Chapter 3 : Synthetic Fibres and Plastics - Ncert Solutions for Class 8 Science CBSE

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Chapter 3 - Synthetic Fibres and Plastics Excercise 41

Solution 1

fibres which are prepared by man using chemicals are called synthetic fibres. These are made by joining small units called monomers into long chains. Examples of synthetic fibres are rayon, nylon, polyester, acrylic, etc.

Concept insight: Synthetic fibres are prepared from chemicals.

Solution 2

Rayon is different from synthetic fibres because:

 (b) It is obtained from wood pulp.

Solution 3

(a) Artificial and man made.

(b) Petrochemicals.

(c) Polymer.

Solution 4

Nylon fibres are very strong. They are used for making ropes which are used for climbing rocks and for making parachutes. Their usage shows that nylon fibres have high tensile strength.

Concept insight: Nylon has very high tensile strength.

Solution 5

The characteristics that make plastics favourable for storing food items are:

(i) Light weight

(ii) Lower price

(iii) Good strength

(iv) Easy handling

Concept insight: Plastics are man made substances. These are light weight, economical and have good tensile strength.

Solution 6

There are two types of plastics: Thermosetting plastics and Thermoplastics.

Thermosetting plastic

Thermoplastic

Thermosetting plastic cannot be bent easily. It may break when forced to bend.

Thermoplastic can be bent easily.

Thermosetting plastic cannot be softened by heating. Thus, it cannot be reshaped once moulded.

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

 

Solution 7

(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.

Concept insight: Thermosetting plastics once shaped cannot be remolded again and again. Thus they do not change there shape on heating again.

Solution 8

Cannot be recycled

Can be recycled

Telephone instruments

Plastic toys

Cooker handles

Plastic chairs

Electrical switches

Carry bags

 

Plastic covering on electrical wires

 

Ball point pens

 

Plastic bowls

Solution 9

Rana should buy shirts made from cotton. This is because cotton is a good absorber of water. It absorbs the sweat coming out of our body and exposes it to the environment thereby cooling our body.

Concept insight: Synthetic fibres do not absorb water and also do not let air to pass through.

Solution 10

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:

1. Plastics don't react with acids or bases which are kept in them. The cleansing chemicals that we use at home are stored in plastic bottles, instead of metal containers.

2. Plastics don't react with air and water. For example: chairs, buckets all are made from plastics.

Concept insight: Plastics are non reactive in nature so they do not corrode.

Solution 11

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 so, it should be made from plastic. While the bristles should be soft and flexible so, they should be made from nylon.

Solution 12

We should avoid plastics because:

  1. Plastics are non-biodegradable. Once introduced into the environment, they take several years to decompose. Plastics cause environmental pollution.
  2. 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 bags choke their respiratory system and can even prove fatal. Therefore, we should avoid plastics as far as possible.

Chapter 3 - Synthetic Fibres and Plastics Excercise 42

Solution 1

A

B

(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

Solution 2

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.

Concept insight: Raw material for manufacturing of synthetic fibers is obtained by distillation of crude oil.

Solution 3

We will design a circuit to see that thermoplastics are poor conductors of electricity.

Materials needed for activity: bulb, some wires, a battery, a piece of metal, and a plastic pipe.

Procedure: 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.

Concept insight: As plastics do not have any free electrons so, they do not conduct electricity.

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