Monday, 13 June 2016

TEXTBOOK ANSWERS AND SOLUTIONS OF CBSE CLASS VIII SCIENCE Chapter-3 SYNTHETIC FIBRES AND PLASTICS


Exercise

1) Explain why some fibres are called synthetic.

ANSWER:-There are some fibres that are prepared by man by using chemicals. These are called synthetic fibres.

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)

3) Fill in the blanks with appropriate words.

(a) Synthetic fibres are also called __artificial__ or __man-made__ fibres.
(b) Synthetic fibres are synthesised from raw materials 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 of its light weight, lower price and good strength.

6) Explain the difference between thermoplastic and thermosetting plastics.

ANSWER:-
Thermosetting plastic-:
Thermosetting plastic cannot be bent easily. It may break when forced to bend.
Thermosetting plastic cannot be softened by heating. Thus, it cannot be reshaped once moulded.

Thermoplastic:-
Thermoplastic can be bent easily.
Thermoplastic can be softened easily by heating. Thus, it can be reshaped.

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) Categorize 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:-Cannot be recycled:-Telephone instruments, Cooker handles, Electrical switches.

Can be recycled:-Plastic toys, Plastic chairs, carry bags, Plastic covering on electrical wires, Ball point pens, plastic bowls.

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:- Cleansing chemicals that we use at home are stored in plastic bottles, instead of metal container.

11) Should the handle and bristles of a tooth brush 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 are non-biodegradable. Once introduced into the environment, they take several years to decompose. Plastics add to the environmental pollution. They cannot be burnt as when burnt, they release poisonous gases. Therefore plastics should be avoided.

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 stockings               b) Used for making parachutes and
(iii) Rayon                              c) Used to make non-stick cookware
(iv) Nylon                               d) Fabrics do not wrinkle easily

ANSWER:-
i)----------------d)
ii)----------------c)
iii)----------------a)
iv)-----------------b)

14) ‘Manufacturing 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.




Written By Badal Paul
The author is a passionate blogger, writer and educationalist. This blog is created to ease troubles of the Cbse students. The author holds multiple blogs and the writer of the Novel KALCHAKRA, OOM AND THE CHOSEN FIVE.