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NCERT Exemplar Class 8 Science Chapter 3 Synthetic Fibres and Plasticst are part of NCERT Exemplar Class 8 Science. Here we have given NCERT Exemplar Class 8 Science Chapter 3 Synthetic Fibres and Plastics. https://www.cbselabs.com/ncert-exemplar-problems-class-8-science-synthetic-fibres-and-plastics/

NCERT  Exemplar Class 8 Science Chapter 3 Synthetic Fibres and Plastics

Multiple Choice Questions
Question. 1 Pick the synthetic fibre out of the following.
(a) Cotton (b) Nylon (c)Jute (d)Wool
Answer. (b) Nylon Is a synthetic fibre. It is made without using any natural raw material. Cotton, jute and wool are natural fibres.

Question. 2 Which of the following is a source of rayon?
(a) Wool (b) PET (c) Wood pulp (d) Silk
Answer. (c) Wool pulp
Rayon is obtained by the chemical treatment of wool pulp (which contains cellulose).

Question. 3 Polycot is obtained by mixing
(a) nylon and wool (b) polyester and wool
(c) nylon and cotton (d) polyester and cotton
Answer. (d) Polycot is obtained by mixing polyester and cotton.
Polycot = polyester + cotton

Question. 4 Which is a thermosetting plastic?
(a) Melamine (b) Polythene –
(c) PVC (d) Nylon
Answer. (a) Melamine is a thermosetting plastic which when moulded once, cannot be softened by heating.

Question. 5 The material similar to silk in appearance is
(a) nylon (b) rayon
(c) polyester (d) terylene
Answer. (b) Rayon resembles silk in appearance, therefore, rayon is also called artificial silk.

Question. 6 The most suitable material for the preparation of handles of cooking utensils is (a) polythene (b) PVC
(c) nylon (d) bakelite
Answer. (d) Bakelite is used for making the handles of various cooking utensils because it is a poor conductor of heat and does not become soft on getting heated.

Question. 7 Which of the following is not a common property of plastics?
(a) Non-reactive
(b) Light in weight
(c) Durable
(d) Good conductor of electricity
Answer. (d) Plastics are non-reactive, light in weight and durable but they do not conduct electricity.

Question. 8 Which of the following represents the correct match for items in Column A with those in Column B.

Codes
(a) (a)-(ii), (b)-(iii), (c)-(i)
(b) (a)-(iii), (b)-(i), (c)-(ii)
(c) (a)-(ii), (b)-(i), (c)-(iii)
(d) (a)-(iii), (b)-(ii), (c)-(i)
Answer. (c) Nylon —-> Fibre , PVC —->Thermoplastic , Bakelite —->Thermosetting plastic

Question. 9 Which of the following groups contains all synthetic substances?
(a) Nylon, tprylene, wool
(b) Cotton, polycot, rayon
(c) PVC, polythene, bakelite
(d) Acrylic, silk, wool
Answer. (c) PVC, polythene and bakelite are synthetic substances.

Question. 10 The material which is commonly used for making kitchen containers is
(a) PVC (b) acrylic
(c) teflon (d) PET
Answer. (d) PET is ‘poly-ethylene’ terephthalate’ PET as a plastic is very light weight and is commonly used for making kitchen containers.

Very Short Answer Type Questions
Question. 11 Cotton is a natural polymer. What is its chemical name?
Answer. Chemical name of natural polymer of cotton is cellulose.

Question. 12 A synthetic fibre which looks like silk is obtained by chemical treatment of wood pulp. It is, therefore, known as artificial silk. What is its common name?
Answer. Its common name is rayon. Rayon is often regarded as artificial silk.

Question. 13 Terrycot is made by mixing two types of fibres. Write the names of the fibres.
Answer. Terrycot is made by mixing terylene and cotton.

Question. 14 Plastic articles are available in all possible shapes and sizes. Can you tell why?
Answer. Plastics can be easily moulded, so they are used to make a large variety of articles (or objects) having different shapes and sizes.

Question. 15 Plastic is used for making a large variety of articles of daily use and these articles are very attractive. But it is advised to avoid the use of plastic as far as possible. Why?
Answer. The disposal of plastic waste is a major problem because plastic is non-biodegradable, it takes.several years to decompose. Thus, cause environmental pollution. So, we should avoid use of plastics, as for as possible.

Question. 16 Why is it not advisable to burn plastic and synthetic fabrics?
Answer. Burning of plastics and synthetic fabrics produces lots of poisonous gases causing air pollution. So, it is not advisable to burn plastic and synthetic fibres.

Question. 17 Select the articles from the following list which are biodegradable.
(a) Paper
(b) Woollen clothes
(c) Wood
(d) Aluminium can
(e) Plastic bag
(f) Peels of vegetables
Answer. The biodegradable articles are:
(a) paper (b) woollen clothes
(c) wood (f) peels of vegetables

Question. 18 A bucket made of plastic does not rust like a bucket made up of iron. Why?
Answer. Plastic is non-reactive material. When left exposed to moisture and air, it does not react with air and water which are essential for corrosion, therefore does not rust.

Short Answer Type Questions
Question. 19 Rohit took with him some nylon ropes, when he was going for rock climbing. Can you tell why he selected nylon ropes instead of ropes made up of cotton or jute?
Answer. Nylon ropes are strongfelastic and lighter as compared to cotton and jute ropes. A nylon thread is actually stronger than a steel wire of similar thickness.

Question. 20 A lady went to the market to buy a blanket. The shopkeeper showed her blankets made up of acrylic fibres as well as made up of wool. She preferred to buy an acrylic blanket. Can you guess why?
Answer. Acrylic blankets are cheap, light in weight, more durable and are available in variety of colours and designs. They can be easily washed at home.

Question. 21 PVC (polyvinyl chloride) is a thermoplastic and is used for making toys, chappals, etc. Bakelite is a thermosetting plastic and is used for making electrical switches, handles of various utensils, etc. Can you write the major difference between these two types of plastics?
Answer. Thermoplastics get deformed easily on heating and can be bent easily on heating. On the other hand, thermosetting plastics when moulded once cannot be softened on heating.
Thermoplastics —–>Polythene and PVC ‘
Thermosetting plastics —–> Bakelite and melamine

Question. 22 Fill in the blanks.

1. A polymer is a chain of many small units joined together which are
   called…………………….
2. The synthetic fibres are also known as…………………….fibres.
3. The first fully synthetic fibre was…………………… .
4.  A fibre similar to wool is…………………….
5. A plastic used for making crockery is…………………….

Answer.

1. monomers
2. man-made
3. nylon
4. acrylic
5. melamine

Question. 23 Match items in Column A with the terms of Column B.

Answer. The correct matching is as given:
(a)—(iii), (b)—(iv), (c)—(v), (d)—(ii), (e)-(i)

Question. 24 Unscramble the jumbled materials. words given below, related to synthetic
(a) Anory (b) Lopmery
(c) Relyteen (d) Laspict
(e) Yespolter (f) Felton
Answer. (a) Rayon (b) Polymer
(c) Terylene (d) Plastic
(e) Polyester (f) Teflon

Long Answer Type Questions
Question. 25 State whether the following statements are True/False.

1. The fabric tery wool is obtained by mixing terylene and wool.
2. Synthetic fibre do not melt on heating.
3. It is risky to wear synthetic clothes while working in the kitchen.
4.  Most of the plastics are biodegradable.

Answer.

1.  True
2. False, synthetic fibres melt on heating.
3. True, a disadvantage as synthetic fibre is that they melt on heating.
4. False, most of the plastics are non-biodegradable.

Question. 26 Write the importance of synthetic polymers in our life.
Answer. The importance of synthetic polymers in our life are as follows:

1. Nylon It is used for making ropes for rock climbing, fishing nets, raincoats, parachutes and tyre cords, etc.
2. Acrylic It is used for sweaters, track suits, linings for boots and gloves and In furnishing fabrics and carpets.
3. Terylene It is used in textile industry to make clothes like sarees, tapestry and dress material. It is also mixed with natural fibre like cotton and wool to make more variety of clothes.
4. Plastics It is used to store food item, water, milk, pickles, dry food, etc. Plastic containers seem most convenient.
   This is because of light weight, low price, good strength and easy handling as compared to metals. Plastics are used in cars, air crafts and spacecrafts.

Question. 27 Despite being very useful, it is advised to restrict the use of plastic. Why is it so? Can you suggest some methods to limit its consumption?
Answer. It is advised to restrict the use of plastic because of the following reasons:

1. The articles mades of plastics are non-biodegradable. They do not decompose (or rot) easily. This causes a great problem in the disposal of plastic wastes.
2. The burning of plastic wastes gives out harmful gases which pollute the air. So, it is not advisable to dispose of the used plastic articles by burning.

Question. 28 Write an activity to show that synthetic fibres are stronger than the cotton fibres.
Answer. Activity Take threads of cotton and nylon (synthetic fibres) of about same length and thickness.. Hang cotton, thread from a stand. Attach a pan at the other end of the thread.
Add weights, one by one, to the pan until the thread breaks (see fig.) note the total weight required to break the cotton thread. This gives a measure of the tensile strength of the thread. Repeat the same with nylon thread.

Conclusion
Much more weight is required to break the nylon thread showing that nylon thread has a grater strength that the cotton thread.

