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Class 8th-Science: Chapter 3-Synthetic Fibers and Plastics Ncert

Q1.  Explain why some fibres are called synthetic.

Ans.  The synthetic fibres are made by human beings. That is why these are called synthetic or man-made fibres. Some examples of synthetic fibres are rayon, nylon, poiyester, acrylic, etc.

Q2.  Mark the correct answer.

      (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.

Q3.  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) petro-chemicals

         (c) polymer

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

Ans.  Nylon fibers are very strong, elastic and light that’s why they are used for making parachutes and ropes for rock climbing. It is scientifically tested that a nylon thread is stronger than a steel wire. 

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

Ans.  Plastic containers are favoured for storing food because of the following reasons:

1. They have light weight.

2. They are easy to handle.

3. They have good strengh.

4. They are generally cheaper than metals.

5. They are durable.

Q6.  Explain the difference between thermoplastic and thermosetting plastics.

Ans.  Thermoplastics

1. The plastic which gets deformed easily on heating and can be bent easily are known as thermoplastics.

2. For Example: Polythene, PVC

3. They are used for manufacturing toys, combs and various types of containers.

Thermosetting plastics

1. Those plastics which when moulded once, can not be softened by heating are called thermosetting plastics.

2. For Example: Bakelite and Melamine

3. Bakelite

(i) Bakelite is a poor conductor of heat and electricity.

(ii) It is used for making electrical switches, handles of various utensils and screw drivers etc.

Melamine

(i) Melamine is a versatile material. It resists fire and can tolerate heat better than other plastics.

(ii) It is used for making floor tiles, kitchenware and fabrics which resist fire.

Q7.  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.

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.

Ans.  Can be Recycled: Plastic toys, Carry bags, Bail point pens, Plastic bowls, Plastic covering on electrical wires, Plastic chairs.

         Cannot be Recycled: Telephone instruments, Cooker handles, 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.

Ans.  Rana should buy cotton shirts for summer. Because cotton absorbs the sweat coming out of the body and the pores in cotton allows easy evaporstion and keeps our body cool. Whereas synthetic shirts don’t absorb sweat, so they are very uncomfortable to wear in summers. 

Q10.  Give examples to show that plastics are noncorrosive in nature.

Ans.  Plastics are non-corrosive in nature. They do not react with water, air and the materials contained in it. That is why they are used to store various kinds of material, including many chemicals.

For example:

1. The cleaning chemicals that we use at home are stored in plastic bottles, instead of metal containers.

2. Many containers like bucket, mug, water bottles. Food containers are made by using plastics.

Q11.  Should the handle and bristles of a tooth brush be made of the same material? Explain your answer.

Ans.  The handle and bristle of a tooth brush should be made of different material because The handles of brushes should be made of hard material so that it can give a strong grip, but bristle of a tooth brush should be made of soft material so that is does not harm the gums. 

Q12.  ‘Avoid plastics as far as possible’. Comment on this advice.

Ans.  We should avoid plastics as far aas possible due to the following reasons:

1. Since plastic is non-biodegradable, because it takes several years to decompose, it is not environment friendly. It causes enviromental pollution. 

2. Besided, the burning process in the synthetic material is quite slow and it does not get completely burnt easily. In the process it releases lots of poisonous fumes into the atmosphere causing air pollution.

3. Sometimes cows and other animals may eat plastics present in the gaebage which may choke their respiratory system and even cause death.

4. Plastic bage drained in the water bodies cause threat to that aquatic life also.

5. The polybags carelessly thrown here and there are responsible for clogging the drains and sewage pipes thus causing lot of trouble.

Q13.  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(b) Used for making parachutes and stockings
(iii) Rayon(c) Used to make non-stick cookwares
(iv) Nylon(d) Fabrics do not wrinkle easily

Ans.  (i) (d), (ii) (c), (iii) (a), (iv) (b)

Q14.  /Manufacturing synthetic fibres is actually helping conservation of forests’. Comment.

Ans.  The synthetic fibres are man-made fibres. They are made up of petrochemicals. So. Manufacturing synthetic fibres does not depend upon plants or animals. These synthetic fibres fulfill the need of people upto great extent. Thus, the forests are not destroyed to manufacture clothes and other items. So, indirectly, we can come to a conclusion that manufacturing synthetic fibres is actually helping conservation of forests.

Q15.  Describe an activity to show to show that thermoplastic is a poor conductor of electricity. 

Ans.  If we make an experimental set up using copper wire, a thermoplastic object, a bulb and a battery, as shown in the figure, the bulb does not glow.

However, if the thermoplastic object is removed from the circuit, the bulb glows up. This shows that thermoplastics are bad conductor of electricity.   

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