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

Short Answer Type

Question 1. What is polymerisation ?
Answer:
Polymerisation is the process in which a large number of small molecules called monomers join together to form a very large molecule called a polymer.
It can be written as n(monomer) gives (monomer)_n, where n is a whole number.
In simple words: Small molecules join to make a long molecule: that is polymerisation.

Teacher's Note:
1. Mention both monomer and polymer in the definition.
2. Example: ethene gives polythene.

Teacher's Note:
1. Name the raw material in each column.
2. Rayon is the standard example of the semi synthetic type.

Question 3. Why is rayon called artificial silk ?
Answer:
Rayon is shiny and lustrous and looks like silk. It can also copy the feel of silk, but it is made from wood pulp in a factory. So it is called artificial silk.
In simple words: Rayon looks like silk but is man-made.

Teacher's Note:
1. Say 'shiny and lustrous' in the answer.
2. Rayon is also cheaper than silk.

Question 4. Why are ropes made of nylon used in cranes and elevators ?
Answer:
Nylon ropes have very high tensile strength and are elastic. They can bear heavy loads without breaking and do not lose strength after repeated use, so they are used in cranes and elevators.
In simple words: Nylon is strong and elastic and can hold heavy loads.

Teacher's Note:
1. Name both tensile strength and elasticity.
2. Polyester ropes are used for the same reason.

Question 5. A strand of wool burns with the odour of burning hair while that of cotton burns with the odour of burning paper. Why ?
Answer:
Wool is an animal fibre. It is made of protein, like hair, so it smells like burning hair when it burns.
Cotton is a plant fibre made of cellulose, the same material as paper, so it gives the smell of burning paper.
In simple words: Wool is protein like hair; cotton is cellulose like paper.

Teacher's Note:
1. Connect the smell to the material of the fibre.
2. This burning test helps to identify fibres.

Question 6. Write the general properties of plastics.
Answer:
1. Plastics are poor conductors of heat, so they are used for handles of cooking vessels and for foam in refrigerators.
2. They are poor conductors of electricity, so they cover wires and electrical outlets.
3. They are insoluble in water, so bottles and buckets are made of plastic.
4. They melt and catch fire easily (most plastics), though melamine is fire resistant.
5. They do not react with air and water and do not corrode like metals, so they store chemicals.
6. They are light, strong, durable and can be moulded into any shape.
In simple words: Plastics are light, strong, non-conducting, non-corroding and easy to mould.

Teacher's Note:
1. Give one use with each property.
2. Six points are enough for full marks.

Question 7. Discuss the problems associated with plastic disposal.
Answer:
Plastics are non-biodegradable, so they are not decomposed by micro-organisms and most disposal methods pollute.
1. When buried, plastic stops rain water seeping into the soil, forms puddles and harms plants.
2. Plastic waste thrown on roads and in lakes and dumps becomes a home for disease-causing germs.
3. In water bodies, it pollutes water and the toxic substances can kill fish or stop their reproduction.
4. When burnt, it gives toxic gases and smoke that pollute the air.
5. Animals, especially cows, swallow polybags and wrappers, which choke them or line their stomach and can cause death.
In simple words: Plastic does not rot, so it pollutes soil, water and air and harms animals.

Teacher's Note:
1. Start with the reason: plastic is non-biodegradable.
2. Cover soil, water, air and animals.

Question 8. Write the types of waste.
Answer:
Wastes are of two types.
1. Biodegradable wastes: wastes that are broken down into simpler, less harmful substances by micro-organisms, for example kitchen waste, paper, cotton cloth and wood.
2. Non-biodegradable wastes: wastes that are not easily broken down by natural processes, for example metal cans, plastic bags and glass.
In simple words: Biodegradable wastes rot; non-biodegradable wastes do not.

Teacher's Note:
1. Give two examples of each.
2. Use two bins, green and blue, to keep them apart.

Question 9. What is the solution of the problems associated with plastic disposal ?
Answer:
Two methods can reduce the hazards of plastic waste.
1. Reduce the use of plastics: use cotton or jute bags, and collect biodegradable and non-biodegradable wastes separately.
2. Recycle plastics: collect, sort and process plastic waste to make new articles. For example, polyester soft drink bottles can be melted and spun into fibres.
Following the 4 R principle (reduce, reuse, recycle, recover) is the best habit.
In simple words: Use less plastic and recycle the plastic we must use.

Teacher's Note:
1. Name both reducing and recycling.
2. Mention the green and blue bins as an example of segregation.

Teacher's Note:
1. Use a table and give examples.
2. Thermosetting plastics are harder and stronger.

Question 11. What do you mean by polymerisation ? Give the examples of natural and artificial polymers.
Answer:
Polymerisation is the process in which many small monomer molecules combine to form a polymer.
Natural polymers: cellulose, protein, starch.
Artificial (synthetic) polymers: PVC, polypropylene, nylon, polyester.
In simple words: Joining many monomers makes a polymer; cellulose is natural and PVC is synthetic.