Question. 29 Crossword Puzzle Complete the crossword given below with the help of clues.
Across
1. Substance used as synthetic wool. (7)
2. A plastic used for making containers and carry bags. (9)
3. Substance made up of large number of smaller molecules. (7)
4. Another name for this compound is artificial silk. (5)
Down
5. A type of fibre obtained naturally from cocoon. (4)
6. A synthetic fibre classified as polyester. (8)
7. A polymer used for making ropes. (5)

Answer.

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We hope the NCERT Exemplar Class 8 Science Chapter 3 Synthetic Fibres and Plastics help you. If you have any query regarding NCERT Exemplar Class 8 Science Chapter 3 Synthetic Fibres and Plastics, drop a comment below and we will get back to you at the earliest.

NCERT Exemplar Class 8 Science Chapter 2 Micro organisms: Friend and Foe are part of NCERT Exemplar Class 8 Science. Here we have given NCERT Exemplar Class 8 Science Chapter 2 Micro organisms: Friend and Foe. https://www.cbselabs.com/ncert-exemplar-problems-class-8-science-micro-organisms-friend-foe/

NCERT Exemplar Class 8 Science Chapter 2 Micro organisms: Friend and Foe

Multiple Choice Questions
Question. 1 Which of the following reproduces only inside a host cell?
(a) Bacteria (b) Virus
(c) Amoeba (d) Fungus
Answer. (b) Virus is a micro organism, which is in an inactive or deal form outside the body of a host. It reproduces or replicates only when it enters a host and reaches its cells.

Question. 2 A disease in human beings caused by virus is
(a) typhoid (b) influenza
(c) dysentery (d) cholera
Answer. (b) Influenza is an infectious disease caused by the influenza virus. It is also known as flu whose symptoms includes high fever, running nose, sore throat, headache, coughing and muscle pain etc. Typhoid, dysentry and cholera are bacterial infections.

Question. 3 Pathogenic micro organisms present in host cells are killed by medicines called
(a) pain killer (b) antibodies
(c) antibiotics (d) vaccines
Answer. (c) Antibiotics are substances that may inhibit or kill the growth of different microbes.
Micro organisms causing infection in host bodies are killed by consumption of these antibiotics.

Question. 4 The two micro organisms which live in symbiotic association in lichens are
(a) fungus and protozoa (b) alga and bacteria
(c) bacteria and protozoa (d) alga and fungus
Answer. (d) Lichens is an association between a fungal and an algal species. It is mutually benefiting association where algae provides food to the tongue fungus return gets shelter from it.

Question. 5 The gas released during the preparation of bread is
(a) oxygen (b) carbon dioxide
(c) nitrogen (d) sulphur dioxide
Answer. (b) Carbon dioxide (CO₂) is released during the preparation of bread.
This is the process of fermentation in which yeast cells rapidly produces CO₂, which causes risening of the dough used in preparation of bread.

Question. 6 The disease caused by a protozoan and spread by an insect is
(a) dengue (b) malaria
(c) polio (d) measles
Answer. (b) Malaria is the disease which is caused by the spread of a protozoan, i.e. Plasmodium.
It is spread in healthy individuals by the bite of a female Anopheles mosquito carrying this Plasmodium in their mouth (saliva).

Question. 7 Paheli dug two pits, A and B, in her garden. In pit A, she put a polythene bag packet with some agricultural waste. In pit B, she dumped the same kind of waste but without packing it in a polythene bag. She, then covered both the pits with soil. What did she observe after a month?
(a) Waste in pit A degraded faster than that in pit B
(b) Waste in pit 6 degraded faster than that in pit A
(c) Waste in both pits degraded almost equally
(d) Waste in both pits did not degrade at all
Answer. (b) Waste in pit B degraded faster than that in pit A because packet B was kept open, so 02 could go in which helped the microbes in decomposing the waste materials easily.In packet A, since it was sealed, 02 could not enter and thus, microbial degradation did not occur.

Very Short Answer Type Questions
Question. 8 Unscramble the jumbled words underlined in the following statements.
(a) Cells of our body produce santiidobe to fight pathogens.
(b) Curbossulite is an air-borne disease caused by a bacterium.
(c) Xanrhat is a dangerous bacterial disease.
(d) Yeasts are used in the wine industry because of their property of meronettinaf.
Answer. (a) Antibodies are specific molecules produced against invading microbes.
(b) Tuberculosis is an infectious air-borne disease caused by Mycobacterium tuberculosis.
(c) Anthrax is an infectious disease of humans and animals caused by Bacillus anthracis.
(d) Fermentation is a process occurring in yeast which converts sugar into gases (C02) and alcohols.

Question. 9 Suggest a suitable word for each of the following statements.
(a) Chemicals added to food to prevent growth of micro organisms.
(b) Nitrogen-fixing micro organism present in the root nodules of legumes.
(c) Agent which spreads pathogens from one place to another.
(d) Chemicals which kill or stop the growth of pathogens.
Answer. (a) Preservatives are the chemicals added in food. They prevent microbial infection without altering the taste or appearance.
(b) Rhizobium are the nitrogen fixing bacteria which fix the atmospheric nitrogen into that form (i.e. nitrates, nitraties) which is usable for plants.
(c) Carrier/Vector are the modes of transmission of a disease.
(d) Antibiotics are the antimicrobial agents for inhibiting or killing the growth of microbes.

Question. 10 Match the names of scientists given in Column I with the discovery made by them given in Column II.

Answer. The correct matching is as given:
(a) —(iii), (b)—(ii), (c)—(iv),(d)—(i)
(a) Louis Pasteur discovered fermentation in 1857 by demonstrating that microbes cause souring of milk.
(b) Robert Koch is known for his role in identification of the causing agent of anthrax.
(c) Edward Jenner is known as father of Immunology. He discovered the vaccine for smallpox in 1798.
(d) Alexander Fleming discovered penicillin antibiotic, which is used extensively in treatment of various diseases.

Question. 11 Name one commercial use of yeast.
Answer. Baking bread/manufacturing of alcoholic drinks is the commercial use of yeast.

Question. 12 Name the process in yeast that converts sugars into alcohol.
Answer. Fermentation is the process by which yeast converts sugars into alcohol.

Question. 13 Preservatives are used in kitchen on daily basis. List a few of them. Preservatives prevent the spoilage of food lives for a long time from microbial infestation.
Answer. Vinegar common salt and oil are common preservatives used in kitchen.

Question. 14 Why should we avoid standing dose to a tuberculosis patient while he/she is coughing?
Answer. Tuberculosis is an air-borne disease, which is easily spreads when the infected person coughs. As coughing spreads germs in the air and these germs remain suspended in air until inhaled by person present in promixity of the patient who is suffering from the disease. That is why we should avoid standing close to a TB patient.

Question.  15 Polio drops are not given to children suffering from diarrhoea. Why?
Answer. If the child is suffering from diarrhoea, the polio drops that is an oral vaccine for fighting against polio virus may be excreted out because of frequent motions.
Thus, the child becomes susceptible to polio inaction as the vaccine becomes ineffective to fight against invading pathogen.

Question. 16 Paheli watched grandmother making mango pickle. After she bottled the pickle, her grandmother poured oil on top of the pickle before closing the lid. Paheli wanted to know why oil was poured? Can you help her understand why?
Answer. The oil poured on the pickle forms a barrier between the pickle and air. This prevents the bacteria present in air from entering jar and attacking the pickle and spoiling it. Thus, increasing the shelf life of pickles.

Short Answer Type Questions
Question. 17 Match the micro organisms given in the Column I to the group to which they belong in Column II.

Answer. The correct matching is as given:
(a) —(iv), (b)—(iii), (c)—(i), (d)—(ii)
(a) Lactobacillus is a bacteria, which causes the formation of curd from milk.
(b) Aspergillus is a fungi from which penicillin antibiotic is extracted, i.e. A. niger.
(c) Splrogyra is a filamentous green spiral in shape, groung as green slimy mats on surface of water bodies.
(d) Paramecium is a unicellular organism, a protozoan found in water bodies.

Question. 18 Classify the following into friendly and harmful micro organisms. Yeast, malarial parasite, Lactobacillus, bread mould, Rhizobium, Bacillus anthracis.
Answer. Friendly micro organisms The microbes which benefit humans and other organisms in one way on the other are considered as friendly microbes.

Question. 19 While returning from the school, Boojho ate chaat from a street hawker. When he reached home, he felt ill and complained of stomachache and fell ill. What could be the reason?
Answer. The reason could be that the chaat was contaminated by pathogenic micro organisms. The unhygienic conditions present near the shop becomes the breeding place for microbes, flies, etc.
The pathogens can be transmitted to places by flies and other vectors, even the utensil used for serving could have been contaminated by the pathogenic micro organism. If such microbial contaminated food is consumed several disease like diarrhoea, dysentery, cholera, etc., may occur.

Question. 20 What will happen to ‘pooris’ and ‘unused kneaded flouri if they are left in the open for a day or two?
Answer. The ‘unused kneaded flour’ if left in warm conditions, gets infected by microbes which causes fermentation and spoils the flavour, texture, etc., of the flour. The pooris would remains in relatively good condition because these were deep fried in heated oil that kills microbes.

Question. 21 (a) Name two diseases that are caused by virus.
(b) Write one important characteristic of virus.
Answer. (a) Influenza and chickenpox are two diseases caused by virus in humans.
(b) Virus are dead when present in the environment. They can reproduce only inside the cells of an infected person (as host).

