Friday, 15 July 2011

CBSE Class VIII (8th) - Science Chapter 3. SYNTHETIC FIBRES AND PLASTICS | Lesson Exercises

CBSE Class VIII (8th) - Science Chapter 3. SYNTHETIC FIBRES AND PLASTICS

Question 1. Explain why some fibres are called synthetic.

Answer : While natural fibres are obtained from plants and animals, synthetic fibres on the other hand, are  obtained by human beings from the chemical processing of petrochemicals. Petrochemicals when processed, results in different type of synthesised products like Petrol, Kerosene, Grease, Bitumen and polymers. Synthetic fibres are made from Synthetic polymers.  That is why these are called synthetic or man-made fibres.

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

Question 3. Fill in the blanks with appropriate words:

Answer :
(a) Synthetic fibres are also called  artificial or  man-made 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.

Answer : Nylon fibres are strong, elastic and light in weight. It has a  lustrous and easy to wash.
Nylon fibres are used to make many articles such as socks, ropes, tents, toothbrushes, car seat belts, sleeping bags, curtains etc.Nylon is also used for making parachutes and ropes for rock climbing . If we compare strength of a nylon thread  and a steel wire of same thickness or diameter,  a nylon thread is stronger than steel wire as it can support more  weight  than a steel wire.

Question 5. Explain why plastic containers are favoured for storing food.

Answer : Plastic containers are mostly used  for storing a food item, water, milk, pickles, dry food along with others like medicines and chemicals . Due to light weight, lower price, good strength, durability and easy handling plastic containers are convenient for use. Metals like iron get rusted when left exposed to moisture
and air. But plastics do not react with water and air. They are not corroded easily. That is why they are used to store various kinds of material, including many chemicals.After use or breakage, later they can be sold as scrap and be recycled to make new products again.

Question 6. Explain the difference between the thermoplastic and thermosetting plastics.

Answer : Plastic is also a polymer like the synthetic fibre. All plastics do not have the same type of arrangement of units. In some it is linear, whereas in others it is cross-linked.

Thermoplastics: Plastic which gets deformed easily on heating and can be bent easily are known as thermoplastics. Thermoplastics do not undergo chemical change in their composition when heated and can be molded again and again. Polythene and PVC are some of the examples of thermoplastics. These are used for manufacturing toys, combs and various types of containers.

Thermosets: Plastics which can be moulded only once, and after solidification,  can not be softened by heating,  are called thermosetting plastics. Two examples are Bakelite and melamine. Bakelite is a poor conductor of heat and electricity. It is used for making electrical switches, handles of various utensils, etc. Melamine is a versatile material. It resists fire and can tolerate heat better than other plastics. It is used for making floor tiles, kitchenware and fabrics which resist fire.

Question 7. Explain why the following are made of thermosetting plastics.
(a) Saucepan handles
(b) Electric plugs/switches/plug boards

Answer :
(a) Saucepan handles :  Saucepan handles are made from a type of thermosetting plastics, called Bakelite. It is  is a poor conductor of heat and electricity. In cooking, while using Saucepan with a Bakelite handle protect us from heat burns and also if we are cooking on a electric stove then it also saves us from electric shocks.

(b) Electric plugs/switches/plug boards. They are also made from a type of thermosetting plastics, called Bakelite. As stated above, It is  is a poor conductor of heat and electricity. It resists fire and can tolerate heat better than other plastics. Hence it prevent possible fire in electrical installations due to over heating and also nsaves us from recieving electric shock while using them.

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.

Answer :
Can be recycledCannot be recycled

  1. plastic toys
  2. carry bags
  3. ball point pens
  4. plastic chairs

  1. Telephone instruments
  2. plastic bowls
  3. plastic covering on electrical wires
  4. electrical switches

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.

Answer : Rana should buy a cotton shirt made for summer. As synthetic material soak less water than the natural fabrics like cotton, this will result in discomfort to him as water from  body perspiration will not be soaked by the the shirt made of synthetic material. Where as shirt made from cotton will soak the perspiration very rapidly and transfer it to outside, for surface vaporization, this will give him cooling comfort.

Question 10. Give examples to show that plastics are noncorrosive in nature.

Answer : metals like iron get rusted when left exposed to moisture and air. But plastics do not react with water and air. They are not corroded easily. That is why they are used to store
various kinds of material, including many chemicals.

Question 11. Should the handle and bristles of a tooth brush be made of the same material? Explain your answer.

Answer : The handle and bristles of a tooth brush should not be made of the same material. As handle require strength and stiffness so it should be made from plastic. Where as bristles are soft and elastic so they should be made from nylon.

Question 12. ‘Avoid plastics as far as possible’. Comment on this advice.

Answer : Plastic is non-biodegradable product. Plastic takes several years to decompose, it is not environment friendly. It causes environmental pollution. Besides, 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.We should avoid the use of plastics as far as possible. We should make use of bags made of cotton or jute when you go for shopping. In our homes, offices and industries, the biodegradable and non-biodegradable wastes should be collected separately and disposed off separately.

Question 13. Match the terms of column I correctly with the phrases given in column II.

Column I Column II
(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


Answer :

Column I Column II
(i) Polyester (d) Fabrics do not wrinkle easily
(ii) Teflon (c) Used to make non-stick cookwares
(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.
Answer : Manufacturing synthetic fibres is actually helping conservation of forests as
 
Question 15. Describe an activity to show that thermoplastic is a poor conductor of electricity.

Answer :

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CBSE Class VIII (8th) - Science Chapter 3. SYNTHETIC FIBRES AND PLASTICS | Lesson Exercises

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