NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics

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Updated at 2021-05-07


NCERT solutions for class 8 Science Chapter 3 Synthetic Fiber And Plastics Exercise

Q.1 Explain why some fibres are called synthetic.



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

The fibers that are made by humans through chemical synthesis are called as synthetic fibres.

They are made from raw materials such as petroleum based chemicals or petrochemicals.

They consist of polymers which has many repeating units known as monomers.

A monomer is a single unit that forms bond with other identical monomers which leads to the formation of a polymer. This process is called as polymerization.

Polymer is a large molecule which is formed when several units of monomers are linked together.

Some of the examples of synthetic fibres are nylon, rayon, acrylic, polyester etc.

Q.2 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.



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

(b) it is obtained from wood pulp.

EXPLANATION:-

Rayon is a cellulose fiber which is made up of a raw material called wood pulp.

It is a regenerated cellulose fiber that is made from natural sources of cellulose, such as wood and other agricultural products.

It has the same molecular structure as cellulose.

It is obtained from beech, pine, or bamboo tree.

Wood pulp is a lignocellulosic fibrous material prepared by chemically separating cellulose fibres from wood.

The wood pulp is converted into soluble compound which is chemically solidified to form synthetic fiber.

As rayon is manufactured from naturally occurring polymers, it is considered different from synthetic fibres.

Q.3 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 __________



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

(a) Synthetic fibres are also called artificial or manmade fibres.
(b) Synthetic fibres are synthesized from raw material called petrochemicals.
(c) Like synthetic fibres, plastic is also a polymer.

EXPLANATION:-

(a) Synthetic fibers are made from synthesized polymers of small molecules while natural fibers are made from naturally formed polymers.
The compounds that are used to make synthetic fibers come from raw materials such as petroleum-based chemicals or petrochemicals while for natural fibers, the raw materials are obtained from natural sources.
Hence synthetic fibers are called artificial or man-made fibers as their raw materials are artificially synthesized using various chemicals and their products.

(b) Petrochemicals are the chemical products obtained from petroleum or crude oil. Some chemical compounds that are made from petroleum, are also obtained from other fossil fuels, such as coal or natural gas.
Petroleum based chemicals are obtained by refining petroleum or crude oil by the use of advanced technology. These petroleum based chemicals are used to produce consumer products such as aspirin, detergents, shampoo, pesticides, gasoline, synthetic fibers, rubbers, insulating materials, paints, polyester clothes, automobiles, and many other things that we use in our daily life.

(c) All plastics are polymers, but not all polymers are plastic.
Plastic polymers includes polyethylene (PE), polypropylene (PP), epoxy and polyester (PS) which are derived from petroleum hydrocarbons or petroleum products.

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



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

Nylon fibres are very strong in the following ways:-

1) It is used in making ropes which are used for climbing rocks.

2) It is also used in making parachutes.

3) It is used as bristles in toothbrushes which provide flexibility and remains strong against water forces.

4) It is used to make fishnets which can remain strong and prevent the fish from getting released into the water.

5) Nylon wire is used in electrical appliances because it proves stronger than steel wire.

Q.5 explain why plastic containers are favored for storing food.



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

Plastic containers are favour for storing for food because of the following reasons:-

1) It is a non-reactive substance that do not react with the ingredients of the food items.

2) Plastic containers are light in weight, are less expensive, provide good strength and is easy for handling.

3) Plastic packaging allows food to travel long distances and ensures that large amounts of food do not get wasted. Different plastic have different functional properties such as being safe for food, flexible and chemical & heat resistant.

Q.6 Explain the difference between thermoplastic and thermosetting plastics.



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

THERMOSETTING PLASTIC:

(a) Thermosetting plastics are synthesized by condensation polymerization.
(b) Thermosetting plastics have high melting points and tensile strength.
(c) Thermosetting plastic has more molecular weight.
(d) Examples : Bakelite , vulcanized rubber etc.

THERMOPLASTIC:

(a) Thermoplastic can be synthesized by the process called addition polymerization.
(b) Thermoplastics have low melting points and low tensile strength.
(c) Thermoplastic has less molecular weight.
(d) Examples : Teflon , nylon etc.

Q.7 Explain why the following are made of thermosetting plastics.
(a) Saucepan handles
(b) Electric plugs/switches/plug boards



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

(a) Thermosetting plastics are used to make saucepan handles because they are bad conductors of heat. On receiving heat, thermoplastic gets melted and then it becomes hard for the person to handle the saucepan while, the thermosetting plastic do not melt and often breaks down without getting reformed upon cooling. So the thermosetting plastic does not harm the person holding it upon receiving heat.

(b) Bakelite is a kind of thermosetting plastic which is a bad conductor of heat and electricity. So it does not have free electrons to conduct electricity by allowing flow of current through it. Hence to protect the person from getting electric shocks or burns, bakelite is used for making electric plugs, switches, plug boards, etc.

Q.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.



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

Can be recycled:-
Plastic chairs, plastic bowls, plastic toys, plastic covering on electrical wires, carry bags, ball point pens

Cannot be recycled:-
Cooker handles, electrical switches, telephone instruments

EXPLANATION:-

Plastic chairs, plastic bowls, plastic toys, plastic covering on electrical wires, carry bags and ball point pens, are all categorized into thermoplastic which can be recycled even after molding. As we know that plastic is non-biodegradable so their disposal would cause plastic contamination of soil, water and air. Plastic disposal in water can clog the waterways and is thus harmful for aquatic life. Plastic disposal in soil can cause harm to soil organisms resulting in soil pollution and plastic burning contaminates the air constituents thus causing air pollution. Hence the only option to avoid plastic contamination is recycling of plastic materials.

