Class 8 Science Chapter 3 Notes

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Class 8 Science Chapter 3 Notes

Chapter Name: - Synthetic Fibres And Plastics

Class 8 Science Chapter 3 Notes

Section - A (1 Marks Questions)

Explain the following Keywords

A type of synthetic fibre that appears to resemble wool. They are available in a variety of colours.

A fibre obtained by chemical treatment of wood pulp and having property similar to that of silk is called rayon or artificial silk.

Nylon is another man-made fibre that is strong, elastic, light, and easy to wash.

Plastic is also a polymer like synthetic fibre. Polythene (Poly+ethene) is an example of plastic. It is used for making commonly used polythene bags and many other household items.

An easy to wash, wrinkle-free synthetic fibre. Terylene is a popular polyester.

A large single unit, made by combining many small units of synthetic fibres is called a polymer. The word polymer comes from two Greek words: poly meaning many and mer meaning part/unit.

Polythene (Poly+ethene) is an example of plastic. It is used for carrying goods and items.

A fibre obtained by chemical treatment of wood pulp and having property similar to that of silk is called rayon or artificial silk.

 It is mixed with cotton to make bedsheets and mixed with wool to make carpets.

The artificial fibres that are made by human beings.

Synthetic fibre is also a chain of small units joined together. Each small unit is actually a chemical substance.

A popular polyester that can be drawn into very fine fibres. Also, it can be woven like any other yarn.

Such plastic which gets deformed easily on heating and can be bent easily are known as thermoplastic.

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

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Class 8 Science Chapter 3 Notes

Section - B (Text Questions - Answer)

Question 1

Explain why some fibres are called synthetic.

Some fibres are called synthetic because they are artificially prepared by a number of processes using petrochemicals.

Types of synthetic fibres are:

Rayon

Nylon

Polyester and acrylic

Question 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.

Option: (b) it is obtained from wood pulp.

Question 3

Fill in the blanks.

(a) Synthetic fibres are also called man-made or synthetic fibres.

(b) Synthetic fibres are synthesised from raw material called petrochemicals.

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

Question 4

Give examples which indicate that nylon fibres are very strong.

Nylon fibres are stronger than steel wire of the same length and thickness. An example of this fact is the activity described below:

1. Take an iron stand with a clamp. Take a cotton thread and tie it to the clamp so that it hangs freely from it.

2. At the free end suspend a pan so that weight can be placed on it.

3. Add weight one by one till the thread breaks. Note down the total weight required to break the thread.

4. Repeat the same activity with threads of wool, polyester, silk, and nylon.

5. We find the weight required to break the nylon thread is maximum.

This is why the nylon is used for making parachutes and ropes for rope climbing.

Question 5

Explain why plastic containers are favoured for storing food.

The plastic containers are favoured for storing food as they seem most convenient. It is because of their light weight, lower price, good strength, and easy handling.

Question 6

Explain the difference between thermoplastic and thermosetting plastics.

The difference between thermoplastic and thermosetting plastics.

Thermoplastic

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

2. Polythene and PVC are examples of thermoplastics.

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

Thermosetting plastics

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

2. Bakelite is used for making electrical switches, handles of various utensils.

3. Melamine is used for making floor tiles, kitchenware, and fabrics that resist fire.

Question 7

Explain why the following are made of thermosetting plastics.
(a) Saucepan handles

Thermosetting plastic is a poor conductor of heat. This means it does not allow the heat to pass through it. The saucepan handles are made from this plastic to prevent heat to pass from the pan to the hands of the users.

Also, it resists fire and can tolerate heat better than other plastics.

(b) Electric plugs/switches/plug boards

Thermosetting plastic is a poor conductor of heat and electricity. Also, it resists fire and can tolerate heat better than other plastics.

This is why it is used for making electric plugs/ switches/ plug boards.

Question 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.

The materials that are made from thermoplastic can be recycled and the materials that are made from thermosetting plastics can not be recycled. Therefore,

Materials that can be recycled are: – 

plastic toys, carry bags, ball point pens, plastic covering on electrical wires, plastic chairs, 

Materials that cannot be recycled are: –

Telephone instruments, cooker handles, plastic bowls, electrical switches.

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Class 8 Science Chapter 3 Notes

Question 9

Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.

Rana is advised to buy the cotton shirts for the summer. The cotton absorbs the body sweat and pores in the cotton allow the evaporation to take place. Also, cotton clothes make us feel better in the summer by allowing proper air circulation.

Whereas the synthetic fibres soak less water and restrict the easy circulation of air through their very tiny pores.

Question 10

Give examples to show that plastics are noncorrosive in nature.

Plastics are non-corrosive in nature as they do not corrode easily. It is because they do not react with water, air, and various kinds of materials, including chemicals. They are used for packing tablets and store many strong chemicals. 

Question 11

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

No, the handle and bristles of a tooth brush should not be made of the same material. The bristles of the tooth brush should be made of soft materials so that it does not harm the teeth and gums.

On the other hand, the handles of the tooth brush should be made of hard materials to give a good and firm grip.

Question 12

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

Plastic is a useful substance but like all other substances, it has its own pros and cons (advantages and disadvantages).

It is a  non-biodegradable substance. It can not be decomposed through natural processes, such as actions of bacteria. Plastics should be avoided as far as possible as it takes several years to decompose. It is not environmental-friendly also. It is very harmful to the environment as it releases poisonous fumes on burning.

By using the 5R principle (Reduce, reuse, recycle, recover, and refuse) we can develop an environment-friendly habit.

Question 13

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

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

Question 14

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

The synthetic fibres are used for making a variety of cloth materials, home used items, kitchenwares, electrical appliances, the interior of vehicles, and many more. To produce this much amount of natural fibre would have put an extra burden on the natural substances.

Whereas, The synthetic fibres are artificially made with the chain of chemical processes on petrochemicals. No cutting of forests and the killing of animals are involved in the manufacturing of synthetic fibres. Therefore, we can say that manufacturing synthetic fibres is actually helping the conservation of forests.

Question 15

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

1. Make an electrical circuit with the help of a battery, connecting wires, and a bulb. Make sure that the connections are correct and the bulb is glowing.

2. Cut one of the wires and connect any conducting material (conductor) in between. We observe that on connecting the conductor the bulb still glows.

3. In place of the conductor, connect a piece of a thermoplastic in the circuit.

4. We observe that the bulb does not glow anymore.

This activity indicates that thermoplastics are poor conductors of electricity.

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Class 8 Science Chapter 3 Notes

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