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NCERT Solutions for Class 8th: Ch 1 Crop Production
and Management Science
NCERT Solutions for Class 8th: Ch 1 Crop Production and Management Science

Exe
1. Select the correct word from the following list and fil in the blanks.

float, water, crop, nutient, preparation

(a) The same kind of plants grown and cultivated on a large scale ata place Is called

(©) The frst step before growing crops ls of he sai

(c) Damaged seeds would_on top of water.

(@) For growing a crop, suficient sunight and and from the sol
are essential

Answer:

(a) The same kind of plants grown and culivated on a large scale ata place is called
co.

(b) The frst step before growing crops is preparation of the soil
(c) Damaged seeds would float on top of water.

(4) For growing a crop, sufficient sunight and water and nutients from the soll are
essential

2. Match items in column A with those in column E.

A 8
() Khanterops (a) Food for cattle
(i) Rabicrops (0) Urea and super phosphate

(ii) Chemical fertilisers (c) Animal excreta, cow dung, urine and plant waste
(iv) Organic manure — (0) Wheat, gram, pea
(e) Paddy and maize

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Answer
A 8

() Khanterops (e) Paddy and maize

(i) Rabicreps (a) Wheat, gram, pea

(li) Chemical feriisers (b) Urea and super phosphate
(1) Organic manure — (c) Animal excreta, cow dung, urine and plant waste

3. Give two exemples of each,
(a) Khan crop
(b) Rabi crop

Answer
(a) Kharif crop: Paddy, maize
(b) Rabi crop: Wheat, gram.

4. Wite a paragraph in your own words on each of the fallowing,
(a) Preparation of sol

(b) Sowing

(©) Weeding

(4) Threshing

Answer
(a) Preparation of solis the fist step before growing of crop. It helps to turn the soil and
loosen it to allow the root to penetrate deep into it. The loosening ofthe Sol helps in the
‘growth of several soll microbes, earthworms etc, which enrich the sol with humus and
other essential nutrients. The process of turning and loosening is called ploughing. This
is done using a plough, hoe and cultvators,

(b) Sowing isthe process of putting the seeds into the sol for growing crops. We need to
Use quality seeds for sowing. Sowing is done manually are mechanical equipment ike
‘Seed Dril. Seeds of few plants Ike rice is frst grown in a separate area and then
transplanted in the fields

(6) Ina field many other undesirable plants may grow along with the crop, These
undesirable plants are called weeds. Farmers use many ways to remove weeds.
Methods to control weeds are: Tiling before sowing of crops, physical removal of weeds

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by uprooting or cutting them close tothe ground, using certain chemicals called
weedicides etc.

(6) Threshing is the process of separating grains or seeds from chaft.Itis done after
harvesting the crop. Itis usualy caried out with the help of a machine known as
‘Combine’. This machine Is a combined harvester and thresher. It harvests plants as well
as cleans grains.

5. Explain how ferlsers are diferent from manure

Answer

Manure isa natural substance prepared
by the decomposition of animal excreta
and plant wastes,

Manure is known to have a large
‘quantity of organic materials and very
litle amount of plant nurients,

Fertlisers are commercially availabe plant
putrients.

‘They can be organic or inorganic in nature.

‘They ensure healthy growin and development
‘of plants by providing them with nitrogen,
phosphorus, potassium, etc.

‘They help in enriching the soll with
organie matter and nutrients.

‘The addition of fertlisers to the soll requires
special guidelines such as dose timo, post
‘addition precautions, etc. to be followed,

‘The action of manure does not
require any special guidelines,

A fetiiser does not provide any humus to the
sol

Manure provides humus to the soll and
Increases sol fer.

ts excessive use causes water pollution. It
cannot replenish organic matter of soi.

Ttprotects the environment and helps In
recycling farm waste.

6. What i

Irigation? Describe two methods of imgation which conserve water.

Answer
‘Supply of water to crops at appropriate intecals is called inigation. The time and
frequency ofimigation varies according to diferent seasons, cops, and soll types.

‘Two methods of imigation which help in conservation of water are;

+ Sprinkler system: This system is more useful on uneven land, having fewer water
‘supplies. In this method, water is supplied using pipes to one or more central locations
Within the feld. When water is allowed to flow under high pressure with the help of a
pump, it gets sprinkled on the crops.

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—+ Drip system: In his system, water is delivered at or near the roots of plants, drop by
drop. This is the most efficient method ofigation as there Is no wastage of water at all
‘This method is important in areas where water availabilty is poor.

7. wheat is sown in te Kharf season, what would happen? Discuss.