Long Answer Type Questions
Question. 22 Observe the figure and answer the questions that follows.
(a) Write the name of the disease.
(b) Name the causative agent of this disease?
(c) How does the disease spread from one plant to another?
(d) Name any two plant diseases and the microbes that cause them.

Answer. (a) The disease is yellow vein mosaic of the lady’s finger/okra. The disease is characterised by alternating green and yellow patches and chlorosis in veins. Severity of disease leads to complete yellowing of leaves.
(b) Bhindi yellow vein mosaic virus.
(c) The disease transmission occurs via the insects sitting on anther of diseased plant and moving to others,
(d) Plant disease occurring commonly are:

1. Common rust Puccinia graminis (fungus) brown pustules which can be • elongated or ovule develops on both the leaf surfaces. On rupturing they release spores.
2. Citrus canker Xanthomonas spp (bacteria) lesions on leaves, stems and fruits of citrus trees which fruit and leaves drops prematurely from trees.

Question. 23 How do vaccines work?
Answer. Vaccines contain dead or weakened microbial strains of a particular disease. When a vaccine is introduced into a heathy body. It produces specific cells against the pathogen.
These cells, are called antibodies and they becomes active when the pathogen attack our body. The body fights and kills them by producing specific set of reactions.
These antibodies remain in the body for life long and protect against the microbe when microbe enters the body again.
Vaccines can be given either orally (in the forms of drops) or injected in the body.

Question. 24 Observe the set-up given in figure and answer the following questions.

1. What happens to the sugar solution in A?
2. Which gas is released in A?
3. What changes will you observe in B when the released gas passes through it?

Answer.

1. Yeast grow very rapidly on the sugar containing substrates. It multiplies and increases its number and in process converts the sugar into alcoholic products with the release of CO₂ gas.
   In flask A, fermentation is occurring due to the presence of yeast and its utilisation of sugar.
2. Carbon dioxide is released in A during fermentation process.
3. The gas released due to the fermentation of sugar in set up is CO₂. When CO₂ is passed over lime water in set up S its color changes, i.e. the lime water turns milky.

Question. 25 Observe the figure and answer the following questions.

1. Name the microorganism and the group to which it belongs.
2. Name the foodjtem on which the organism grows.
3. Does it grow well in dry or in moist conditions?
4. Is it safe to eat infected bread?

Answer.

1. The microorganism is Rhizopus, also known as the bread mould. It belongs to the microbial group of fungi.
2. The Rhizopus grows well on moist and stale bread.
3. This organism grows well in moist conditions. Dry conditions inhibit its growth.
4. No, Intake of fungus infected bread can cause harm by producing poisonous chemicals or toxins which adversely effects the proper functioning of our body.

Question. 26 Give reasons for the following.

1. Fresh milk is boiled before consumption while processed milk is stored in packets and can be consumed without boiling.
2. Raw vegetables and fruits are kept in refrigerators, whereas jams and pickles can be kept outside.
3. Farmers prefer to grow beans and peas in nitrogen deficient soils.
4. Mosquitoes can be controlled by preventing stagnation of water though they do not live in water. Why?

Answer.

1. Fresh milk is boiled before consumption to kill the micro organisms that could be present in it. But packed milk is pasteurised and boiled at 70°C for 30 sec followed by sudden : chilling then stored.
   Thus, it can be consumed without boiling. Pasteurisation kill all micro organisms for making it free of any contamination.
2. Raw vegetables and fruits get easily infected by micro organisms and get spoiled. They are kept in refrigerator at low temperature that inhibits growth of microbes. Jams and pickles contain sugar and salt as preservatives.
   These preservatives forms a protective layer and prevent spoilage of these products by inhibiting the growth of various micro organisms.
3. Beans and peas are leguminous plants as they have Rhizobium, a bacteria in their root nodules. This bacteria can fix atmospheric nitrogen to enrich the soil with nitrogen and increase its fertility.
   Farmers grow leguminous plants in between the major crops, a practice called crop rotation to restore the used nutrients of the soil.
4. Though mosquitoes, live on land, water is their breeding ground their larvae grow in water. Water stagnation is collecting of water in a small area, without any activity. Mosquitoes lay egg on such a surface.
   If water stagnation is prevented, their larvae cannot survive, thus reducing mosquito population.

Question. 27 How can we prevent the following diseases?

1. Cholera
2.  Typhoid
3. Hepatitis-A

Answer.

1. Cholera An infectious disease caused by bacteria Vibrio cholerae. It occurs due to the consumption of contaminated or unhygienic food and water. It can be prevented by maintaining personal hygiene, good sanitation practice, consumption of clean drinking water, etc.
2. Typhoid An acute illness caused by bacteria Salmonella typhi. It occurs by ingestion of typhoid bacterium through food, water, fomite, etc. Preventions includes consumption of hygiene and properly cooked meals, filtered or boiled water, etc.
3. Hepatitis-A A disease caused by hepatitis. A virus. Symptoms of disease appears after 2 weeks of infection. Prevention includes proper vaccinasation drinking boiled water, wash hands before eating, etc.

Question. 28 Complete the following cycle given as figure by filling the boxes (1), (2),(3), (4).

Answer. The cycle given above represents the nitrogen cycle occurring in the environment.
The atmospheric nitrogen cannot be up taken by plants and animals as such, this is corrected by some bacteria and blue-green algae present in the soil. These microbes fix the atmospheric nitrogen into usable components which are utilised by other living ’organisms.
Nitrogen cycle checks the percentage of nitrogen in the atmosphere and maintains it at a constant level.

1. Lightning fixes nitrogen.
2. Nitrogen fixing bacteria and blue-green algae fix atmospheric nitrogen.
3. Nitrogenous waste from excretion and death.
4. Bacteria turns compounds of nitrogen into gaseous nitrogen.

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We hope the NCERT Exemplar Class 8 Science Chapter 2 Micro organisms: Friend and Foe help you. If you have any query regarding NCERT Exemplar Class 8 Science Chapter 2 Micro organisms: Friend and Foe, drop a comment below and we will get back to you at the earliest.

NCERT Exemplar Class 8 Science Chapter 4 Materials : Metals and Non-Metals are part of NCERT Exemplar Class 8 Science. Here we have given NCERT Exemplar Class 8 Science Chapter 4 Materials : Metals and Non-Metals. https://www.cbselabs.com/ncert-exemplar-problems-class-8-science-materials-metals-and-non-metals/

NCERT  Exemplar Class 8 Science Chapter 4 Materials : Metals and Non-Metals

Multiple Choice Questions
Question. 1 Which of the following is not a metal?
(a) Copper (b) Sulphur (c) Aluminium (d) Iron
Answer. (b) Copper, aluminium and iron are metals but sulphur is not a metal.

Question. 2 The substance that will be flattened on beating with a hammer is
(a) crystal of iodine (b) lump of sulphur
(c) piece of coal 4 (d) zinc granule
Answer. (d) Metals can be beaten into thin sheets (or flattened) with a hammer. Non-metals break into small pieces when hammered. So, zinc granule will be flattened on beating with a hammer.

Question. 3 Boojho has learnt that non-metals on beating with a hammer are generally broken into pieces. Which of the following is non-metal?
(a) Iron nail (b) Aluminium wire
(c) Copper plate (d) Piece of coal
Answer. (d) Piece of coal is a non-metal which broke into small pieces when hammered.

Question. 4 Materials which can be drawn into wires are called ductile. Which of the • following is not a ductile material?
(a) Silver (b) Copper (c) Sulphur (d) Aluminium
Answer. (c) Ductility is a characteristic property of metals. Sulphur is a non-metal, so it is not
ductile. When stretched, sulphur break into pieces and do not form wires.

Question. 5 Metals are generally hard. Which of the following metals is an exception ! and can be cut with a knife?
(a) Iron (b) Sodium (c) Gold (d) Magnesium
Answer. (b) Metals are generally hard. This means that most of the metals cannot be cut easily. But sodium metal is soft and can be easily cut with a knife.

Question. 6 Metals are generally solid. Which of the following metals is in the liquid state at room temperature?
(a) Mercury (b) Silver (c) Aluminium (d) Sodium
Answer. (a) Mercury metal is in the liquid state at room temperature.

Question. 7 Metals generally react with dilute acids to produce hydrogen gas. Which one of the following metals does not react with dilute hydrochloric acid? (a) Magnesium (b) Aluminium (c) Iron (d) Copper
Answer. (d) The less reactive metals like copper, silver and gold do not react with dilute hydrochloric acid to produce hydrogen gas.

Question. 8 Which of the following reacts with cold water vigorously?
(a) Carbon (b) Sodium (c) Magnesium (d) Sulphur
Answer. (b) The vigour (or intensity) of reaction of a metal with water depends on its chemical reactivity. Sodium is a very reactive metal, so it reacts violently even with cold water.

Question. 9 The metal which produces hydrogen gas on reaction with dilute hydrochloric acid as well as sodium hydroxide solution is (a) copper (b) iron (c) aluminium (d) sodium
Answer. (c) Aluminium reacts with hydrochloric acid as well as sodium hydroxide solution to produce hydrogen gas.

Question. 10 Which of the following non-metals reacts and catches fire on exposure to air?(a) Phosphorus (b) Nitrogen » (c) Sulphur (d) Hydrogen
Answer. (a) Phosphorus is a non-metal which reacts and catches fire on exposure to air.

