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Page No 43:

Question 1:

Which of the following is a polyester fibre?
(a) rayon
(b) terylene
(c) nylon
(d) acrylic

Answer:

(b) terylene
It is a polymer (synthetic fibre) made up of small chemical molecules called esters.

Page No 43:

Question 2:

Polycot, a blended fibre is made from
(a) polyester and nylon.
(b) polyester and wool.
(c) polyester and acrylic.
(d) polyester and cotton.

Answer:

(d) polyester and cotton
Polyesters are wrinkle-free and do not require ironing. They absorb less water and they dry easily. It is because of these advantages of polyester over cotton that polyester is blended with cotton to form polycot. 

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Question 3:

polythene, polystyrene, nylon, PVC, rayon, terene, acrylic, formica, polycot, melamine
The list of materials mentioned above can be classified as
(a) synthetic fibres and plastics.
(b) polyester fibres and objects made of glass.
(c) man-made materials and plastics.
(d) synthetic fibres and thermoplastics.

Answer:

(a) synthetic fibres and plastics
Nylon, rayon, acrylic, terene and polycot are synthetic fibres. Polythene, polystyrene, PVC, melamine and formica are plastics.



Page No 44:

Question 4:

Which of the following is a thermosetting plastic?
(a) melamine
(b) teflon
(c) polythene
(d) PVC

Answer:

(a) melamine 
Melamine is a thermosetting plastic.

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Question 5:

Which of the following statements is true for rayon?
(a) It resembles wool in appearance.
(b) It is obtained from wood containing large amount of cellulose.
(c) It is a naturally occurring fibre obtained from wood.
(d) All the statements are correct.

Answer:

(b) It is obtained from wood containing large amount of cellulose.
As wood contains huge amount of cellulose, rayon (synthetic fibre) is prepared chemically from wood pulp (cellulose). Rayon looks like silk.

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Question 6:

Anu bought a sweater which was not very expensive. At home, when she burnt a small piece of fibre from the sweater, it smelled like burning plastic. What is the sweater most likely made of?
(a) dacron
(b) acrylic
(c) cashmere
(d) polyester

Answer:

(b) acrylic
Acrylic is a synthetic fibre, which is light, soft and warm. It resembles wool but does not shrink or wrinkle. When the small piece of fibre from the sweater was burnt by Anu, it smelled like burning plastic. Thus, the sweater was not pure wool, but it was made of synthetic acrylic fibre.

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Question 7:

Which of the following correctly lists the raw materials needed to make glass?
(a) sand, sodium bicarbonate, limestone
(b) silicon, sodium bicarbonate, limestone
(c) silicon, sodium carbonate, calcium carbonate
(d) sand, sodium carbonate, calcium carbonate

Answer:

(d) sand, sodium carbonate, calcium carbonate

Sand (silicon dioxide (SiO2), sodium carbonate (Na2CO3) and calcium carbonate (CaCO3) are the three raw materials needed to make glass.

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Question 8:

One characteristic of thermosets is that ___________.
(a) they become soft on cooling
(b) they become hard after heating
(c) they can be repeatedly remoulded into desired shapes
(d) they become soft again and again on reheating

Answer:

(b) they become hard after heating

Thermosetting plastics become soft only the first time; when they are reheated, they become hard
(take firm shape).

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Question 9:

Plastics are used to cover electrical cables because _____________.
(a) they are light
(b) they can be moulded into any shape
(c) they do not conduct heat or electricity
(d) they can coloured by adding pigments

Answer:

(c) they do not conduct heat or electricity
Plastics are bad conductors of heat and electricity (good insulators). Hence, they are used to cover electrical cables.

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Question 10:

Name the following:
(a) the process by which glass is cooled and made tougher
(b) gas given off when PVC is burnt
(c) large molecules made by joining many small molecules together
(d) substances that are easy to mould and can regain their shape
(e) fabrics made by the combination of natural and synthetic fibres
(f) two fabrics made from petroleum products

Answer:

(a) Annealing is the process by which glass is cooled and made tougher.

(b) Hydrogen chloride gas is given off when PVC is burnt.

(c) Polymers are large molecules made by joining many small molecules.

(d) Thermoplastics are easy to mould and they can regain their shape.

(e) Blended fabrics are made by the combination of natural and synthetic fibres.

(f) Polyester fibres like dacron and terene are made from petroleum products.

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Question 11:

Differentiate between thermoplastics and thermosetting plastics.

Answer:

Thermoplastics          Thermosetting Plastics
They become soft on heating. They become soft only the first time; they become hard on reheating.
They can be moulded into different shapes again and again. They cannot be moulded more than once as they take a firm shape once they are heated and cooled.
Some examples are polythene, polyvinyl chloride (PVC), polystyrene, teflon, etc. Some examples are bakelite, melamine, formica, etc.

