NCERT Solutions for Class 8th Science
Chapter 3. Synthetic Fibres And Plastics
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.
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.
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.
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).