Question. 11 Generally metallic oxides are basic and non-metallic oxides are acidic in nature. Solution of which of the following oxides in water will change the colour of blue litmus to red?
(a) Sulphur dioxide (b) Iron oxide
(c) Magnesium oxide (d) Copper oxide
Answer. (a) Sulphur is a non-metal and it forms an acidic oxide, i.e. sulphur dioxide (S02) which turns blue litmus red.

Question. 12 Which of the following property is not responsible for copper to be used as electrical conduction wires?
(a) Ductility (b) Colour
(c) Good conductor of electricity (d) It is solid
Answer. (b) Colour of the copper metal is not responsible for copper to be used as electrical conduction wires.

Very Short Answer Type Questions
Question. 13 Name two soft metals which can be cut with a knife.
Answer. Metals which can easily cut with a knife
(i) Sodium (ii) Potassium
These are soft metals.

Question. 14 Which non-metal is essential for our life and all living beings inhale it during breathing?
Answer. Oxygen is a non-metal which is used by plants and animals (including human beings) for breathing. Thus, oxygen (non-metal) is essential for maintaining our life.

Question. 15 Name two majorTion:metals which are present in fertilisers and enhance the growth of plants.
Answer. Two major non-metals which are present in fertilisers and enhance the growth of plants are: .
(i) Nitrogen (ii) Phosphorus

Question. 16 Which non-metal is used to disinfect water?
Answer. Chlorine which is a non-metal, is used in water purification plants because chlorine has ‘ the ability to kill germs.

Question. 17 A purple coloured non-metal forms a brown solution in alcohol which is applied on wounds as an antiseptic. Name the non-metal.
Answer. Iodine is a non-metal which*.is used to make purple-coloured solution called tincture iodine which is applied on cuts and wounds as an antiseptic.

Question. 18 Zinc sulphate forms a colourless solution in water. Will you observe any colour on adding copper turning in it?
Answer. There is no colour change on adding copper turning in zinc sulphate solution because copper is less reactive than zinc and it cannot displace zinc from its salt solution. So, displacement reaction does not take place.

Question. 19 Why are bells made of metals?
Answer. Metals have sonorous property. Ringing sound produce by metals is known as sonorous. So, the bells are made of metals. .

Question. 20 Which liquid metal is used for making thermometers?
Answer. Mercury metal is used for making thermometers.

Question. 21 Which of the following metals can displace the other two metals from their salt solutions? Zinc, iron, copper
Answer. Zinc is most reactive among three metals and copper is least-reactive metal. Reactivity of iron is in between zinc and copper. So, zinc metal can displace the other two metals from their salt solutions.

Short Answer Type Questions
Question. 22 Paheli bought a statue made up of copper. To her surprise, it acquired a dull green coating after a couple of months. Explain the reason.
Answer. When a copper statue (or copper vessel) is exposed to moist air for long, it acquires a dull green coating. The green material is a mixture of copper hydroxide [Cu(OH)z] and copper carbonate (CuC03) formed due to the reaction of copper with moist air. The following is the reaction.

The green coating is commonly known as ‘basic copper carbonate’ and the formation of green coating on copper objects show the corrosion of copper.

Question. 23 In the given figure, you find that the bulb glows when an iron nail is placed between two ends of wire. Complete the following sentences on the basis of this fact.
(a) is a metal. (b) Metals are good of electricity.
Answer. (a) Iron (b) conductors

Question. 24 If in figure given above, iron nail is replaced by a wooden stick, will the bulb glow or not? Justify your answer.
Answer. No, the bulb will not glow. It is because wood is a non-conductor of electricity.

Question. 25 Paheli prepared a blue coloured solution of copper sulphate in beaker A and placed an iron nail in it. Boojho prepared a yellowish green solution of ferrous sulphate in beaker B and placed a copper wire in it. What changes will they observe in the two beakers after an hour?
Answer. In beaker A₁, a reddish brown layer of copper will deposit on the iron nail and blue coloured solution of copper sulphate will become yellowish green due to the formation of iron sulphate. This is because iron is more reactive than copper and it displaces copper from its salt solution (copper sulphate solution). In beaker B₁, no change is observed because copper being less reactive does not displace iron (more reactive) from ferrous sulphate solution.

Question. 26 A doctor prescribed a tablet to a patient suffering from iron deficiency. The tablet does not look like iron. Explain.
Answer. Iron is a metal but the tablet is not made up of iron metal. It is made up of a salt of iron. So, it does not look like iron.

Question. 27 Match the Column A with Column B.

Answer. The correct matching is as given :
(a)—(iii), (b)—(iv), (c)—(i), (d)—(v), (e)-(ii)

Long Answer Type Questions
Question. 28 State whether the following statement are True/False.

1. The property of metals by virtue of which they can be drawn into wires is called ductility.
2. Metals are good conductors of electricity but poor conductor of heat.
3. Articles made up of metals produce ringing sound when struck hard.
4. Oxides of non-metals and metals are acidic in nature.
5. A less reactive metal displaces a more reactive metal from its salt solution in water.

Answer.

1. True
2. False, metals are good conductors of heat and electricity.
3. True, because metals are sonorous.
4.  False, oxides of non-metals are acidic in nature and oxides of metals are basic in nature.
5. False, a more reactive metal displaces a less reactive metal from its salt solution in water.

Question. 29 Iron is more reactive than copper. Can you write an activity to show this?
Answer. Take about 50 mL of water in a beaker and dissolve 5 g of copper sulphate in it to obtain copper sulphate solution (which is blue in colour). Put a clean iron nail in this solution and keep the beaker undisturbed for some time.
We will find that the blue colour of copper sulphate solution starts fading gradually and the iron nail gets covered with a red-brown layer of copper metal. It is because iron is more reactive than copper, so it displaces copper metal from its solution of copper sulphate.
It is the copper metal set free from its compound which forms a red-brown layer on the surface of iron nail. This shows that iron is more reactive than copper.

Question. 30 Fill in the blanks to complete the following paragraph.The name of the product formed in the reaction of sulphur and is sulphur dioxide gas. When sulphur dioxide is dissolved in sulphurous acid is formed. The sulphurous acid turns litmus paper to Generally, oxides of are acidic in nature. After completing the paragraph, write two questions which you can raise on the basis of this information.
Answer. Oxygen, water, blue, red, non-metals.
Questions may be :

1. Which gas is formed when sulphur reacts with oxygen?
2. What is the nature of oxides of non-metals?

Question. 31 Find out the names of three metals and three non-metals from the box given as figure.

Answer.

Metals Iron, copper, aluminium Non-metals Sulphur, carbon, oxygen

Question. 32 Complete the crossword given in figure with the help of the clues.
Across
1. Which is generally hard, ductile, malleable and sonorous.
2. A metal is called so, it can be drawn into wires.
3. Metal bells are used because of this property.
Down
4. A metal generally used for making jewellery.
5. A metal which -is liquid at room temperature.
6. A metal which reacts with acid as f well as base to form hydrogen gas.
7. Substances used to enhance the growth of plants.
8. Property by virtue of which metals can be beaten into thin sheets.

Answer.

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NCERT Exemplar Class 8 Maths Chapter 13 Playing with Numbers are part of NCERT Exemplar Class 8 Maths. Here we have given NCERT Exemplar Class 8 Maths Chapter 13 Playing with Numbers. https://www.cbselabs.com/ncert-exemplar-problems-class-8-mathematics-playing-numbers/

NCERT Exemplar Class 8 Maths Chapter 13 Playing with Numbers

Multiple Choice Questions
Question. 1 Generalised form of a four-digit number abdc is
(a) 1000a + 100b + 10c + d (b) 1000a + 100c + 10b + d
(c) 1000a + 100b + 10d + c (d) a x b x c x d
Solution. (c) In generalised form, we express a number as the sum of the products of its digits with their respective place values.
abdc is written in generalised form as 1000a + 100b + 10d + c.
i.e. abdc = 1000a + 100b + 10d + c

Question. 2 Generalised form of a two-digit number xy is
(a)x + y (b)10x + y (c)10x-y (d)10y+x
Solution. (b) In generalised form, xy can be written as the sum of the products of its digits with their respective place values, i.e.xy = 10x+ y

Question. 3 The usual form of 1000a + 10b + c is
(a) abc (b) abc0 (c) a0be (d) ab0c
Solution. (c) Given expanded (or generalised) form of a number is 1000a + 10b + c. Then, we have to find its usual form.
We can write it as 1000 x a + 100 x 0 + 10 x b + c
i.e. a0bc, which is the usual form.

Question. 4 Let abc be a three-digit number. Then, abc – cba is not divisible by
(a) 9 (b) 11 (c) 18 (d) 33
Solution. (c) Given, abc is a three-digit number.
Then, abc = 100a + 10b + c
and cba = 100c + 10b + a
abc -eba = (100a + 10b + c)- (100c + 10b + a)
= 100a – a + 10b – 10b + c -100=
= 99a – 99c = 99 (a -c)
= abc-cba is divisible by 99.
=> abc – cba is divisible by 9,11,33, but it is not divisible by 18.