Answer
I wheat is sown inthe kharif season (from June to October), then the whole crop might
get destroyed because of many factors such as lack of optimum temperature,
adaptability, avalabilty of pests, etc. Khan season indudes the rainy season, which is,
not favourable for ne growth of wheat crop. Therefore, wheat crop should not be sown
during this season

8. Explain how sol gets affected by the continuous plantation of crops ina fel,

Answer
I continuous plantation of crops is done in the fields then the sal wll become poor in
necessary nutrients Ike nitrogen, phosphorus, potassium, etc. Plants require nutrients
for their proper growth and functioning. When a farmer continues to grow crops one after
the other, then all nutients availabe in the soll reduce and the crop yield decreases
‘automaticaly

9. What are weeds? How can we control them?

Answer
Weeds are unwanted wild plants that grow in the feld. Weeds compete with the crop for
nutients, light, and space. As a resul, crop plants get lesser nutrients ight, and space
for their development.

‘Some important weeding methods are:

— Weeds can be controlled using weedicides, It is a chemical, which is sprayed in the
fields to kil all available weeds. Weedicides are not harmful to crops,

— Tiling before sowing of crops also helps in removing weeds. Tiling uproots the
‘weeds. The best time forthe removal of weeds is before they produce flowers and
seeds,

— The manual method of removing weeds is withthe help of a khurpi,ttinvolves regular
Lprooting or cutting of weeds dose tothe ground.

10, Arrange the following boxes in proper order to make a flow chart of sugarcane crop
production,

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Farmer
1 2 3 4
FRET Fogg
5 5 7

Answer

per] ren <= Gow]

11. Complete the following word puzzle with the help of clues given below.
Down

1. Providing water to the crops,

2. Keeping crop grains for a long time under proper conditions.

5. Certain plants of the same kind grown on a large scale.

Across:

3. A machine used for cuting the matured crop.

4. Arabi crop thatis also one of the pulses,

6. A process of separating the grain from chaf.
1

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Answer
Down

1. IRRIGATION
2, STORAGE
5. CROP

Across
3, HARVESTOR
4. GRAM

6. WINNOWING

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NCERT Solutions for Class 8th: Ch 2 Microorganisms :
Friend and Foe Science

NCERT Solutions for Class 8th: Ch 2 Microorganisms : Friend and Foe Science

Exercise:
1. Fill in the blanks:

(a) Microorganisms can be seen with the help of a

(b) Blue green algae ix__ direct from air to enhance fertity of sai
(€) Alcohal is produced with the help of

a) Cholera is caused by

Answer
(a) Microorganisms can be seen with the help of a microscope

(©) Blue green algae fx nitrogen direct from ai to enhance ferity of sol.
(€) Alcohol is produced with the help of yeast

(a) Cholera is caused by bacteria.

2. Tick the correct answer:

(a) Yeast is used in the production of
() sugar

(i alcohol

(ii) hydrochloric acid

(1) oxygen

> (i) alcohol

(b) The folowing is an antibiotic
() Sodium bicarbonate

(i) Steptomycin

(ii) Aleonot

(iv) Yeast

> (1) Streptomycin

(€) Carter of malaria-causing protozoan ls
(i) female Anopheles mosquito

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(i) cockroach
(iy housety
(but
» (D female Anopheles mosquito

(a) The most common carter of communicable diseases is
(ent

housety.

(ii) éragontty

iv) spider

> (i) housefly

(e) The bread or ili dough rises because of
(heat

(i) grinding

(li) growth of yeast cells

(iv) kneading

> (ii) grow of yeast cells

(f) The process of conversion of sugar into alcohol is called
(i) nitrogen tiation

(1) moulding

(ii) fermentation

(iv infection

» (i) fermentation

3. Match the organisms in Column! with their action in Column I,
Column! Columnil
[Bacteria |e) [Fing nitrogen

(iy [Rhizobium _|(b)] Setting of curs

(ii) | Lectobacitus | c) | Baking of bread

div) | Yeast (a) [causing malaria

{ [A protozoan_| (e) | Causing cholera

04) | a virus ( [causing AIDS

(g) | Producing antibodies

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Answer
Column! Column!
[Bacteria |(e) | Causing cholera

(iy [Rhizobium _| (a) | Fixing nitrogen

ti | Lactobacitus | (b) | Setting of curs

1) | Yeast (e) [Baking of bread

(u [A protozoan | ca) | Causing mataria

oi | a virus ( [Causing AIDS:

4, Can microorganisms be seen with the naked eye? If not, how can they be seen?

