Goa Board Class 8 Solutions for Science – Synthetic Fibres and Plastics (English Medium)
Page No. 41:
Explain why some fibres are called synthetic.
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.
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.
Rayon is different from synthetic fibres because:
(b) It is obtained from wood pulp.
Fill in the blanks with appropriate words.
(a) Synthetic fibres are also called _______ or _______ fibres.
(b) Synthetic fibres are synthesised from raw materials called _______ .
(c) Like synthetic fibres, plastic is also a _______ .
(a) Artificial and man made.
Give examples which indicate that nylon fibres are very strong.
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.
Explain why plastic containers are favoured for storing food.
The characteristics that make plastics favourable for storing food items are:
- Light weight
- Lower price
- Good strength
- Easy handling
Concept insight: Plastics are man made substances. These are light weight, economical and have good tensile strength.
Explain the difference between thermoplastic and thermosetting plastics.
There are two types of plastics: Thermosetting plastics and Thermoplastics.
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.
Explain why the following are made of thermosetting plastics.
(a) Saucepan handles
(b) Electric plugs/switches/plug boards
(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.
Categorize 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.
Cannot be recycled
|Can be recycled|
Plastic covering on electrical wires
Ball point pens
Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.
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.
Give examples to show that plastics are non-corrosive in nature.
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:
- 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.
- 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.
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 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.
‘Avoid plastics as far as possible’. Comment on this advice.
We should avoid plastics because:
- Plastics are non-biodegradable. Once introduced into the environment, they take several years to decompose. Plastics cause 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 bags choke their respiratory system and can even prove fatal. Therefore, we should avoid plastics as far as possible.
Match the terms of column A correctly with the phrases given in column B.
|Polyester||(a)||Prepared by using wood pulp|
|Teflon||(b)||Used for making parachutes and stockings|
|Rayon||(c)||Used to make non-stick cookware|
|Nylon||(d)||Fabrics do not wrinkle easily|
|(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|
‘Manufacturing synthetic fibers is actually helping conservation of forests’. Comment.
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.
Describe an activity to show that thermoplastic is a poor conductor of electricity.
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.