Class 08 Basic sciences synthetic-materials
A. Answer these Questions
1. What is polymers?
Answer : A polymer is made up of small units called monomers that combine under specific conditions. Example: Cotton, jute, silk and wool.
2. What is the process of the formation of the formation of polymers from its monomers called?
Answer : The process of formation of a polymer from its monomer is called polymerisation.
3. Name three rayon?
Answer : Viscose, cuprammonium and acetate rayon are three widelyused rayons.
4. Mention the five uses of synthetic fibre Other than making clothes?
Answer : Other than making clothes, synthetic fibres are used in making towels, mats, curtains, cushions, furniture covers and mosquito nets
5. What are plastic?
Answer : Synthetic materials that can be moulded into a permanent shape are called plastics
6. What are the polymers is used for making bubblegums , thermocol and Synthetic erasers ?
Answer :
Poly (styrenebutadiene) is used for making bubblegums.
Polystyrene is used for making thermocol.
Synthetic erasers are made from vinyl rubber, neoprene and poly (styrenebutadiene).
7. Name the three polymers that can be obtain from chemical derived from natural gas?
Answer : Polythene, polyvinyl chloride (PVC) and Bakelite are three polymers that can be obtained by using chemicals derived from natural gas.
8. What do you mean by tensile strength of material?
Answer : The force or stress that is required to break a material is called its tensile strength.
9. What are those material called that (a) that attract water (b) repel water
Answer : (a) Materials that attract water are called hydrophilic. (b) Materials that repel water are called hydrophobic.
B. Answer these Questions
1. Why are acrylics not spun from the melt?
Answer : Acrylics are so named because they are made from polymer polyacrylonitrile. They are not spun from the melt because these polymers decompose without melting. Thus, they are dissolved in a specific solvent and the solution is forced through spinnerets to obtain filaments that are spun into yarns.
2. What are Thermoplastics and Thermosetting plastics ?
Answer : Thermoplastics are those plastics that can retain their plasticity even after repeated heatings and coolings. They can also be moulded again and again.
Thermosetting plastics are those plastics that cannot be moulded again after they are set into a particular shape
3. To what use can a polythene and acetylene be put?
Answer : Acetylene is obtained from methane. It is used for making polythene, polyvinyl chloride and acrylics. Propylene is obtained from petroleum and is used for making polypropylene (a plastic).
C. Answer these Questions
1. How are polymers classified? What are the raw material used to manufacture synthetic polymers?
Answer : Polymers are classified into three types depending upon their sources:
(a) Natural polymers
(b) Semisynthetic polymers
(c) Synthetic polymers
The raw materials used to manufacture the synthetic polymers are obtained from natural gas and petroleum, either directly or indirectly.
Methane is present in natural gas, which is converted into acetylene and formaldehyde.
Acetylene is used for making polythene, PVC and acrylics and formaldehyde is used for making bakelite.
Fractional distillation and cracking of petroleum gives acetylene, propylene, butylene, benzene and naphthalene.
Benzene is used for making nylon, butylene is used for making poly (butadienestyrene), naphthalene is used for making polyester and propylene is used for making polypropylene.
2. Mention three advantages and disadvantages of synthetic fibre over natural fibre?
Answer : Advantages of synthetic fibres:
(a) They do not depend on agricultural crops and animal farming unlike natural fibres that are obtained by these methods.
(b) They are cheaper than natural fibres.
(c) They are stronger and more durable than natural fibres.
Disadvantages of synthetic fibres:
(a) They are hydrophobic and do not absorb moisture whereas natural fibres are hydrophilic and are more comfortable to wear.
(b) They are very lustrous unlike natural fibres that have soothing colours.
(c) They melt before burning and may cause severe injuries in case of an accident.
3. Describe how youwould compare the tensile strength of thread of different material?
Answer : Tensile strengths of different materials can be compared by the following activity:
Take a cotton thread. Tie its one end to a hook that is fixed with a rigid body and the other end to the pan of a balance. Now, add some weights to the pan. Keep on increasing the weight in small amounts till the thread breaks. Note the total weight including the weight of the pan required to break the thread.
Repeat the same activity with other materials and note the different weights required to break the threads. By comparing the values of the weights, one can compare the tensile strengths of different materials.
D. Complete sentence.
1. Acrylic fibres compete with wool.
2. Polythene carrybags block drains and sewers.
3. Plastics are generally nondegradable, i.e., they are not environmentally broken down to simpler, harmless substances.
4. On being burnt, plastics give harmful gases.
5. Nylon has a higher tensile strength than copper.
6. On being heated, polyester and nylon melt before they burn.
E. Chose the correct options with explanation
1. (c) Cellulose
Cellulose is a natural polymer.
2. (a) polyester
Polyester is a synthetic polymer that can be stretched several times of its original length.
3. (d) polyester
Polyester is a synthetic fibre and it is hydrophobic.
4. (c) polythene
Polythene is a bad conductor of heat because it is a synthetic polymer.
5. (a) plastic
Plastic is a bad conductor of electricity because it is a synthetic polymer
F. Match the columns
Answer :
A B
(a) Cellulose (v) Glucose
(b) Nylon (iv) Parachutes
(c) Acrylics (i) Artificial wool
(d) Acetate rayon (ii) Semisynthetic fibre
(e) Polythene (iii) Carrybags
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