Question. 5 The sum of all the numbers formed by the digits x, y and z of the number xyz is divisible by (a) 11 (b) 33 (c) 37 (d) 74
Solution. (c) We have, xyz + yzx + zxy
= (100x + 10y + z) + (100y + 10 z+ x) + (100z+ 10x + y) …(i)
= 100x + 10x + x + 10y + 100y + y + z+ 100z+ 10Z
= 111x + 111y + 111z = 111 (x + y + z)
= 3 x 37 x (x + y + z)
Hence, Eq. (i) is divisible by 37, but not divisible by 11,33 and 74.

Question. 6 A four-digit number aabb is divisible by 55. Then, possible value(s) of b is/are
(a) 0 and 2 (b) 2 and 5 (c) 0 and 5 (d) 7
Solution. (c) It is given that, aabb is divisible by 55. Then, it is also divisible by 5.
Now, if a number is divisible by 5, then its unit digit is either 0 or 5.
Hence, the possible values of b are 0 and 5.

Question. 7 Let abc be a three-digit number. Then, abc + bca + cab is not divisible by
(a) a + b + c (b) 3 (c) 37 (d) 9
Solution. (d) We know that, the sum of three-digit numbers taken in cyclic order can be written as 111 (a + b + c).
i.e. abc + pea + cab = 3 x 37 x (a + b + c)
Hence, the sum is divisible by 3, 37 and (a + b + c) but not divisible by 9.

Question. 8 A four-digit number 4ob5 is divisible by 55. Then, the value of b-a is
(a) 0 (b) 1 (0 4 (d) 5
Solution. (b) Given, a four-digit number 4ab5 is divisible by 55. Then, it is also divisible by 11.
The difference of sum of its digits in odd places and sum of its digits in even places is either 0 or multiple of 11.
i.e. (4 + b) – (a + 5) is 0 or a multiple of 11, if 4 + b — a — 5 = 0 => b-a = 1

Question. 9 If abc is a three-digit number, then number abc -a-b-c is divisible by
(a) 9 (b) 90 (c)10 (d) 11
Solution. (d) We have, abc= 100a + 10b+c
.-. abc – a – b-c = (100a + 10b + c)-a – b-c = 100a – a + 10b – b = 99a + 9b = 9(11a + b)
Hence, the given number abc – a – b-c is divisible by 9.

Question. 10 A six-digit number is formed by repeating a three-digit number. For example, 256256, 678678 etc. Any number of this form is divisible by (a) 7 only (b) 11 only (c) 13 only (d) 1001
Solution.

Question. 11 If the sum of digits of a number is divisible by three, then the number is always divisible by (a) 2 (b) 3 (c) 6 (d) 9
Solution. (b) We know that, if sum of digits of a number is divisible by three, then the number must be divisible by 3, i.e. the remainder obtained by dividing the number by 3 is same as the remainder obtained by dividing the sum of its digits by 3.

Question. 12 If x +y + z = 6 and z is an odd digit, then the three-digit number xyz is
(a) an odd multiple of 3 (b) an odd multiple of 6
(c) an even multiple of 3 (d) an even multiple of 9
Solution. (a) We have, x + y+ z= 6 and z is an odd digit. Since, sum of the digits is divisible by 3, it will also be divisible by 2 and 3 but unit digit is odd, so it is divisible by 3 only.Hence, the number is an odd multiple of 3.

Question. 13 If 5A + 53 = 65, then the values of A and B is
(a) A = 2,8 = 3 (b) A = 3,8 = 2 (c)A = 2,8 = 1 (d)A=1,8 = 2
Solution.

Question. 14 If A3 + 8B = 150, then the value of A + B is
(a) 13 (b) 12 (c) 17 (d) 15
Solution. (a) We have, A3+ 8B = 150
Here, 3 + B = 0, so 3 + B is a two-digit number whose unit’s digit is zero.
.-. 3+B = 10=>B = 7
: Now, considering ten’s column, A+ 8 + 1 = 15
= A + 9 = 15
=> A = 6
Hence, A+B=6+7 = 13

Question. 15 If 5A x A = 399, then the value of A is
(a) 3 (b) 6 (c) 7 (d) 9
Solution. (c) We have, 5A x A = 399
Here, A x A= 9 i.e. A x A is the number 9 or a number whose unit’s digit is 9. Thus, the number whose product with itself produces a two-digit number having its unit’s digit as 9 is 7.
i.e. A x A = 49 => A=7
Now, 5 x A + 4 = 39
=> 5 x 7+4 = 39
So, A satisfies the product.
Hence, the value of A is 7.

Question. 16 If 6A x B = > 488, then the value of A-B is
(a)-2 (b) 2 (c) -3 (d) 3
Solution. (c) Given, 6A x B = A86
Let us assume, A = 1 and S = 3 Then, LHS = 61 x 3 = 183 and RHS = 183 Thus, our assumption is true.
A-6 = 1-3=-2

Question. 17 Which of the following numbers is divisible by 99?
(a) 913462 (b) 114345 (c) 135792 (d) 3572406
Solution. (b) Given a number is divisible by 99.
Now, going through the options, we observe that the number (b) is divisible by 9 and 11 both as the sum of digits of the number is divisible by 9 and sum of digits at odd places = sum of digits at even places.

Fill in the Blanks
In questions 18 to 33, fill in the blanks to make the statements true.
Question. 18 3134673 is divisible by 3 and————-.
Solution. 9
3134673 is divisible by 3 and 9 as sum of the digits, 3+1+3+4+6+7 + 3 = 27 is divisible by both 3 and 9.

Question. 19 20 x 3 is a multiple of 3, if the digit x is——–or——— or————.
Solution. 1,4,7
We know that, if a number is a multiple of 3, then the sum of its digits is again a multiple of 3, i.e. 2+0+x+3 is a multiple of 3.
x + 5 = 0, 3, 6, 9, 12, 15 But, x is a digit of the number 20 x 3.
x can take values 0, 1,2, 3,……..9.
=> x + 5 = 6 or 9 or 12
Hence, x = 1 or4 or 7

Question. 20 3 x 5 is divisible by 9, if the digit x is————.
Solution. 1
Since, the number 3 x 5 is divisible by 9, then the sum of its digits is also divisible by 9. i.e. 3 + x + 5 is divisible by 9.
=>x + 8 can take values 9,18, 27,…
But x is a digit of the number 3 x 5, so x = 1.

Question. 21 The sum of a two-digit number and the number obtained by reversing the digits is always divisible by———–.
Solution. 11
Let ab be any two-digit number, then the number obtained by reversing its digits is ba.
Now, ab + ba = (10a + b) + (10b + a) = 11a + 11b = 11(a + b)
Hence, ab + ba is always divisible by 11 and (a + b).

Question. 22 The difference of two-digit number and the number obtained by reversing its digits is always divisible by ———-.
Solution. 9
Let ab be any two-digit number, then we have
ab – ba = (10a + b)- (10b + a)
= 9a – 9b = 9(a – b)
Hence, ab – ba is always divisible by 9 and (a – b).

Question. 23 The difference of three-digit number and the number obtained by putting the digits in reverse order is always divisible by 9 and——-.
Solution. 11
Let abc be a three-digit number, then we have
abc -cba = (100a + 10b + c)- (100c + 10b+ a) ; = (100a – a) + (c – 10Cc)
= 99a – 99c = 99(a -c)
= 9 x 11 x (a – c)
Hence, abc – cba is always divisible by 9,11 and (a – c).

Question. 24

Solution.

Question. 25

Solution.

Question. 26

Solution.

Question. 27 1 x 35 is divisible by 9, if x =———.
Solution.

Question. 28 A four-digit number abed is divisible by 11, if d + b =———–or————-.
Solution. a + c,12(a + c)
We know that, a number is divisible by 11, if the difference between the sum of digits at odd places and the sum of its digits at even places is either 0 or a multiple of 11.
Hence, abcd is divisible by 11, if (d + b)- (a + c) = 0,11,22, 33,…
=> d + b = a + c or d + b = 12(a + c)

Question. 29 A number is divisible by 11, if the differences between the sum of digits at its odd places and that of digits at the even places is either 0 or divisible by ————-.
Solution. 11
By test of divisibility by 1,1, we know that, a number is divisible by 11, if the sum of digits at odd places and even places are equal or differ by a number, which is divisible by 11.

Question. 30 If o three-digit number abc is divisible by 11, then——–is either 0 or multiple of 11.
Solution. (a+c)-b
Since, abc is divisible by 11, the difference of sum of its digits at odd places and that of even places is either zero or multiple of 11, i.e. (a + c) – b is either zero or multiple of 11.

Question. 31 If A x 3 = lA then A =————.
Solution.

Question. 32 If B x B = AB, then either A = 2, B = 5 or A =———- B =———-.
Solution.

Question. 33 If the digit 1 is placed after a two-digit number whose ten’s is t and one’s digit is u, the new number is———–.
Solution. tu1
Given, a two-digit number whose ones digit isu in and tens digit isu. If the digit 1 is placed after this number, then the next number will be tu1.

True/False
In questions 34 to 44, state whether the given statements are True or False.
Question. 34 A two-digit number ab is always divisible by 2, if b is an even number.
Solution. True
By the test of divisibility by 2, we know that a number is divisible by 2, if its unit’s digit is even.

Question. 35 A three-digit number abc is divisible by 5, if c is an even number.
Solution. False
By the test of divisibility by 5, we know that if a number is divisible by 5, then its one’s digit will be either 0 or 5, i.e. the numbers ending with the digits 0 or 5 are divisible by 5.

