic is non-biodegradable. This means that plastic waste is getting accumulated in our environment. Accumulation of plastic waste is creating a huge problem for us. Hence, we should avoid plastic as far as possible. We should switch over to biodegradable alternatives wherever possible. For example; we should use shopping bags made up of jute or cloth instead of plastic.

Question 13: Match the terms of column I correctly with the phrases given in column II.

Column IColumn II
(a) Polyester(1) Prepared by using wood pulp
(b) Teflon(2) Used for making parachutes and stockings
(c) Rayon(3) Used to make non-stick cookwares
(d) Nylon(4) Fabrics do not wrinkle easily

Answer: (a) 4, (b) 3, (c) 1, (d) 2

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

Answer: Synthetic fibres have replaced natural fibres for most uses. It has resulted in reducing our dependency on natural fibres. Many natural fibres come from plants and hence plants need to be cut to obtain these fibres. Less demand for natural fibres means there is reduced need to cut trees. It has helped in conservation of forests. Hence, it can be said that manufacturing synthetic fibres is actually helping conservation of forests.

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

Answer: For this, take a small piece of thermoplastic, some copper wires, two electric cells (from torch) and an electric bulb (from torch).