Answer
‘Microorganisms are too small so they cannot be seen with naked eye. They can be seen
with the help of a magnifying glass or microscope.

5. What are the major groups of microorganisms?

Answer
There are five major groups of micro-organisms:
+ Bacteria - They are single celled disease-causing micro-organisms. They can be
Spiral or rod- shaped,

+ Fungi - They are mostly mufceluiar disease-causing microbes. Bread moulds are
‘common examples of fungi
Protozoa - They mainly include organisms such as Amoeba, Plasmodium.ete. They
can be unicellular er muttcellular.
— Virus Viruses are disease-causing microbes that reproduce only inside the host
‘organism
— Algae - They include multcalllar, photosynthetic organisms such as Spirogyra,
(Chlamydomonas, etc.

6. Name the microorganisms which can fix atmospheric nitrogen inthe soi

Answer
Bacteria such as Rhizobium and certain blue-green algae can fix atmospheric nitrogen in

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the sol.
7.Wite 10 lines on the usefuness of microorganisms in our Ives.

Answer
\Micro-organisms are too small tobe seen through naked eyes. However, they are vital to
plants and the environment — Lactobacillus is used to form curd from milk

— Rhizobium present in the roots of pulse plants fx nitrogen from air and supply
nitrogen compounds to the pulse plants.

>> Micro-arganisms are also used in winemaking, baking, picking, and other food
making processes.

_ Alcoholic fermentation by yeast is widely used in the preparation of wine and bread.
— Microbes are used to reduce pollution, For example, decomposers such as bacteria
‘and fungi break down dead bodies and excreta to form Inorganic compounds, which can
be absorbed by plants.

+ Microbes also play an important role in the preparation of medicines. Antibiotics are
Chemicals produced by micro-organisms to kil bacteria. Peniciin is an antibotic made
from Peniclium,

— Bacteria presentin our intestine helps in proper digestion and release Vitmain 8.
which is absorbed by intestine,

— Many vaccines are prepared from micro-organisms. These vaccines are given to
children to protect tiem from disease.

— Certain microbes are also used inthe biological treatment of sewage and industrial
emuents,

— Yeast is used in making ils, bhaturas, bread, pastries and cakes.

8. Write a short paragraph on the harms caused by microorganisms.

Answer
\Micro-organisms cause diseases in animals. For example, in humans, bacteria cause
diseases such as tuberculosis, cholera, typhoid, etc. In cat, he foot and mouth
disease is caused by a virus. Also, several microbes cause diseases in plants. For
example, the productivity of wheat, orange, apple, ete is reduced due to microbial
diseases in plants. Certain microbes, on entering into our body, produce toxic
substances. This leads to food poisoning, Some micro-organisms such as fungus spol
our food. For example, bread when left unused under moist conditions gets spoilt by
fungus, producing a white cottonike growth on the bread,

9. What are antibiotics? What precautions must be taken while taking antibiotics?

Answer

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Antibiotics are medicines produced by certain micro-organisms to kil other disease-
causing micro-organisms,

Precautions to be taken while using antibiotics:

— Antblotics should be taken under the supervision of a well qualified doctor.

— Course (intake) of antibitics should be completed as per the prescription given by
the doctor.

— Antbiotics should be taken in the right amount and at te right ime, A wrong dose of
antibiotics makes the drug ineffective. Also, excessive consumption of drugs may kil the
Useful bacteria present in our body

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NCERT Solutions for Class 8th: Ch 3 Synthetic
and Plastics Science

res

NCERT Solutions for Class 8th: Ch 3 Synthetic Fibres and Plastics Science

Excorcis
1. Explain why some fibres are called synthetic,

Answer
‘Some fbres are called synthetic fibres because they are man-made. These fibres made
by the combination of diferent types of chemicals in industries and their nature are also
different in respect of natural fibre

2. Mark the correct answer.
Rayon is diferent fram synthetic res because

(a)ithas a sikke appearance.

()itis obtained from wood pulp.

(€) its flores can also be woven Ike those of natural bres.
> (b) tis obtained from wood pulp.

3. Fil in the blanks with appropriate words
(a) Synthetic bres are also called or fibres
(b) Synthetic fbres are synthesised from raw materials called
(€) Uke synthetic fibres, plastic Is also a

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

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

(©) Uke synthetc bres, plastic is also a polymer.

4, Give examples which indicate that nylon fibres are very strong,

Answer

Nylon fibers are very strong thus they are used to prepare ropes for mountaineering

‘They are also used to prepare parachutes.

5. Explain why plastic containers are favoured for storing food.

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Answer
Plastic containers are favoured for storing food because
= The plastics do not react withthe food stored in them,
— The plastics are light weight and are strong

— They are easy to handle and safe

=> They are cheap and ductile.