Question. 36 A four-digit mmbeFabcd is divisible by 4, if ab is divisible by 4.
Solution. False
As we know that, if a number is divisible by 4, then the number formed by its digits in unit’s and ten’s place is divisible by 4.

Question. 37 A three-digit number abc is divisible by 6, if c is an even number and a + b + c is a multiple of 3.
Solution. True
If a number is divisible by 6, then it is divisible by both 2 and 3. Since, abc is divisible by 6, it is also divisible by 2 and 3. Therefore, c is an even number and the sum of digits is divisible by 3, i.e. multiple of 3.

Question. 38 Number of the form 3N + 2 will leave remainder 2 when divided by 3.
Solution. True
Let x = 3N + 2. Then, it can be written as.
x = (a multiple of 3) + 2
i.e. x is a number which is 2 more than a multiple of 3
i.e. x is a number, which when divided by 3, leaves the remainder 2.

Question. 39 Number 7N+1 will leave remainder 1 when divided by 7.
Solution. True
Given, a number of the form 7N + 1 = x (say)
Here, we observe that * is a number which is one more than a multiple of 7. i.e. when x is divided by 7, it leaves the remainder 1.

Question. 40 If a number a is divisible by b, then it must be divisible by each factor of b.
Solution. True
Given, a is divisible by b.
Let b = p1•p2, where p1 and p2 are primes.
Since, a is divisible by b, a is a multiple of b
i.e. a = mb
=> a = m.p1.p2
or a=cp2=dp1, where c = mp1, d = mp2
=>a is a multiple of p1 as well as p2.
Hence, a is divisible by each factor b.

Question. 41 If AB x 4 = 192, then A + 6 = 7.
Solution.

Question. 42 If AB + 7C = 102, where \(B\neq 0\), \(C\neq 0\), then A + B + C = 14.
Solution.

If B= C = 1, lf8 = 5,C=7, A = 2 and A +B+C=2 + 5+ 7 = 14

Question. 43 If 213 x 27 is divisible by 9, then the value of x is 0.
Solution. False
Given, 213 x 27 is divisible by 9, so sum of its digits is also divisible by 9.
i.e. 2 1 + 3 + x + 2 + 7 — 0, 9,18, 27, 36,…
=> x+ 15= 0,9,18,27,36,…
=> x + 15 = 18 [x is a digit of a number]
=> x= 3

Question. 44 In N + 5 leaves remainder 3 and \(N\div 2\) leaves remainder 0, then \(N\div 10\) leaves remainder 4.
Solution.

Question. 45 Find the least value that must be given to number a, so that the number 91876a2 is divisible by 8.
Solution. Given, 91876a2 is divisible by 8.
Since, we know that, if a number is divisible by 8, then the number formed by last 3 digits is divisible by 8.
So, 6a2 is divisible by 8.
Here, a can take values from 0 to 9.
For a = 0, 602 is not divisible by 8.
For a = 1, 612, which is not divisible by 8.
For a = 3, 632 is divisible by 8.
Hence, the minimum value of a is 3 to make 91876a2 divisible by 8.

Question. 46

Solution.

Question. 47 If 1AB + CCA = 697 and there is no carry-over in addition, find the value of A + B + C.
Solution.

Question. 48 A five-digit number AABAA is divisible by 33. Write all the numbers of this form.
Solution. Given, a number of the form AABAA is divisible by 33. Then, it is also divisible by 3 and
11, as if a number a is divisible by b, then it is also divisible by each factor of b.
Since, AABAA is divisible by 3, sum its digits is also divisible by 3. i.e. 4 + 4 + 8 + A + .4 = 0,3, 6,9…
or 4/4 + 8 = 0, 3, 6 9,… …(i)
From Eq. (i), we have
Further, the given number is also divisible by 11, therefore (2/4 + 8) – 2A = 0,11,22,…
B=Q 11,22,…
8 = 0 [v8 is a digit of the given number]
4/4 = 12or 24 or 36 A= 3, 6 9
Hence, the required numbers are 33033, 66066 and 99099.

In questions 49 to 60, find the value of the letters in each of the following questions.
Question. 49

Solution.

Question. 50

Solution.

Question. 51

Solution.

Question. 52

Solution.

Question. 53

Solution.

Question. 54

Solution.

Question. 55

Solution.

Question. 56

Solution.

Question. 57

Solution.

Question. 58

Solution.

Question. 59

Solution.

Question. 60

Solution.

Question. 61 If \(27\div A\)= 33, then find the value of A
Solution. We observe that, 4 x 3 can never be a single digit number 2, so 4 x 3 must be a two-digit number, whose ten’s digit is 2 and unit’s digit is the number less than or equal to 4. Therefore, the value of 4 can be 9, as the values of 4 from 1 to 8 do not fit.

Question. 62 212 x 5 is a multiple of 3 and 11. Find the value of x.
Solution. Since, 212 x 5 is a multiple of 3,
2 +1 + 2 +x+5 = 0, 3, 6,9,12,15,18,
=> 10 + x = 0, 3, 6,………..
=> x =2, 5, 8 …(i)
Again, 212×5 is a multiple of 11, (2 + 2 + 5) — (1 + x) = 0,11,22, 33
=> 8 – x = 0,11,22,…
=> x = 8 …(ii)
From Eqs. (i) and (ii), we have ‘
x = 8

Question. 63 Find the value of k, where 31K2 is divisible by 6.
Solution. Given, 31k2 is divisible by 6. Then, it is also divisible by 2 and 3 both.
Now, 31K2 is divisible by 3, sum of its digits is a multiple of 3.
i.e. 3+ 1 + k + 2 = 0, 3, 6, 9,12,…
=> k+ 6 = 0, 3,6, 9,12
=> k = 0 or 3, 6, 9

Question. 64 1y3y6is divisible by 11. Find the value of y.
Solution. It is given that, 1y3y6 is divisible by 11.
Then, we have (1 + 3 + 6) – (y + y) = 0,11,22,…
=> 10-2y= 0,11,22,…
=> 10-2y = 0 [other values give a negative number]
=> 2y = 10
=> y= 5

Question. 65 756x is a multiple of 11, find the value of x.
Solution. We are given that, 756x is a multiple of 11. Then, we have to find the value of x.
Since, 756x is divisible by 11, then (7 + 6) – (5 + x) is a multiple of 11,
i.e. 8-x = 0,11,22,…
=> 8- x = 0
=> x = 8

Question. 66 A three-digits number 203 is added to the number 326 to give a three-digits number 5b9 Which is divisible by 9. Find the value of b – a.
Solution.

Question. 67 Let E = 3, B = 7 and A = 4. Find the other digits in the sum

Solution.

Question. 68 Let D = 3, L = 7 and A = 8. Find the other digits in the sum

Solution. In the first column, we have
3 + S + 8, which is definitely a two digits number whose unit’s digit is 7.
S must be 6.
Now, in second column, 2A +1 = 16+1 = 7 [1 is carry forward]
In third column, M + 1 is a 2 digit number, therefore M must be 9.
Then, M + 1 = 9+ 1 = 10 6 = 1, U = 0
Hence, S = 6, M = 9, 6 = 1 and U = 0

Question. 69 If from a two-digit number, we subtract the number formed by reversing its digits then the result so obtained is a perfect cube. How many such numbers are possible? Write all of them.
Solution. Let ab be any two-digit number. Then, the digit formed by reversing it digits is ba.
Now, ab-ba = (10a+b)-(10b +a)
=(10a-a)+(b-10b)
= 9a – 9b = 9(a – b)
Further, since ab-ba is a perfect cube and is a multiple of 9.
.-.The possible value of a – b is 3.
i.e. a = b + 3
Here, b can take value from 0 to 6.
Hence, possible numbers are as follow.
For b = 0, a = 3, i.e. 30
For b = 1, a = 4, i.e. 41
For b =2, a = 5, i.e. 52
For b = 3, a = 6, i.e. 63
For b = 4, a = 7, i.e. 74
For b = 5,a = 8, i.e. 85
For b = 6, a = 9, i.e. 96

Question. 70 Work out the following multiplication.

Use the result to answer the following questions.
(a) What will be 12345679 x 45?
(b) What will be 12345679 x 63?
(c) By what number should 12345679 be multiplied to get 888888888?
(d) By what number should 12345679 be multiplied to get 999999999?
Solution.

Question. 71 Find the value of the letters in each of the following.

Solution.

