NCERT CLASS 8 SCIENCE CH 3

 

         SYNTHETIC FIBER AND PLASTIC

1. Explain why some fibers are called synthetic.

ANS:

Some fibers are called synthetic fibers because they are man-made fibers prepared using chemicals. These are made of small units that join together to form long chains. Some examples of synthetic fibers are nylon, rayon, acrylic, polyester etc.

2. Mark the correct answer.

Rayon is different from synthetic fibers because

(a) it has a silk-like appearance.

(b) it is obtained from wood pulp.

(c) its fibers can also be woven like those of natural fibers.

ANS:

The answer is (b) it is obtained from wood pulp.

3. Fill in the blanks with appropriate words.

(a) Synthetic fibers are also called __________ or __________ fibers.

(b) Synthetic fibers are synthesized from the raw material called __________.

(c) Like synthetic fibers, plastic is also a __________

ANS:

(a) Synthetic fibers are also called artificial or man-made fibers.

(b) Synthetic fibers are synthesized from a raw material called petrochemicals.

(c) Like synthetic fibers, plastic is also a polymer.

4. Give examples which indicate that nylon fibers are very strong.

ANS:

Following are the examples that indicate nylon fibers are very strong:

• Parachutes and ropes of rock climbing are made of nylon
• Nylon is used in making seat belts, fishing nets and tire cords.
• Nylon is used in making sports accessories like rackets.

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

ANS:

Plastic containers are favored 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.

ANS:

The difference between thermosetting plastics and thermoplastics are as follows:

Thermosetting plastics	Thermoplastics
It cannot be bent; it will break if we attempt to bend thermosetting plastics	Thermoplastic can be bent easily
On heating thermosetting plastics, they cannot be softened. This is the reason it cannot be reshaped once it is molded.	On heating the thermoplastics, it becomes softened and can be molded and reshaped easily.

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

(a) Saucepan handles

(b) Electric plugs/switches/plug boards

ANS:

a) Thermosetting plastics are used to make saucepan handles because they are bad conductors of heat, and also, on heating, these plastics do not get softened.

b) Bakelite is a kind of thermosetting plastic which is a bad conductor of heat and electricity. Because of this property, it is used for making electric plugs, switches, plug boards, etc.

8. Categories 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.

ANS:

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.

ANS:

Rana should buy a cotton shirt for the summer and not a synthetic shirt. Cotton is a good absorber of water. So it can soak the sweat coming out of the body and expose it to the environment. Thus, it helps in evaporating sweat and helps in cooling the body.

10. Give examples to show that plastics are non-corrosive in nature.

Ans:
The literal meaning of non-corrosive is resistant to get destroyed by chemical action.
Following are the examples that show that plastics are non-corrosive in nature.

• Plastic containers do not react with items stored in it.
• They do not get rusted when exposed to moisture and air.
• They do not decompose when left in open for a long period.

Question 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 toothbrush should not be made of the same material. This is because our gums are soft and the bristles should be made of soft material so that it does not harm the gums. On the other hand, the handles should be made up of hard material so that it can give a firm grip.

Question 12.
‘Avoid plastics as far as possible’. Comment on this advice.
Ans:
Plastics must be avoided as far as possible. The materials made of plastics are non-biodegradable. The use of plastics has a bad effect on the environment. When the plastics are burnt, it releases a lot of poisonous fumes into the atmosphere causing air pollution. These plastic materials when eat up by the animals (like cows), choke their respiratory system. This can cause death of these animals. The waste plastic articles thrown here and there carelessly get into dirty water drains and sewers, and block them. In a nutshell, plastics can be considered a threat to our environment.

Question 13.
Match, the terms of column A correctly with the phrases given in column B.

A	B
(i) Polyester	Prepared by using wood pulp
(ii) Teflon	Used for making parachutes and stockings
(iii) Rayon	Used to make non-stick cookware
(iv) Nylon	Fabrics do not wrinkle easily

Ans:
(i) (d)
(ii) (c)
(iii) (a)
(iv) (b)

Question 14.
‘Manufacturing synthetic fibers is actually helping the conservation of forests’. Comment.
Ans:
In the manufacturing of synthetic fibers, we use only chemical substances and no natural materials, thus, in turn, we conserve forests.

Question 15.
Describe an activity to show that thermoplastic is a poor conductor of electricity.
Ans:
Arrange a circuit as shown in the given figure. Leave a gap between two ends of the wire. Place a the-Mo plastic in the gap. Observe the bulb.