NCERT Solutions for Class 8 Science Synthetic Fibres and Plastics [Old Book]

NCERT
SOLUTIONS
CLASS - 8TH

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Class : 8th
Subject : Science
Chapter : 3
Chapter Name : Synthetic Fibres And Plastics

Q1 Explain why some bres are called synthetic.

Answer. There are some bres that are prepared by man by using chemicals. These are called
synthetic bres. These are made of small units that join together to form long chains. Examples of
synthetic bres are rayon, nylon, polyester, acrylic, etc.

Q2 Mark (✓ ) the correct answer.
Rayon is different from synthetic bres because
(a) it has a silk-like appearance.
(b) it is obtained from wood pulp.
(c) its bres can also be woven like those of natural bres.

Answer. Rayon is different from synthetic bres because
(a) it has a silk-like appearance.
(b) it is obtained from wood pulp. (tick)
(c) its bres can also be woven like those of natural bres.

Q3 Fill in the blanks with appropriate words.
(a) Synthetic bres are also called __________ or __________ bres.
(b) Synthetic bres are synthesised from raw material called __________ .
(c) Like synthetic bres, plastic is also a __________

Answer. (a) Synthetic bres are also called arti cial or man-made bres.
(b) Synthetic bres are synthesised from raw materials called petrochemicals
(c) Like synthetic bres, plastic is also a polymer

Q4 Give examples which indicate that nylon bres are very strong.

Answer. Nylon bres are very strong. It is used for making ropes used for climbing rocks and for
making parachutes. Their usage shows that nylon bres have high tensile strength.

Q5 Explain why plastic containers are favoured for storing food..

Answer. The characteristics that make plastics favourable for storing food items are:
(i) Lightweight
(ii) Lower price
(iii) Good strength
(iv) Easy handling

Q6 Explain the difference between thermoplastic and thermosetting plastics

Q7 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.

Q8 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.

Q9 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.

Q10 Give examples to show that plastics are non corrosive in nature

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.

Q11 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 exible.

Q12 ‘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. Plastic bags thrown in the garbage dump are swallowed by
animals like cows. These plastic bags choke their respiratory system and can even prove fatal.
Therefore, we should avoid plastics as far as possible.

Q13 Match the terms of column A correctly with the phrases given in column B

Page : 42 , Block Name : Exercise

Q14 ‘Manufacturing synthetic bres is actually helping conservation of forests’. Comment.

Answer. Raw materials for natural bres 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 bres helps in the conservation of forests.

Page : 42 , Block Name : Exercise

Q15 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 rst with
the metal and then with the plastic pipe (as shown in the gure). 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.

Page : 42 , Block Name : Exercise