chemistry

NCERT CLASS 8^THSYNTHETIC FIBRES AND PLATSTICS

Question: 1. Explain why some fibres are called synthetic.

Answer: Some fibres are made in factories and hence they are called synthetic.

Question: 2. 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

Answer: (b) It is obtained from wood pulp

Question: 3. Fill in the blanks with appropriate words:

(a) Synthetic fibres are also called ______ or _______ fibres.

Answer: man-made or artificial

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

Answer: petrochemicals

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

Answer: polymer

Question: 4. Give examples which indicate that nylon fibres are very strong.

Answer: Nylon is used in making ropes which are used for heavy-duty purposes; like mountaineering and in parachute. These examples show that nylon fibres are very strong.

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

Answer: Plastic is a non-reactive and non-corroding material. So, plastic containers do not change the flavour or quality of food. Due to this, plastic containers are favoured for storing food.

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

Answer: The shape of thermoplastic can be easily changed by heating again, while the shape of thermosetting plastic cannot be altered once it has been mould into a particular shape. Thermoplastic is used in making toys, combs, containers, etc. Thermosetting plastic is used for making those items which need to withstand high temperature; such as handles of cooking utensils.

Question: 7. Explain why the following are made of thermosetting plastics.

(a) Saucepan handles

Answer: Saucepan handles have to withstand high temperature during cooking and thermosetting plastic is ideal for that.

(b) Electric plugs/switches/plug boards

Answer: Electric plugs/switches/plug-boards need to be durable and should withstand continuous use over many years. Moreover, in case of some heating due electric current; they should not melt. Hence, these devices are made from thermosetting plastic.

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 recycled: Plastic toys, carry bags, ball point pens, plastic bowls, , plastic covering on electrical wires, plastic chairs
• Cannot be recycled: Telephone instruments, cooker handles, 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: Cotton has better absorbent property than synthetic fibre. During summer, cotton cloth can absorb sweat. The sweat would give cooling effect to the body, due to evaporation. Thus a cotton shirt would be much more comfortable than a shirt made from synthetic fibre. Hence, Rana should buy cotton shirts for summer.

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

Answer: Pickles usually come packed in plastic sachets or glass bottles. Pickles contain acids, yet no harm is done to the plastic sachet. Sanitary cleaners and some acids; which are used as bathroom cleaner are sold in plastic bottles. These examples show non-corrosive nature of plastic.

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 be made of different materials. The material which is used for making toothbrush handle should be hard and flexible; because the handle has to withstand lot of stress and has to reach every corner of mouth. The material which is used for making bristles should be soft, flexible yet durable; because the bristles should not damage the tooth or gum but should last longer.

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

Answer: Plastic has gained popularity because of its versatile nature. But because of its unmatched popularity; in terms of variety of usage; it has turned into a curse as well. We are creating too much of plastic waste around us. Plastic is non-biodegradable and many forms of plastic cannot be recycled. This is creating a burden on the environment around us. Hence, the statement, “Avoid plastics as far as possible” is a correct statement.

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

Question: 14. ‘Manufacturing synthetic fibres is actually helping conservation of forests’. Comment.

Answer: Forests provide many important raw materials from which we make various useful items. Discovery of synthetic fibres has resulted in many things; which were earlier made from wood; are now being made from plastic. For example; plastic chairs have replaced wooden chairs in a big way. Similarly, wooden cartons and crates have been replaced by plastic cartons and crates. The way plastic has replaced wood from many items, it is definitely minimizing the need to cut forests. Thus, it can be said that manufacturing synthetic fibres is actually helping conservation of forests.

Question: 15. Describe an activity to show that thermoplastic is a poor conductor of electricity.

Answer: For this, take a discarded plastic toy, a torch which is in working order. Switch the torch on to check that it is in working condition. From the plastic toy, cut a small piece about the size of a coin. Now insert this coin between two cells of the torch. Then tighten the lid of the torch. Now, switch on the torch. You will notice that the bulb does not glow. This happens because the plastic coin; between two cells; does not allow current to pass between two cells. This shows that thermoplastic is a poor conductor of electricity.