Tuesday, 19 June 2012

CBSE Class 8 - Science - Synthetic Fibres and Plastics - NCERT Solutions #eduvictors

Synthetic Fibres and Plastics

NCERT Chapter Solutions and Q & A

Q1: Explain why some fibres are called synthetic?

Answer: Some fibres are called synthetic because they are not made from natural fibres. Instead, they are made by chemical processing of petrochemicals. Synthetic fibres are made up of very large units called polymers. Some of the synthetic fibres are rayon, nylon and acrylon.

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 ✓

SPI Coding System for Plastics
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Answer: (b) it is obtained from wood pulp

Q3: Fill in the blanks with appropriate words:

Answer:

(a) Synthetic fibres are also called man-made or artificial fibres.

(b) Synthetic fibres are synthesised from raw materials called petrochemicals.

Q4: Give examples which indicate that nylon fibres are very strong.

Answer: Nylon is a very strong fibre that is why nylon is used for making parachutes, rock climbing ropes, fishing nets seat belts etc. A nylon thread is actually stronger than a steel wire.

Q5: Explain why plastic containers are favoured for storing food.

Answer: Plastic containers are favoured for storing food for the following reasons:

Plastic containers are lightweight so they are easy to handle.
The price of plastic containers is very less as compared to other containers.
They have good strength.
Plastic containers are durable.

Q6.Explain the difference between thermoplastic and thermosetting plastics.

Answer:

	Thermoplastics	Thermosetting plastics
1	Plastics which can be deformed and bend easily on heating are known as thermoplastics.	Plastics that when moulded once cannot be bent and deformed on heating are known as thermosetting plastics
2	They are used for making toys, containers, etc	They are used for making switches, handles of utensils, floor tiles etc.
3	Example: PVC, Polythene	Bakelite and Melamine

Q7.Explain why the following are made of thermosetting plastics.

(a) Saucepan handles

(b) Electric plugs/switches/plug boards

Answer:

(a) Saucepan handles: Saucepan handles are made from thermosetting plastics because these are bad conductors of heat and electricity. They do not bend or deform on heating.

(b) Electric plugs/switches/plug boards: Electric plugs/switches/plug boards etc. are made up of thermosetting plastics because thermosetting plastics are bad conductors of heat and electricity. It does not allow the current to pass through it.

Q8: Categorise the materials of the following products into ‘can be recycled and ‘cannot be recycled’:

Telephone instruments, plastic toys, cooker handles, carry bags, ballpoint pens, plastic bowls, plastic covering on electrical wires, plastic chairs, and electrical switches.

Answer:

	Can be recycled	Cannot be recycled
1	Plastic toys carry bags ballpoint pens plastic chairs plastic covering on electrical wires, plastic bowls	cooker handles electrical switches telephone instruments

Q9: Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, give your reason.

Answer: Rana is advised to buy a cotton shirt for summers because cotton absorbs the sweat and the pores in cotton allow easy evaporation and keep our body cool, whereas synthetic shirts don’t absorb sweat and they are very uncomfortable to wear in summers.

Q10: Give examples to show that plastics are noncorrosive in nature.

Answer:

We can store various kinds of chemicals and other materials in plastic containers because they are non-reactive and do not react with water and air.
Teflon, a kind of plastic is used as a coating to manufacture non-stick cookware.
Melamine is used for making floor tiles and kitchenware.
Bakelite is a poor conductor of heat and electricity so it is used for making electric switches and handles of the utensils.

Q11: Should the handle and bristles of a toothbrush be made of the same material? Explain your answer.

Answer:

The bristles of a toothbrush should be made of soft material so that it does not harm the gums and the handle should be made of hard material so that it can give a firm grip. Therefore, the bristles are made up of nylon fibres which are strong, elastic, lustrous, lightweight and easy to handle and the handle is made of hard plastic as it is a strong, durable, and poor conductor of heat and electricity.

Q12: "Avoid plastics as far as possible". Comment on this advice.

Answer:

Although plastics have become important in our daily life, we should avoid them as far as possible.

Plastics are non-biodegradable and take a lot of time to decompose.
Plastics are not eco-friendly. By burning plastics, poisonous fumes are released that cause air pollution.
Sometimes cows and other animals may eat plastics present in the garbage which may choke their respiratory system and even cause death.
The plastic left in the soil results in soil pollution.
Plastic bags drained into the water bodies cause a threat to the aquatic life also.
Polybags choke the drains and sewage pipes thus causing a lot of trouble.

So we should use plastic judiciously and give preference to other alternatives like jute bags etc.

Q13: Match the terms of column I correctly with the phrases given in column II.

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

Answer: i-d, ii-c, iii-a, iv-b

Q14: Explain, ‘Manufacturing synthetic fibres are actually helping conservation of forests’.

Answer: 'Manufacturing synthetic fibres are actually helping conservation of forests’ because synthetic fibres are man-made fibres. These fibres are made from petrochemicals. For manufacturing synthetic fibres cutting trees or killing of animals is not required and hence we conserve our natural environment.

Q15: Describe an activity to show that thermoplastic is a poor conductor of electricity.

Answer: Take a small piece of thermoplastic material and connect it as shown in the figure When we close the switch, no current passes and the bulb does not glow.