NCERT solutions for class 8 Science Chapter 3 Synthetic Fiber And Plastics

Solution for Exercise

Q.1 Explain why some fibres are called synthetic.


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Answer :

Some fibres are called as synthetic fibers because these are manmade fibers prepared by using chemicals. Some of the examples of synthetic fibres are nylon, rayon, acrylic, polyester etc.

Q.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.


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Answer :

(b) it is obtained from wood pulp.

Q.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 __________


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Answer :

(a) Synthetic fibres are also called __artificial__ or ___manmade___fibres.
(b) Synthetic fibres are synthesised from raw material called __petrochemicals__.
(c) Like synthetic fibres, plastic is also a ___polymer___.

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


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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.

Q.5 explain why plastic containers are favored for storing food.


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Answer :

Plastic containers are favour for storing for food because it is non-reactive substance and will not react with the ingredients of the food items.

Q.6 Explain the difference between thermoplastic and thermosetting plastics.


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Answer :

Thermo setting plastic
(a) Cannot be bent, it will break if we attempt to bend thermo setting plastic.
(b) on heating it can not be softened
(c) it can not be reshaped
ThermoPlastic
(a) Thermoplastic can be bent easily.
(b) on heating it can be soften .
(c) it can be reshaped after heating.

Q.7 Explain why the following are made of thermosetting plastics.
(a) Saucepan handles
(b) Electric plugs/switches/plug boards


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Answer :

(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.

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.


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Answer :

Can be recycled;
Plastic chairs, plastic bowls, plastic toys, The plastic covering on electrical wires, carry bags, ball point pens
Can not be recycled
Cooker handles, electrical 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.


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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.

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


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Answer :

Plastics are not corroded even if they come in contact with strong chemicals. This is because of their non-reactive nature with most materials. For example, the cleansing chemicals that we use at home are stored in plastic bottles, instead of metal containers.

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


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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.

Q.12 'Avoid plastics as far as possible'. Comment on this advice.


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Answer :

We should avoid plastics as far as possible because plastics are non-biodegradable in nature. Once introduced into the environment they cause pollution. If burnt they release poisonous gases and the the plastic bags thrown in the garbage dumb are swallowed by animals choke their respiratory system and causes fatalities in animals.

Q.13 Match the terms of column A correctly with the phrases given in column B
A 1) polyester
2) Teflon
3)Rayon
4)Nylon
a) prepared by using wood pulp
b)Used for making parachutes and stockings
c) Used to make non-stick cookwares
d) Fabrics do not wrinkle easily


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Answer :

(i) Polyester (d) Fabrics do not wrinkle easily
(ii) Teflon (c) Used to make non-stick cookwares
(iii) Rayon (a) Prepared by using wood pulp
(iv) Nylon (b) Used for making parachutes and stockings

Q.14 ‘Manufacturing synthetic fibres is actually helping conservation of forests’. Comment.


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Answer :

The manufacturing of synthetic fibers is helpful in the conservation of forests because if we use natural fibers, the raw materials for them have to be derived from the plants, which require cutting off lots of trees. This proves to be helpful in the conservation of forests.

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


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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



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