6. Explain the difference between thermoplastic and thermosetting plastics.

Answer

Thermosetting plastic Thermoplastic

‘Thermosettng plastic cannot be bent easily. It

Maps ‘Thermoplastic can be bent easily.

‘Thermosetting plastic cannot be softened by
heating, Thus, it cannot be reshaped once
moulded,

‘Thermoplastic can be softened easily
by heating. Thus, it can be reshaped.

7. Explain why the folowing are made of thermosetting plastics.
(a) Saucepan handies
(b) Electric plugs/switchesiplug boards

Answer
(a) The handies of saucepans are made of thermosetting plastics because its a bad
conductor of heat and so, do not get heated up while cooking, Soi is easier to handle
the utensil wile cooking. Moreover it does not get softened due to heat.

(b) Electrical plugs, switches and plug-boards are made up of thermosetting plastics
because its a bad conductor of electricity. I does not allow the electric current to pass
{through it, hus safe in using in electric appliances,

8. Categorize the materials of the folowing products into ‘can be recycled! and ‘cannot
be recycled!

Telephone instruments, plastic toys, cooker handles, carry bags, bal point pens, plastic
bows. plastic covering on electrical wires, plastic chairs, electrical switches.

Answer

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Cannot be recycled Gan bo recycled
Telephone Instruments Plastic toys
Cookerhandies Plastic chairs
Electrical switches Cary begs

Plastic covering on electrical wires
Ball point pens

Plastic bows

9. Rana wants to buy shits for summer. Should he buy cotton shirts or shirts made from
synthetic material? Advise Rana, giving your reason,

Answer
Rana should buy shirts made from cotton. This Is because cotton Is a good absorber of
water. It can soak the sweat coming out of our body and expose it othe environment.
‘Thus, it helps in evaporating the liquid (sweat), hereby cooling our body

10, Give examples to show that plastics are non-corrosive in nature,

Answer
Plastics are nancorrosive In nature because itis a nonreactive substance. It does not
react with air, water, minerals and other chemicals. Soin present days most of the
Containers, utensils and laboratory equipments are made with plastics to store food,
chemicals ete.

11. Should the handle and bristles of a tooth brush be made of the same material?
Explain your answer.

Answer
No. The handle and bristles of a tooth brush should be made of different materials. The
handle of a toothbrush should be hard and strong, while the bristles should be soft and
flexible.

12. ‘Avoid plastics as far as possible. Comment on this advice.

Answer
Plastics are non-blodegradable materials, They take several years to decompose. Itis

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not environment friendly and causes environment palluion. Also burning process of
plastics is quite slow. In the process of burning it releases lots of poisonous gas into the
almosphere causing air palluton. Some plastics can't be recycled and so, itis very
dificut to fnaly dispose off such materials. Polythene bags, carelessiy thrown in an
eating garbage can cause death for the animals like — cows, dogs etc or sometimes
even become a cause for clogging of the drains.

13, Match the terms of column A correcty with the phrases given in column B.

A 8
@ [Polyester
[Tetton | (b) | Used formating parachutes and stockings

Prepared by using wood pulp.

iy [Rayon _|(c) [uses to make non-stick cookware

ti [ton _| ta) | Fabrics do not wrinkle easly

Answer

A 8

(© |Powyester | (a) | Fabrics do not wrinkle easily

Gi [renon [co

Used to make non-stick cookware

iy [Rayon _ |(a)| Prepared by using wood pulp

(0) [ton | 0) | Used tor making parachutes and stockings

14, Manufacturing synthetc fbres Is actually helping conservation of forests. Comment.

Answer
‘The natural fibers are obtained from plants and animals, whereas the synthetic bers are
‘obtained by chemical processing of petrochemicals. Unlike natural fibers, manufacturing
of synthetic fibers do not require to cut trees or hunting of any animal. In this way
‘manufacturing synthetic fibers Is actually helping conservation forests

15, Describe an actvity to show that thermoplastic is a poor conductor of electcty

Answer

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We will design a circuit to see that thermoplastics are poor conductors of electricity, We
need a bulb, some wires, a battery, a piece of metal, and a plastic pipe. Set up the circuit
first withthe metal and then with the plastic pipe (as shown inthe figure). After you
‘switch on the current, you will observe that the bulb glows in the former case. In the
latter case, the buib does not glow. Hence, a plastic pipe (which isa thermoplastic) is
showin to be a poor conductor of elect.

