NCERT Solutions for Class 7 Science - Synthetic Fibres and Plastics

NCERT TEXTBOOK QUESTIONS SOLVED
 

Q. 1. Explain why some fibres are called synthetic.
 Ans. Some fibres are called synthetic fibres because they do not occur in the nature.

Q. 2. Chose 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.
 Ans. (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 raw material called ______.
 (c) Like synthetic fibres, plastic is also a ______.
 Ans. (a) man-made, artificial.
(b) Petrochemicals.
(c) polymer.
 

Q. 4. Give examples which indicate that nylon fibres are very strong.
 Ans. Nylon fibres are strong so they are used for making parachutes and ropes for rock climbing.
 

Q. 5. Explain why plastic containers are favoured for storing food.
 Ans. Advantages of plastic containers:
(a) Plastics do not react with food items.
(b) Plastics are strong and light.
(c) They are easy to handle and safe.

Q. 6. Explain the differences between thermoplastics and thermosetting plastics.
 Ans. Differences:

Q. 7. Explain why the following are made of thermosetting plastics.
 (a) Saucepan handles
 (b) Electric plugs/switches/plug boards 
 Ans. (a) The handles of saucepan are made of thermosetting plastic because it is a bad conductor of heat and do not get heated up while cooking.
(b) Electric plugs/switches/plug boards are made up of thermosetting plastic, because it is a bad conductor of electricity. The electric current does not pass through such plastics.

Q. 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.
 Ans. Can be recycled: Toys, carry bags, plastic bowls, ball point pen, plastic chairs, electric wire covering. Cannot be recycled: cooker handles, electric switches, telephone instruments.
 

Q. 9. Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.
 Ans. Rana should buy cotton shirts for summer because cotton is a bad conductor of heat. It does not allow the transmission of heat from or to the body, thus protects body from heat. It has more capacity to hold moisture than the synthetic clothes. So, it retains the sweat of the body and keeps it cool. So Rana should buy cotton shirts.

Q . 10. Give examples to show that plastics are noncorrosive in nature.
 Ans. Plastics are noncorrosive in nature:
(i) They do not react with any substance.
(ii) Plastics do not react with air and water which are essential for corrosion.
(iii) They do not show any chemical reaction.

Q . 11. Should the handle and bristles of a toothbrush be made of the same materials? Explain your answer.
 Ans. No, handle and bristles of a toothbrush should not be made of the same material because the handle should be hard while bristles should be made of the soft material. Bristles should be soft so that it does not harm the gum of teeth. The handle gives the firm grip so it should be made of hard material.

Q . 12. ‘Avoid plastics as far as possible’. Comment on this advice.
 Ans. Avoid the use of plastics as far as possible. Plastics are non-biodegradable materials. So use of plastics is harmful for our environment. The plastics cannot be finally disposed off. Thus, plastics should be avoided as far as possible.

Q . 13. Match the terms of Column A correctly with the phrases given in Column B.

Ans.

Q . 14. ‘Manufacturing synthetic fibres is actually helping conservation of forests’. Comment.
 Ans. The natural fibres required the raw materials from plants and animals. So they lead cutting of trees and killing of animals. Synthetic fibres are made up of chemicals and these chemicals are not available in forests. So manufacturing synthetic fibres is actually helping conservation of forests.

Q . 15. Describe an activity to show that thermoplastic is a poor conductor of electricity. Ans. Observe the electrical wires. These wires have plastic covering which show that plastics are poor conductors. The handles of screw drivers are made of plastics. These observations show that thermoplastic is a poor conductor of electricity.