Question. 72 If 148101B095 is divisible by 33, find the value of B.
Solution. Given that the number 148101S095 is divisible by 33, therefore it is also divisible by 3 and 11 both.
Now, the number is divisible by 3, its sum of digits is a multiple of 3. i.e. 1 + 4+ 8+1 + 0+1 + B+ 0+ 9+ 5 is a multiple of 3.
29 + B = 0, 3, 6,9,…
=> B=1,4,7 …(i)
Also, given number is divisible by 11, therefore
(1+ 8 + 0+ B + 9)-(4 + 1+ 1+ 0 + 5)=0, 11,22, …
=> (18 + B) -11 = 0,11,22
B+ 7 = 0,11,22
=> B+7 = 11 => B = 4 …(ii)
From Eqs. (i) and (ii), we have B = 4

Question. 73 If 123123A4 is divisible by 11, find the value of A.
Solution. Given, 12312344 is divisible by 11, then we have (1 + 3 + 2 + 4) – (2 + 1 + 3 + 4) is a multiple of 11.
i.e. (6+ 4)-10 =0,11,22,,..
=> A-4 = 0,11,22,…
=> A-4 = 0 [A is a digit of the given number]
=> A = 4

Question. 74 If 56 x 32y is divisible by 18, find the least value of y.
Solution. It is given that, the number 56 x 32y is divisible by 18. Then, it is also divisible by each factor of 18.
Thus, it is divisible by 2 as well as 3.
Now, the number is divisible by-2, its unit’s digit must be an even number that is 0, 2,4, 6, Therefore, the least value of y is 0.
Again, the number is divisible by 3 also, sum of its digits is a multiple of 3. i.e. 5 + 6 + x + 3 + 2 + y is a multiple of 3
=> 16 + x + y = 0, 3, 6, 9,…
=> 16+ x = 18
=>x = 2, which is the least value of x.

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NCERT Exemplar Class 8 Science Chapter 1 Crop Production and Management are part of NCERT Exemplar Class 8 Science. Here we have given NCERT Exemplar Class 8 Science Chapter 1 Crop Production and Management. https://www.cbselabs.com/ncert-exemplar-problems-class-8-science-crop-production-management/

NCERT Exemplar Class 8 Science Chapter 1 Crop Production and Management

Multiple Choice Questions
Question . 1 Which one of the following conditions is not essential to grow maize?
(a) High temperature (b) Humidity
(c) Low temperature (d) Rainfall
Answer. (c) Maize is a kharif crop, which requires high temperature, rainfall and humidity to grow, while rabi crops require low temperature for their growth,

Question. 2 Propagation of ginger is generally done using
(a) seed (b) stem (rhizome)
(c) root (d) leaf
Answer. (b) The ginger is a modified underground stem, which is propagated vegetatively by stem (rhizome) cultivation. It does not propagate by roots, leaves or seeds.

Question. 3 Which of the following statements is not true for organic manure?
(a) It enhances water holding capacity of soil
(b) It has a balance of all plant nutrients
(c) It provides humus to soil
(d) It improves texture of soil
Answer. (b) Organic manures are obtained from decomposition of plant and animal wastes that are rich in nutrients and humus.
But all nutrients might not be present in a proportionate amount in it, thus will not solve : the problem if soil is particularly deficient in one or more specific nutrient.

Question. 4 The term used for the process of separation of grains from chaff is
(a) sieving (b) winnowing
(c) threshing (d) hand picking
Ans. (c) The process of separating the edible part of the harvested grains (crop) by loosening and removing the scaly, inedible chaff surrounding the grain.
The process utilises a machine called ‘combine’ which is a combination of harvester and a thresher.

Question. 5 Read the statements given below.

1. Seeds require moisture for germination.
2. Plants can absorb nutrients mostly in dissolved form.
3. Irrigation protects crops from both frost and hot air currents.
4. Irrigation improves soil texture.

Choose combination of statements which indicate need to irrigate crops.
(a) (1) and (2) (b) (1), (2) and (3)
(c) (1), (2), (3) and(4)-,(d) (1) and (3)

Answer. (a) The combination of statements (1) and (2) indicate need for irrigation.
Irrigation is the supply of water to crops at regular intervals as water or moisture is necessary for seed germination, growth and other physiological processes. Moreover, the plants readily absorb nutrients in dissolved form along with water through root hairs of roots.
It does not protect crops from frost and hot air currents and manure not irrigation improves soil texture.

Question. 6 Which of the following tools would a farmer use to remove weeds from the field?
(a) Hoe (b) Plough (c) Axe (d) Cultivator
Answer. (a) Hoe is a simple tool used for removing weeds from the field and loosening of soil, it works like a blade, pulled by the animals In the field. Plough is used for tilling of soil, axe is used for cutting trees and cultivator is another tool implemented for ploughing,

Question. 7 Which of the following is not true for fertilisers?
(a) They increase the yield
(b) Their excessive use disturbs the balance of nutrients in soil
(c) They are generally used in small quantity
(d) They are environment friendly
Answer. (d) Fertilisers are not environment friendly as they are chemical substances rich in particular nutrient, Though, they increase the crop yield, the repeated and excessive use of fertilisers in the same field reduces the soil fertility, It also pollutes water bodies as they get dissolved and washed away by rain and also seeps in the water table posing threats to humans.

Question. 8 Given below are statements about the harmful effects of weeds on crop plants.

1. They interfere in harvesting.
2. They help crop plants to grow healthily.
3.  They compete with crop plants for water, nutrients, space and light.
4. They affect the plant growth.

(a) (1), (3) and (4)
(b) Only (3)
(c) (3) and (4)
(d) (1), (2), (3) and (4)

Answer. (a) Statements (1), (3) and (4) are correctly suggesting the harmful effects of weeds. Weeds are the undesirable plants that grow along the main crops.
Removal of weeds is necessary because they interfere and affect the major crop’s growth, by competing for light, water, nutrients and space.They also interfere during harvesting procedure.
Weeds do not help in healthy growth of crop plants.

Question. 9 The process of loosening and turning of soil is called
(a) irrigation and manuring
(b) digging and winnowing
(c) tilling and ploughing
(d) harvesting and storage
Answer. (c) The process of loosening and turning of soil is called tilling and ploughing . It is the first step in preparation of soil for cultivation of crop. Ploughing is done using a plough, made of wood or iron.

Question. 10 The monsoon season in our country is during the months
(a) April-December
(b) June-September
(c) November-March
(d) January-May
Answer. (b) The monsoon season in India is during the months June to September, The crops grown in this season are called kharif crop, e.g. paddy, maize, cotton, etc.
The kharif crops sown in June are harvested towards the end of rainy season (October-November).

Question. 11 The system of irrigation where water is supplied drop by drop near the roots of plants, is called
(a) pulley system
(b) drip system
(c) sprinkler system
(d) lever system
Answer. (b) Drip system is the method of irrigation which conserves water and avoids wastage as water is made to fall drop by drop at the position of roots. It is best technique for watering fruit plants, trees and gardens.
Sprinkler system is also a modern method of irrigation that efficiently uses water in the form of sprinklers. It is useful for uneven lands, while pulley and lever system are traditional methods of irrigation where a lot of water is wasted.

Very Short Answer Type Questions
Question.12 Pick out the odd one from the following words given in the box and give reason for it. Plough, Seed drill, Hoe, Chain pump. Sickle
Answer. Seed drill is the odd one in the above given set of tools as it is a modern agricultural tool used to sow seeds at uniform distances and to properly cover them with soil.
Rest of the tools, i.e. plough (used for tilling), hoe (for removal of weeds and loosening of soil), chain pump (irrigation method) and sickle (tool for removing weeds) are all traditional implements used in agriculture.

Question. 13 If you are given a dry piece of land for cultivation, what will you do before sowing the seeds?
Answer. If the field is dry for cultivation of crops, the soil preparation is done accordingly before sowing of crops. Preparation of dry soil includes adequate watering to restore the moisture content and then tilled and ploughed to allow growth of microbes and aeration, Finally, the soil is turned and its crumbs are levelled and manures are mixed. The soil is ready for sowing of crops in the field.

Question. 14 State whether the following statements are True/False

1. Using good quality seed is the only criterion to get high yield.
2. Growing different crops in different seasons in the same field will deplete the soil of nutrients.
3. Ail crop plants are sown as seeds in the field.
4.  Cells of root nodules of leguminous plants fix nitrogen.
5. Freshly harvested grains must be dried before shoring.

Answer.

1.  False, apart from good quality seeds, using appropriate agricultural practices are important for getting higher yield.
2. False, it actually enriches the soil. Different crops utilises different nutrients which permits the replenishment of used nutrients. This method is also called as crop rotation.
3. False, some crop plants need transplantation.
4. False, Rhizobium (bacteria) present in the cells of root nodules of leguminous plants fix nitrogen.
5. True, the freshly harvested grains must be dried. The moisture content in them can spoil the grains or microbial infection may occur.
   Drying reduces’ the moisture content in grains, thus preventing the attack of different organisms.

Question. 15 During which months do farmers grow mustard in India?
Answer. Farmers grow mustard during the months from October-March. Mustard is a rabi crop that is sown in beginning of winters (October-November) as they need low temperature and harvested at the end of winter (April-May).

Question. 16 Which activity of the farmer can promote growth of earthworms and microbes in the field?
Answer. Ploughing of the soil during soil preparation creates spaces in the soil thus, leading to aeration and the soil also loosens which promotes the growth and survival of earthworms and microbes and release of nutrients in the soil

Short Answer Type Questions
Question. 17 Beera wants to practice crop rotation in his field. Suggest a rabi crop and a kharif crop which will replenish his field with nitrogen. Which crop replenishes nitrogen and why?
Answer. Rabi crops are wheat, pea, mustard while kharif crops are maize, paddy (rice) or soyabean, These are grown in different seasons and therefore, can very well be rotated alternatively. Pea and soyabean are leguminous plants which harbour bacteria, i.e, Rhizobium in their nodules, thus help in fixing nitrogen.
These nitrogen fixing plants can replenish nitrogen in the field and hence, Beera can easily practice crop rotation.

Question.18 Match the agricultural implements given in column A with their use given in column B.

