CBSE Class 8 Science Advanced Chemistry Synthetic Clothing Material and Polymer Long Answer Questions Set 01

Long Answer Type

Question 1. What is viscose process ?
Answer:
The viscose process is the method used in Europe to make rayon (viscose rayon) from wood pulp.

Steps:
1. Wood is changed into wood pulp, which is cellulose.
2. Cellulose is treated with sodium hydroxide to give alkali cellulose.
3. Alkali cellulose is treated with carbon disulphide, giving an orange-coloured suspension that looks like small beads.
4. Sodium hydroxide is added to this orange suspension to form a thick, sticky viscose solution.
5. The viscose solution is pushed through the fine holes of a spinneret (like a shower head) into a bath of sulphuric acid.
6. In the acid, the solution hardens into cellulose filaments, which are spun into rayon yarn on a winding spool.
In simple words: Wood pulp is turned into a sticky viscose solution, which is forced into acid to harden as rayon thread.

Teacher's Note:
1. Write the steps in order.
2. Name the spinneret and the sulphuric acid bath.

Question 2. What are the different steps involved in the process of fabric formation from petroleum products ?
Answer:

1. Polymer solution: the polymer made from petroleum chemicals is dissolved or melted to form a thick liquid, which is stored in a tank.
2. Pumping and filtering: a pump pushes the liquid through a filter that removes impurities.
3. Coagulating: the liquid is forced through the holes of a spinneret into the spin bath, where it hardens into fine fibres.
4. Stretching: the fibres are stretched so that they become strong.
5. Washing: the fibres pass through a wash tank and are washed.
6. Applying fibre finish: a finishing liquid is sprayed on them.
7. Drying: the fibres pass over heated drum rolls and dry.
8. Take-up: the fibre is wound on a take-up package, or cut into short pieces (staple), and then spun into yarn and woven or knitted into fabric.
In simple words: The polymer liquid is spun into fibres, stretched, washed, finished, dried and wound up; then yarn and fabric are made.

Teacher's Note:
1. Follow the five headings of the figure: coagulating, stretching, washing, applying finish, drying.
2. The last step turns fibre into yarn and fabric.

Question 3. Give two clothing-related applications of each of the following : (i) Nylon (ii) Polyester (iii) Acrylic.
Answer:
(i) Nylon: sarees, socks and stockings. It is also blended with wool to increase strength.
(ii) Polyester: sarees and dress materials; Terrywool (with wool) for suits and Terrycot (with cotton) for skirts and shirts.
(iii) Acrylic: sweaters, socks and shawls (and blankets).
In simple words: Nylon: sarees and socks; polyester: dresses and suits; acrylic: sweaters and shawls.

Teacher's Note:
1. Two uses are enough for each fibre.
2. Acrylic is the wool substitute.

Question 4. Discuss the limitations associated with synthetic fibre.
Answer:
1. Most synthetic fibres absorb very little moisture, so they become sticky when we sweat. Sweat is trapped between the fibre and the skin, whereas in natural fibres it evaporates and the skin can breathe.
2. Most synthetic fibres melt very easily, so they should not be worn in the kitchen or the laboratory. If the cloth catches fire, the molten fabric sticks to the skin.
3. They need very careful ironing, since a hot iron melts them.
4. They are not as comfortable to wear as natural fibres.
5. Synthetic fibres are non-biodegradable, so their waste adds to pollution.
In simple words: Synthetic fibres do not absorb sweat, melt easily and do not rot.

Teacher's Note:
1. Number the points; four points are enough.
2. The best fibres are blends that combine the advantages of both types.

Question 5. List two uses of each of the following fibre in applications other than clothing : (i) Nylon (ii) Polyester (iii) Acrylic.
Answer:
(i) Nylon: tents, umbrellas, parachutes and tarpaulins; tooth brush bristles; fishing nets, climbing ropes, seat belts and sleeping bags; strings of badminton and tennis racquets.
(ii) Polyester: light sails of boats; conveyor belts; curtains; polyester film (mylar) for magnetic recording tapes.
(iii) Acrylic: carpets and blankets; car tops, boat covers and filter material.
In simple words: Nylon makes ropes and parachutes, polyester makes sails and tapes, acrylic makes carpets and covers.

Teacher's Note:
1. Choose any two from each list.
2. Do not repeat clothing uses.

Question 6. Explain the 4R principal
Answer:
The 4R principle is a set of four habits to reduce waste, especially plastic waste.
1. Reduce: use fewer things, for example carry cotton or jute bags instead of plastic bags.
2. Reuse: use the same article again, for example refill a bottle or use a container again.
3. Recycle: send waste to be processed into new articles, for example polyester bottles melted and spun into fibres.
4. Recover: take back useful material or energy from waste.
A responsible citizen should follow these four steps in daily life to protect the environment.
In simple words: Reduce, reuse, recycle and recover are four steps to cut down waste.

Teacher's Note:
1. Give one example for each R.
2. The principle applies most to plastic.

Question 7. Explain the statement 'Plastics are non-corrosive'.
Answer:
Corrosion is the slow eating away of a metal by air, water or chemicals, as in the rusting of iron.
Plastics do not react with air, water, acids or alkalis, so they do not rust or corrode.
For this reason plastic pipes, tanks and buckets last long, and plastic bottles and containers can store water and a variety of chemicals safely. Natural fibres such as cotton and wool, in contrast, are affected by acids and alkalis.
In simple words: Plastics do not react with air and water, so they do not rust.

Teacher's Note:
1. Define corrosion first.
2. Give storage of chemicals as the use.

Question 8. Give reasons why : (a) refrigerators and freezers have a plastic foam core ? (b) electrical wires have a plastic covering ? (c) cooking pans have plastic handles ?
Answer:
(a) Plastic is a poor conductor of heat. The plastic foam core stops heat from outside from entering, so the inside stays cold.
(b) Plastic is a poor conductor of electricity. It insulates the wire, so we do not get an electric shock and the current does not leak.
(c) Plastic such as Bakelite is a poor conductor of heat and does not melt, so the handle stays cool and can be held safely.
In simple words: Plastics block heat and electricity, so they are used for insulation.

Teacher's Note:
1. Use 'poor conductor' in every part.
2. Part (c) needs the word thermosetting.

Teacher's Note:
1. Learn the table of monomers, plastics and uses.
2. PS: thermocol; PVC: pipes.

Question 10. What is plastic and explain types of plastics ?
Answer:
A plastic is a polymeric material that can be easily moulded or set into any desired shape and size.

(a) On the basis of structure:
1. Linear plastics: monomers join in long straight chains that pack closely, for example HDPE and PVC.
2. Branched chain plastics: side branches of different length grow from the main chain, for example LDPE.
3. Cross-linked plastics: the chains are joined to each other in a three-dimensional network, so they are hard and rigid, for example Bakelite and melamine-formaldehyde.
(b) On the basis of reaction to heat:
1. Thermoplastics: soften on heating and harden on cooling, and can be re-moulded again and again, for example polythene, PVC and polystyrene.
2. Thermosetting plastics: harder and stronger, and once set they cannot be softened or melted by reheating, for example Bakelite and melamine.
In simple words: Plastics are grouped by chain structure (linear, branched, cross-linked) and by their reaction to heat.

Teacher's Note:
1. Draw or describe the three structures.
2. Give two examples for each type.