Cooker handles, electrical switches, telephone instruments are all categorized into thermosetting plastic which cannot be recycled once molded into a particular shape. Cooker handles, electrical switches, telephone instruments are made up of bakelite (thermosetting plastic) which is a very hard and tough plastic.

Q.9 Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

Rana should buy cotton shirts rather than buying synthetic ones, for summer.

During summer season, the water cycle starts where the evaporation of water takes place in a continuous manner. As there is an increase in evaporation therefore more water molecules start accumulating in the air. As a result, when we starting perspiring / sweating , there remains no space for our sweat which is water molecule, to evaporate and hence it remains on our body surface.

Now if one wears cotton clothes then cotton being a good absorber, tends to absorb the sweat quickly and release it to the environment thus making us feel cool.

While synthetic clothes couldn't release the sweat quickly, so it keeps the body warm which becomes unbearable for summer days.

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



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

Corrosion takes place when the material loses electron and gets oxidized.

Corrosion is seen in metals as they tend to lose electrons to oxygen or water molecules present in the air.

While in case of plastic, they don't have free electrons to lose. So they do not react with air and water which are essential for corrosion.

As they tend to remain unreactive with strong chemicals, hence cleansing chemicals are stored in plastic bottles and not in metal containers.

Q.11 Should the handle and bristles of a tooth brush be made of the same material? Explain your answer



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

No, the handle and bristles of a tooth brush should be made up of different materials. The handle of a toothbrush should be hard and strong, while the bristles should be soft and flexible.

The bristles of a toothbrush are made up of nylon. Because nylon is a synthetic fiber which is soft for the teeth while strong and flexible for usage as it doesn't degrade in water or with the types of ingredients that are found in toothpaste.

The handle of a toothbrush is made up of plastic so that it can be injected into molds because it exhibits plasticity. The most common plastics used are polypropylene and polyethylene.

Q.12 'Avoid plastics as far as possible'. Comment on this advice.



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

1) Plastics are non-biodegradable so they take centuries to decay depending upon their composition. When the plastics are disposed indecently, it gets washed away into different water reservoirs. Thus it gets clogged in the waterways and float on reservoirs, hence polluting and making them undrinkable and unusable. Plastic contamination affects adversely to the environment thus harming humans, animals and plants.

2) Plastics have low melting point, so they can't be used as protective barrier for furnaces. Plastics like polystyrene, acrylics, polyethylene and nylons are highly flammable but as they are commonly used in packaging and in home and office appliances so this makes a way for a easy fire hazard.

3) Plastics have a short useful life-time as compared to other materials. This short life-time usage of plastics results in their piling up of unwanted garbage in the office, home or waste yards. Although some of the plastics are recycled but most of them remain uncollected in dump sites and pollute the environment.

So we should avoid plastics as far as possible.

Q.13 Match the terms of column A correctly with the phrases given in column B
A 1) polyester
2) Teflon
3)Rayon
4)Nylon
a) prepared by using wood pulp
b)Used for making parachutes and stockings
c) Used to make non-stick cookwares
d) Fabrics do not wrinkle easily



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

(i) Polyester------------(d) Fabrics do not wrinkle easily
(ii) Teflon-------------- (c) Used to make non-stick cookwares
(iii) Rayon------------- (a) Prepared by using wood pulp
(iv) Nylon-------------- (b) Used for making parachutes and stockings

EXPLANATION:-

(i) Polyester fabrics do not wrinkle easily as they are more durable and wrinkle-resistant because they don't absorb water efficiently and thus does not get wrinkled upon squeezing.

(ii) Teflon is used to make non-stick cookwares because of PTFE (polytetrafluoroethylene) coating, which exhibits non-stick property and this helps the cookwares from sticky fat and oil like substance.

(iii) Rayon is prepared by using wood pulp. Wood pulp is obtained from beech, pine, or bamboo tree. It is a lignocellulosic fibrous material prepared by chemically separating cellulose fibres from wood.

(iv) Nylon is used for making parachutes and stockings because it exhibits excellent wind resistance, has good elasticity, is mildew (a fungus) resistant, and is comparatively cheaper.

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



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

Natural fibres are synthesized from the raw materials that are obtained from nature.

These raw materials include cotton, kapok, and coir fibres originating as hairs borne on the seeds or in the inner walls of the fruit.

Other examples of raw materials are flax, hemp, bamboo, sisal, jute etc. All of these are either plant products or tree products.

So the heedless use of these plant based raw materials for manufacturing natural fibres leads to the deforestation of many forests, thus affecting our biological ecosystem adversely.

As the raw materials for synthetic fibres are petroleum based chemicals or petrochemicals so there will be neither deforestation nor the imbalance of biological ecosystem.

Hence manufacturing synthetic fibres is more preferred over natural fibres as manufacture of synthetic fibres are actually helping in the conservation of forests

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



NCERT Solutions for Class 8 Science Chapter 3 Synthetic Fiber And Plastics


Answer :

A circuit is made in both cases, thus highlighting the behaviour of thermoplastics as follows:

1st Case:-

A circuit is made using a bulb and a battery which are connected with wires. When the current is allowed to pass through the circuit, it is observed that the bulb glows which shows that all the materials have free electrons and thus can conduct electricity by allowing the current to pass through them.

2nd Case:-

A circuit is made using a bulb , a battery and a PVC pipe which are connected with wires. When the current is allowed to pass through the circuit, it is observed that the bulb does not glow which shows that PVC pipe cannot conduct electricity. Thus we can conclude that a thermoplastic (PVC pipe in this case) is a poor conductor of electricity.

Thermoplastic is a poor conductor of electricity because it doesn't have free electrons which can conduct electricity.



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