3. Synthetic Fibres And Plastics Science class 8 in English Medium ncert book solutions Text Book Exercise

Exercise

Q1. Explain why some fibres are called synthetic?

Answer:

Since man-made fibers are obtained by the synthetic of petrochemicals, they are called synthetic fibres.

Q2. 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.

Answer:

 (b) it is obtained from wood pulp.

Q3. Fill in the blanks with appropriate words:

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

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

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

Answer:

(a) artificial, man-made.

(b) polymer.

(c) petrochemicals.

Q4. Give examples which indicate that nylon fibres are very strong. 

Answer:

They are used to make Parachutes,and ropes for rockclimbing. 

Q5. Explain why plastic containers are favoured for storing food.

Answer:

The three main advantages of using plastic containers for storing food are: 

(I) plastic has light weight, 

(II) good strength,and 

(III) are easy to handle. 

Q6. Explain the difference between the thermoplastic and thermosetting plastics.

Answer:  

Thermoplastics Plastic:

(I) These are the plastics which gets deformed easily on heating and can be bent easily.

Examples: Polythene and P.V.C. 

(II) These are used for manufacturing toys, combs, car grills and various types of containers.

Thermosetting plastics:

(I) These are the plastics which when moulded once,cannot be softened by heating.

Examples: Bakelite and melamine.

(ii) (a) Bakelites are used for making electrical switches,handles of various utensils,etc.

(b)Melamines are used for making floor tiles,kitchen-wares and fabrics,which resist fire.

Q7. Explain why the following are made of thermosetting plastics.

(a) Saucepan handles

(b) Electric plugs/switches/plug boards

Answer

:(a) As baklite (a kind of thermosetting plastic) is a bad conductor of heat, therefore, saucepan handles are made up of it.

(b) Bakelite (a kind of thermosetting plastics) is also a bad conductor of electricity, so these electricity appliances are made up of it.

Q8. Categorise the following  materials   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:

(i) Can be recycled: Telephone instruments, plastics toys, ball point pens, plastics bowls.(As these are thermoplastics, so these can be recycled).

(II) Cannot be recycled: Cooker handles,carry bags, plastic covering on electric wire, plastic chairs, etc. (As these are thermosetting plastic, these cannot be recycled). 

Q9. Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason?

Answer:

I would advise Rana to buy cotton shirts absorb the sweat and thus endorse cooling, Besides, they also provide aeration.

Q10. Give examples to show that plastics are noncorrosive in nature?

Answer:

The plastics are noncorrosive, that is, do not react or do not have chemical reaction with the materials contained in it. That is why many containers like bucket, mug, water bottles, food containers are made by using plastics. 

Q11. Should the handle and bristles of a tooth brush be made of the same material? Explain your answer?

Answer:

The handles and bristles of a toothbrush should be made of a material which has light weight, good strength and is hygine. But the bristles should be soft enough so that they do not harm gums while the handle should be quite stuff so that it may not get bent while brushing. 

Q12. ‘Avoid plastics as far as possible’. Comment on this advice?

Answer:

Since plastic takes several years to decompose, it is not decompose, it is not enviorment friendly. It causes enviormental pollution. Besides, its burning releases lots of poisoms fumes into the atmosphere causing air pollution. So, we should aviod the use of plastic as far as possible.

Q13. Match the terms of column A correctly with the phrases given in column B.

(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 njot wrinkle easily.

Answer:    

Q14. ‘Manufacturing synthetic fibres is actually helping conservation of forests’. Comment?

Answer: The synthetic  fibres are made up of petrohemicals. So, manufacturing synthetic fibres does not depend upon plants. These synthetic fiders cater the need of people upto great extent. Thus, the forests are not destroyed to manufacture clothes and other items. So, indirectly, we can come to a conclusion that manufacturing synthetic fiber is actually helping conservation of forests.

Q15. Describe an activity to show that thermoplastic is a poor conductor of electricity?

Answer: If we make an experimental set up using copper wire, a thermoplastic object, a bulb and a cell, as shown in the figure given below, the bulb does not glow.

however, if thermoplastic object is removed from the circuit, the bulb glows up. This shows that thermoplastic objects are bad conductor of electricity.