Teacher's Note:
1. Define first, then list examples.
2. Cellulose and protein are the examples given in the book.

Question 12. Why melamine is called thermosetting plastics ?
Answer:
Melamine is a thermosetting plastic because once it has been moulded and set into shape by heating, it cannot be softened or melted again on reheating. The change on setting is permanent, and it keeps its shape even at high temperature.
In simple words: Melamine sets permanently on heating and cannot be remelted.

Teacher's Note:
1. 'Thermo' means heat and 'setting' means fixed.
2. Hardness and high polish help in making crockery.

Question 13. Which material is used in making kitchen utensil ?
Answer:
Kitchen utensils are made of metals such as steel, aluminium or copper for the body. Non-stick pans have a coating of Teflon, handles are made of Bakelite (a thermosetting plastic), and unbreakable dinner-ware is made of melamine.
In simple words: Metals make the body of utensils; plastics are used for coatings, handles and crockery.

Teacher's Note:
1. The book gives plastic uses; add the metals yourself.
2. Name Teflon, Bakelite and melamine.

Question 14. Why bakelite is used in preparation of handle of cooking utensil ?
Answer:
Bakelite is a thermosetting plastic, so it does not melt or soften when heated, and it is a poor conductor of heat. The handle therefore stays cool and does not burn the hand.
In simple words: Bakelite does not melt and does not conduct heat, so the handle stays cool.

Teacher's Note:
1. Give both reasons: no melting and poor conduction.
2. Recall the activity with two saucepans.

Question 15. Why recycling is the best solution to get rid of non biodegradable waste ?
Answer:
Non-biodegradable waste does not decay. Burning it pollutes the air, and burying or dumping it spoils soil and water.
Recycling collects and processes the waste to make useful new things, so it saves material, reduces waste and does not pollute.
In simple words: Recycling reuses waste that would otherwise stay for years.

Teacher's Note:
1. Compare recycling with burning and burying.
2. Mention the example of bottles turned into fibres.

Question 16. Give the examples of biodegradable and non-biodegradable wastes.
Answer:
Biodegradable: cotton cloth (2 to 5 months), woollen clothes (about a year), kitchen waste (1 to 2 weeks), paper (10 to 30 days) and wood (10 to 15 years).
Non-biodegradable: metal cans such as tin or aluminium (about 100 to 500 years) and plastic bags (several years).
In simple words: Food, paper and cloth rot; metal cans and plastics do not.

Teacher's Note:
1. Add the approximate time if you remember it.
2. Time to decay is different for each material.

Question 17. Give examples to show that plastic are non corrosive in nature.
Answer:
Plastics do not react with air or water, so they do not rust or corrode.
1. Plastic buckets and mugs last for years in water without rusting, while iron buckets rust.
2. Plastic water pipes and tanks do not corrode.
3. Bottles and containers of plastic can store acids, alkalis and other chemicals safely.
In simple words: Plastics do not rust, so they are used to hold water and chemicals.

Teacher's Note:
1. Contrast with iron, which rusts.
2. Use everyday examples.

Question 18. What is melamine. How it is obtained ?
Answer:
Melamine is a hard, high-polish thermosetting plastic. It is an organic compound with a 1,3,5-triazine ring that is a trimer of cyanamide.
It is obtained by joining melamine with formaldehyde to form melamine-formaldehyde resin, which sets into a cross-linked solid.
It is used for unbreakable crockery and decorative articles and to make fire-fighters' uniforms fire resistant.
In simple words: Melamine reacts with formaldehyde to give a hard, fire-resistant thermosetting plastic.

Teacher's Note:
1. Mention thermosetting and uses.
2. The triazine detail is beyond Class 8; a short mention is enough.

Question 19. What is spandex and how it is obtained ?
Answer:
Spandex is a synthetic fibre with exceptional elasticity. It is also called Lycra or elastane, and it is stronger and more durable than natural rubber.
It is a copolymer: a soft polyester or polyether part is linked with polyurethane units, and the polymer is spun into fibre. It was invented in 1958 by the chemist Joseph Shivers.
It is mixed with cotton and other fibres for stretch fabrics such as swimwear, caps and T-shirts.
In simple words: Spandex is a very stretchy copolymer of polyester and polyurethane.

Teacher's Note:
1. Say 'copolymer' and 'elastic'.
2. Give two uses.

Question 20. Polymer is prepared in the laboratory by which methods ?
Answer:
Polymers are prepared by two methods of polymerisation.
1. Addition polymerisation: monomers with a double bond add to each other with nothing lost, for example ethene to polythene.
2. Condensation polymerisation: monomers join with loss of a small molecule such as water, for example the preparation of nylon and Bakelite.
In simple words: Polymers are made by addition or condensation polymerisation.

Teacher's Note:
1. This topic is beyond the Class 8 book; learn only the two names and an example each.
2. Addition: ethene to polythene.