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Question 12:

Give scientific reasons for the following:
(a) Plastic containers used for microwave cooking can be made of thermosetting plastic.
(b) Thermoplastics are not used for making saucepan handles.
(c) Bakelite is used to make plugs and sockets.

Answer:

(a) Thermosetting plastics remain hard on reheating and can sustain high temperatures. As the containers used for microwave cooking should be able to sustain reheating without causing any damage to the food kept in them, these containers can be made of thermosetting plastics.

(b) Thermoplastics become soft on heating. As saucepans are used for cooking, their handles should be able to sustain large amount of heat without melting. Therefore, thermoplastics are not used for making saucepan handles.

(c) Thermosetting plastics remain hard even at high temperatures. They are also insulators of heat and electricity. As plugs and sockets may get heated up while they are being used, Bakelite, a thermosetting plastic, is used for making them.

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Question 13:

What problems can be caused by the overuse of plastics?

Answer:

The overuse of plastics leads to many problems like environmental pollution, health hazards, etc. Plastics are not biodegradable and take thousands of years to degenerate. This results in environmental pollution. When plastics are burnt, they release hydrogen chloride gas. This hydrogen chloride gas gets combined with the moisture in the environment to form hydrochloric acid, which is toxic and can cause problems to the human body. Recycling of plastics is very costly. Even though biodegradable plastics have been developed by scientists, they are costly and are not used on a large scale.

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Question 14:

Are these statements true or false? Rewrite the false ones correctly.
(a) Polymers are small molecules made by stringing together several monomers.
(b) Blended fabrics contain only synthetic fibres.
(c) Dusters made of nylon or other synthetic materials are good for use in the kitchen.
(d) Polyester fibres are obtained from petroleum products.
(e) Terylene, a synthetic fibre, can be used instead of wool.
(f) Rayon, which is obtained from cellulose, is a natural fibre.

Answer:

(a) False
The correct statement should be:
​Polymers are large molecules made by stringing together several monomers (small molecules).

(b) False
The correct statement should be:
Blended fabrics contain both synthetic and natural fibres. They are produced by the combination of synthetic fibres and natural fibres.

(c) False
The correct statement should be:
Dusters made of cotton (natural fibre) are good for use in kitchen. They absorb water and do not catch fire easily. Even if they catch fire, they do not stick to the body of the person who is holding it.

(d) True

(e) False
The correct statement should be:
Acrylic is a synthetic fibre that can be used instead of wool.

(f) False
The correct statement should be:
Rayon, which is obtained from cellulose, is a synthetic fibre. Rayon is prepared chemically from wood pulp (cellulose).



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Question 15:

State three properties of synthetic fabrics.

Answer:

Three properties of synthetic fabrics are as follows:
1. They absorb very less water.
2. They dry quickly and do not require ironing.
3. They are not infected by bacteria or fungus and are resistant to insects.

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Question 16:

Sort out these fabrics into different groups: rayon, nylon, jute, acrylic, terylene, wool, silk, polyester, terry cotton, cotton

Answer:

We can sort the given fabrics into natural, synthetic and blended fibres as shown in the table below:
 

Natural Fibre Synthetic Fibre Blended Fibre
Cotton Rayon Terry cotton
Jute Nylon  
Silk Acrylic  
Wool Polyester  
  Terylene  

Page No 45:

Question 17:

Find the odd one out in the following. giving reasons.
(a) bakelite, teflon, formica, melamine
(b) rayon, nylon, polycot, acrylic
(c) dacron, acrylic, terene, terylene
(d) cotton, jute, silk, rayon
(e) polyester, cotton, ester, nylon

Answer:

(a) The odd one is teflon. It is a thermoplastic, while bakelite, melamine and formica are thermosetting plastics.

(b) The odd one is polycot. It is a blended fabric, but rayon, nylon and acrylic are not.

(c) The odd one is acrylic. It is not a polyester synthetic fibre, while dacron, terene and terylene are polyester synthetic fibres that are manufactured from petroleum products.

(d) The odd one is rayon. It is a synthetic fibre, while cotton, jute and silk are natural fibres.

(e) The odd one is ester. It is a monomer (small molecule) unit, while polyester, cotton and nylon are polymers (larger molecules made up of smaller monomer units).

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Question 18:

Define the following:
polymer, blended fibre

Answer:

Polymers are large molecules formed by the linkage of smaller molecules called monomers. For example, cotton is a naturally occurring polymer that is made up of glucose as monomer units.

Blended fibres are fibres that are obtained by blending natural fibres with synthetic fibres. These fibres have mixed properties of both the fibres involved in its production. For example, polycot is obtained by blending cotton with polyester.



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