NCERT Solutions for Class 8th Science
Chapter 3. Synthetic Fibres And Plastics
Text Book 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.
Q4. Give examples which indicate that nylon fibres are very strong.
Answer: They are used to make Parachutes,and ropes for rockclimbing.
5. 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.
6. Explain the difference between the thermoplastic and thermosetting plastics.
(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.
(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.
7. 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.
8. 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).