NCERT Solutions for Class 8th Science

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).