Answer. The correct matching is as given :
(a) —(iii), (b)—(i), (c)—(ii), (d)—(iv)
(a) Harvesting is the process of cutting matured crops from close to the ground. It is done manually using a sickle.
(b) Ploughing is the process of loosening and turning of soil. It is done with hoe, made of wood or iron.
(c) Sowing is the process of adding seeds of the crop to be grown in field using the seed drill.
(d) Irrigation is the process of watering the fields which favours the germination and growth of seeds, A sprinkler is one method used to water the field.

Question. 19 What are organic foods?
Answer. The crops that are cultivated using organic substances like manures, etc and prohibit the use of harmful chemical substances like fertilisers, pesticides, weedicides, etc., are called organic foods.

Question.20 Crossword Puzzle From the word puzzle given as figure. Find at least eight words which are ‘farmer’s friends’. Classify them into living and non-living.

Answer. From the given puzzle, the farmer’s friends are:
Living Seed, Earthworm, Rhizobium, Microbes, Ox. Living components like seeds, microbes earthworm, etc., helps in growth of crops, enrichment of soil nutrients, etc,, microbes like Rhizobium fix the nitrogen in soil, earthworms loosens the soil.
Non-living Plough, Urea, NPK, Manure, Hoe. Non-living components support the living components and help in better yield of crops. Plough and the hoe are instruments used in ploughing and harvesting.
Manure and urea are used for enrichment of soil in nutrients.

Question. 21 (a) Name the tool used with a tractor for sowing seeds in a field.
(b) What are the advantages of using this tool?
Answer. (a) The tool used with a tractor for sowing seeds in a field at uniform distances is seed drill. Before the introduction of seed drill, the process was manual.
(b) The advantages of seed drill are:

1. Seeds are sown at a uniform distance and depth to avoid over crowding.
2. After sowing, seeds are covered by soil which prevent them from being eaten by birds.
3.  It saves time and labour.
4. Improves the ratio of crop yield by as much as nine times.

Question. 22 (a) Name the practice followed for large scale rearing of farm animals.
(b) What facilities are provided to farm animals?
Answer. (a) The science of rearing, caring, breeding and improvement of domesticated animals is known as animal husbandry. It is the practice of breeding and raising livestock like cows, buffaloes, pigs, horses and sheep, etc. This practice requires good care and management of livestock.
(b) The animals or livestock in animal husbandry practice should be provided with:

1. Appropriate shelter facilities
2. Food and water
3. Good hygienic practices
4. Medical facilities, etc.

Question. 23 Classify the following crops into kharif and rabi crops and write in the tabular column given below: Maize, paddy, mustard, pea, gram, wheat, groundnut, cotton

Answer.The crops which are sown in the rainy season, i.e. from June to September are classified as kharif crops.
The crops sown in the winter season, i.e generally from October to March are called as Rabi crops. From the above given crop list, the kharif and rabi crops can be grouped as

Long Answer Type Questions
Question. 24 Despite favourable climatic conditions, a farmer’s crop failed to give
good yield. Give the possible reasons for this.
Answer. If despite favourable climatic conditions, farmer’s crop failed to give good yield, the possibilities may be

1. selection of crops was not done correctly.
2. he did not use good quality and healthy seeds.
3. his field was not well irrigated.
4. manures/fertilisers were not timely and properly applied.
5. weeds were not removed.
6. seeds were not sown at appropriate distance.
7. the soil was not ploughed and levelled properly.

Question. 25 As a part of eco-club activity, students were asked to raise a kitchen garden in the school premises. They were provided with some materials given in the box. List the other materials you would require. How will you plan the garden? Write the steps.
Khurpi, water-can, spade, shovel
Answer. Besides the given tools, the other items required for raising a garden are seeds and seedling of vegetables, plants from nursery, kitchen waste, manure, water, soil.
Steps for raising the garden:

1. Kitchen waste will be collected and composted in a pit.
2. A patch of land will be identified for the garden, with adequate sunlight available.
3. Soil will be dug up and levelled with the help of a spade.
4. Sowing of seeds/ transplanting of seedlings will be done. Seeds should be uniformly placed with adequate spacing,
5. Select seeds/seedlings as per the season. Water the plants regularly with a water-can.
6. Compost will be applied.
7. Weeds will be removed periodically with the help of khurpi.

Question. 26 Paddy is a major cereal crop in our country.
(a) In which season is paddy cultivated?
(b) Discuss the method of sowing.
(c) What measures must be taken to prevent spoilage and insect attack of harvested grains?
Answer. (a) Paddy is cultivated in rainy season as it is a kharif crop.
(b) For sowing paddy, seeds are first grown in a nursery and later after selecting healthy seed plant lets, they are transplanted in the field. This allows us to select seedling which can survive in harsher environmental conditions for cultivation.
(c) Measures taken to prevent spoilage or damage of harvested crops from insect attack are:

1. Exposing them in sunlight for drying as freshly harvested crops have moisture.
2. Dried neem leaves are added to stored grains, so as to prevent damage from pests.
3. Specific chemical treatments are also available for preventing crops from pest
   infestations.
4. Fumigation of the storage area to remove its insects.

Question. 27 Unscramble the words related to crop production and its management and write in the boxes given beneath them.

Now, using the circled letters, form one word which is an activity carried out in fields, after maturation of crops.
Answer.

1. SILOS Used for storing of harvested grains.
2. THRESHING The process of harvested grains being separated from the chaff.
3. IRRIGATION The process of supplying water to a crop field at different intervals is called irrigation.
4. RIVER The source of water.

The letters in circles are I S HR GATVE Which forms HARVESTING.
Harvesting is the activity carried out in fields after the maturation of crops, i.e. the cutting of matured crop from close to the ground.

Question. 28 Given below is a conversation between two farmers Heera and Beera.
Heera Brother Beera, your maize crops look beautiful. They have grown pretty fast.
Beera Yes, I have sprayed urea this time. What about you?
Heera Well, I am still relying on good old cow dung. I am saving money for buying a tractor. Beera That’s good. Tractor saves a Lot of time and labour.
Heera Yes, it’s been very labour intensive for me and now these weeds have come up.
Beera Try weedicides, they are very effective.
Now, answer the following questions.

1. List the practices which are not environment friendly and why?
2. What is the advantage of modern agricultural implements over traditional ones?
3. Name one weedicide and the precaution, farmers must take during its application.

Answer.

1.  The practices used in field which are not environment friendly are use of fertilisers, pesticides, weedicides and other chemical substances that pose health hazards.
   Tractor though saves labour and time but contributes to air pollution.
2. Modern agricultural implements save both time and labour as compared to traditional equipments.
   They import more efficiency and accuracy in different agricultural procedures like sowing of seed.
3. The example of weedicide is 2,4-D. The precaution taken by farmers, while spraying it on crops is they should properly cover their nose and mouth, so as to avoid inhaling of these chemicals.
   Excessive use is harmful, hence they should limit the application of such chemicals.

Question. 29 For discussion
Despite being one of the growing economies, our country still has a large section of population going hungry and malnourished. Do you think an efficient crop production mechanism is the only solution to this? Discuss in the class what else needs to be done to root out hunger and malnutrition from our country.
Answer. India, a growing economy with agriculture is a main contributor in its growth.
Agriculture is science of cultivating plants, animals and other life forms for food and other sources, Despite large production of crops by agricultural practices, people still go hungry and malnourished. This happens because of inefficient crop production and management.
In India, an increase in production of food grains has been observed from 1960 to 2004. With efficient management, the crop production can be enhanced and improved to feed the country’s poor.
A few steps in this direction can be:

1. Selection of high field seeds for planting The seeds with increased yield and reduced growth period.
2. Management of crop production The financial aspects of agriculture should be made more approachable to farmers at all levels.
3. Information regarding Use of chemicals, manures, cropping patterns, nutrient quality and irrigation facilities should be discussed and propagated among farmers.
4. Protection of crops The agricultural produce should be shared efficiently to reduce the microbial, pest or other organisms from infesting the produce.Pest and weed control measers should be adopted.

A large amount of crops get spoiled during storage by different living and non-living factors.
Living factors include rodents, microbes and insects.
Non-living factors are moisture, temperature and hygiene of storage place.
Keeping these factors in mind, we can reduce the economic losses, increase the marketability and feed the country’s poor.

Question. 30 Terms related to agricultural practices are given below. Rearrange them in the correct sequence, harvesting, sowing, manuring, tilling and ploughing, irrigation, weeding.
Answer. The correct sequence in the series of agricultural practices is:
Tilling and ploughing ——–> Sowing ——> Manuring ——–> Irrigation ———> Weeding —–> Havesting

1. Tilling and ploughing It is the turning and loosening of soil to encourage growth of earthworms and microbes. It is the first step in preparation of soil.
2. Sowing After soil preparation, good and healthy seeds are selected and sowed to considerable depths in soil by either traditional or modern methods.
3. Manuring It is done to add humus, improve texture of soil as well as to replenish nutrients in soil.
4. Irrigation Crop fields are watered at regular intervals to provide moisture for growth and germination of seeds.
5. Weeding Weeding is removal of weeds that grow naturally between crops and compete with them for sunlight, space, water and nutrients. They should be removed before weeds mature, i.e. produce flowers and seeds,
6. Harvesting It is the process of cutting and gathering mature crops. It can be done manually by sickle or by harvester machine.

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