NCERT Solutions Class 8 Science
The NCERT Solutions in English Language for Class 8 Science Chapter – 3 (Synthetic Fibres and Plastics) has been provided here to help the students in solving the questions from this exercise.
Chapter – 3 (Synthetic Fibres and Plastics)
1. Explain why some fibres are called synthetic.
Answer – Since man-made fibres are synthesised from petrochemicals, they are called synthetic fibres. Examples of synthetic fibres are rayon, nylon, polyester, acrylic, etc.
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) it is obtained from wood pulp.
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 which indicate that nylon fibres are very strong.
Answer – Nylon fibres are very strong. It is used for making ropes used for climbing rocks and for making parachutes. Their usage shows that nylon fibres have high tensile strength.
5. Explain why plastic containers are favoured for storing food.
Answer – Plastic containers are favoured for storing food because plastic is non-reactive and will not react with the ingredients of the food items.
6. Explain the difference between thermoplastic and thermosetting plastics.
Answer – The difference between thermosetting plastics and thermoplastics are as follows:
| Thermoplastic plastics | Thermosetting plastics |
|---|---|
| 1. Thermoplastic can be easily bent. | 1. Thermosetting plastics cannot be easily bent. If forced to bend, they will break. |
| 2. Softens on heating | 2. Do not soften on heating |
| 3. Can be moulded and reshaped | 3. Cannot be moulded and reshaped. |
| 4. Expensive | 4. Cheaper |
| 5. Recyclable | 5. Not recyclable |
| 6. Soft and weak | 6. Hard and strong |
| 7. It is used in making toys,combs and various containers. | 7. It is used in making handles of cooking utensils. |
| 8. Example: Polythene and PVC | 8. Example: bakelite and melamine |
7. Explain why the following are made of thermosetting plastics.
(a) Saucepan handles
(b) Electric plugs/switches/plug boards
Answer –
(a) Saucepan handles are made of 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.
8. Categorise the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’.
Telephone instruments, plastic toys, cooker handles, carry bags, ballpoint pens, plastic bowls, plastic covering on electrical wires, plastic chairs, electrical switches.
Answer –
| Can be Recycled | Cannot be recycled |
| Plastic chairs | Cooker handles |
| Plastic bowls | Electrical switches |
| Plastic toys | Telephone instruments |
| The plastic covering on electrical wires | |
| Carry bags | |
| Ballpoint pens |
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 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.
10. Give examples to show that plastics are non-corrosive in nature.
Answer – Plastics are non-corrosive in nature; they do not react even if they come in contact with strong chemicals. For example, the cleaning chemicals that we use at home 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 – No. The handle and bristles of a tooth brush should be made of different materials. The handle of a toothbrush should be hard and strong, while the bristles should be soft and flexible.
12. ‘Avoid plastics as far as possible’. Comment on this advice.
Answer – Plastics should not be used for the following reasons:-
- They are non biodegradable and take several years to decompose.
- It adds to the pollution in the environment.
- By burning plastics, poisonous gases are released that cause air pollution.
- Besides, the burning process in the synthetic material is quite slow and it does not get completely burnt easily.
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 |
Answer –
| 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 of synthetic fibres is actually helping conservation of forests’. Comment.
Answer – Raw materials for natural fibres are mainly derived from plants and this means cutting a lot of trees. This leads to deforestation. But raw materials of synthetic materials are mainly petrochemicals. Hence, manufacturing synthetic fibres helps in the conservation of forests.
15. Describe an activity to show that thermoplastic is a poor conductor of electricity
Answer – We will design a circuit to see that thermoplastics are poor conductors of electricity. We need a bulb, some wires, a battery, a piece of metal, and a plastic pipe. 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.
