NCERT Solutions for Class 8 Science Chapter 3 : Synthetic Fibres and Plastics

Q:

Give examples to show that plastics are noncorrosive in nature.

A:

Plastics are non-reactive and noncorrosive in nature. They are not corroded even by strong chemicals. That is why various kind of chemicals are stored in plastics bottles.

Q:

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 to make non-stick
(iii) Rayon (c) Prepared by using wood pulp
(iv) Nylon (d) Used for making parachutes and stockings

A:

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

Q:

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

A:

Handle and bristle of a tooth brush should not be made of the same material. This is because handle of brush is strong and hard, while bristles are soft and flexible.

Q:

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

A:

Rana should buy cotton shirts. This is because cotton is a good absorber of water. Therefore, it absorbs the sweat (mainly water having dissolved salts) and exposes it to the environment. With the help of atmospheric heat, water present in sweat is evaporated. During evaporation water is lost and this water takes away heat with it. Thus, it helps in evaporating the water (sweat), thereby cooling our body.

Q:

Explain why the following are made of thermosetting plastics.
  1. Saucepan handles
  2. Electric plugs/switches/plug boards

A:

  1. Saucepan handles are made of thermosetting plastics because these plastics do not soften on heating.
  2. Thermosetting plastics are poor conductors of heat and electricity. Therefore, they are used in making electric plugs, switches, plug boards, etc.

Q:

Explain why plastic containers are favoured for storing food.

A:

The plastic containers are favoured for storing food because plastic is non-reactive, corrosion resistant, cheap, durable, strong and light in weight.

Q:

Give examples which indicate that nylon fibres are very strong.

A:

Nylon fibres are strong, elastic and light. In fact they are stronger than steel wire. They are used in making ropes for rock climbing and parachutes. This usage indicates that nylon fibres are very strong.

Q:

Explain why some fibres are called synthetic.

A:

Man-made fibres are called synthetic fibres. These fibres are prepared from petrochemicals with the help of various processes. Examples of some synthetic fibres are nylon, rayon, polyester, acrylic, etc.

Q:

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

A:

Raw material used for making natural fibres is mainly derived from plants. It requires cutting of lots of trees leading to deforestation. On the other hand raw materials required for synthetic fibres are mainly obtained from petrochemicals. It means for manufacturing of synthetic fibres, cutting of trees is not needed. Hence, it can be said that ‘manufacturing synthetic fibres is actually helping in conservation of forest’.

Q:

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

A:

Plastics are non-biodegradable, i.e. they are not decomposed by micro-organisms. Once introduced into the environment they may take several years to decompose. Plastics when burnt produce poisonous gases. They are dangerous to animals like cows, which swallow these bags from garbage. These bags choke the respiratory system of the animals or form a lining in their stomach and may cause their death. Hence, we should avoid plastics as far as possible.

Q:

Fill in the blanks with appropriate words.
  1. Synthetic fibres are also called ______ or ______ fibres.
  2. Synthetic fibres are synthesised from raw materials called _________.
  3. Like synthetic fibres, plastic is also a ________.

A:

  1. artificial,  man-made
  2. petrochemicals
  3. polymer

Q:

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

A:

Rayon is different from synthetic fibres because

  1. it has a silk-like appearance.
  2. it is obtained from wood pulp. (✓)
  3. its fibres can also be woven like those of natural fibres.

Q:

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

A:

We can set two circuits to show that thermoplastic is a poor conductor of electricity. For this we need two bulbs, wires, two batteries, a piece of metal and a pipe made of thermoplastic.

Set up the circuits with metal and plastic pipe separately as shown below.

When you turn the switch on the current flows and the bulb glows in the first circuit (circuit with metal) while, bulb does not glow in the second circuit (circuit with plastic).

This activity confirms that thermoplastic is a poor conductor of electricity.

Q:

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.

A:

Can be recycled

Cannot be recycled

plastic toys

telephone instruments

carry bags

cooker handles

ball point pens

electrical switches

plastic bowls

 

plastic covering on electrical wires

 

plastic chairs

 

Q:

Explain the difference between thermoplastic and thermosetting plastics.

A:

Thermoplastics

Thermosetting Plastics

Thermoplastics can be softened and reshaped when heated, but harden again when cooled.

Thermosetting plastics do not soften on heating. Hence, they can be moulded only once.

Thermoplastics can be bent easily.

Thermosetting plastics cannot be bent easily and may break when forced to bend.

Polythene and PVC are some examples of thermoplastics.

Bakelite and melamine are some examples of thermosetting plastics.

Thermoplastics are used for manufacturing toys, combs and various types of containers, etc.

Thermosetting plastics are used for making electrical switches, handles of various utensils, etc.

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