Meal ‘Thermoplastic

er

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NCERT Solutions for Class 8th: Ch 4 Materials : Metals

and Non-Metals Science

NCERT Solutions for Class 8th: Ch 4 Materials : Metals and Non-Metals Science

Excorcis
1. Which of he following can be beaten into thin sheets?
(a)Zine

(b) Phosphorus.
(6) Sulphur

(4) Oxygen

> (a) Zinc

2. Which of the following statements is correct?
(a) Al metals are ductle,

(b) All non-metats are ductle

(€) Generally, metals are ductie.

(4) Some non-metals are ductile.
> (c) Generally, metals are ductie,

3. Fill in the blanks:

(a) Phosphorus is a very non-metal
(b) Metals are Conductors of heat and
(6) ron is_____ reactive than copper.

(4) Metals react with acids to produce. ges.

Answer
(a) Phosphorus is a very reactive non-metal

(b) Metals are good conductors of heat and electricity
(c) ron is more reactive than copper.

(@) Metals react with acids to produce hydrogen gas.

4, Mark 'T'iftho statement is true and Fifi is false.
(a) Generaly, non-metals react with acids. {)

> False

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(b) Sodium isa very reactive meta. ( )
> Tue
(c) Copper displaces zinc from zinc sulphate solution. ( )
> False

(4) Coal can be drawn into wires.()

> False Metals| Non-
metals

5. Some properties are listed inthe folowing Table. Distinguish

between metals and non-metals on the basis of these properties,

Properties

1. Appearance

2 Hardness

3, Malleabilty

4, Ductity

5. Heat Conduction

6, Conduction of Electricity

Answer

Properties Metals Non-metals

1. Appearance Lustrous Dull

2 Hardness Hard Sor

3, Malleabilty [Can be beaten into tin Cannot be beaten into thin

4 Ductity sheets sheets

5. Heat conduction |Canbe drawn into wires | Cannot be drawn into wires

6. Conduction of [Good conductors of heat | Poor conductors of heat

electicity | Good conductors of Poor conductors of electricity

electricity

6. Give reasons for the following.
(a) Aluminium folls are used to wrap food items.

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(b) Immersion rods for heating liquids are made up of metalic substances.
(c) Copper cannot displace zinc from ts salt solution
(a) Sodium and potassium are stored in kerosene.

Answer
(a) Aluminium fais are used to wrap food items because aluminium metal is malleable.
Therefore, it can be beaten into thin fois.

(b) Metals are good conductors of heat and electricity. Therefore, immersion rods for
heating liquids are made of metalic substances.

(©) À metal can displace a less reactive metal from its salt in an aqueous solution. But
Zinc is more reactive than copper. Therefore, copper cannot displace zinc from its salt
solution

Cuts} ZnSO, (aq) — No reaction

(6) Sodium and potassium are stored in kerosene because they are highly reactive
elements, They can easily catch fre even when in contact with ai.

7. Can you store lemon pickle in an aluminium utensi? Explain,
Answer

Lemon pickle cannot be stored in aluminium utensils because lemon pickle contains
‘acids, which can react with aluminium (metal) Iberating hydrogen gas. This can lead to
the spoiling ofthe picke.

8. Match the substances given in Column A with their uses given in Column B.

A 8
( [God |(a)| mermometers
@ [iron (0) | Electric wire

(ay | Aluminium | (c) | Wrapping food

(6) | Carbon — |(a)| Jewellery

q [Copper |(e) Machinery

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| Mercury |) | Fuet

Answer
A 8

© [Gola |(a)| Jewetery

@ [iron e) | Machinery

iy | Aluminium | (¢) | Virapping food

(1) carbon en [Fuer

(0 [Copper |) | Electic wire

4) | Mercury | (a) | thermometers

8. What happens when
(a) Dilute sulphuric acid is poured on a copper plate?
(b) ron nails are placed in copper sulphate solution?
Wirte word equations ofthe reactions involved,

Answer
(a) When diute sulphuric acid is poured on a copperplate, there will be no reaction
between copper and dilute sulphuric acid as copper is less reactve and hence no
products will be formed,

(b) Iron being more reactive displaces copper from copper sulphate solution In this
reaction, the blue colour of copper sulphate fades and there is deposition of copper on
the iron nal

Iron (Fe) + Copper Sulphate (CuSO.) ~ Iron Sulphate (FeSO,) + Copper (Cu)

10, Salon took a piece of buming charcoal and collected the gas evolved ina test tube,
(a) How will she And the nature of the gas?
(b) Write down word equations of all the reactions taking place in this process.

Answer

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(a) Add a few drops of water in the test tube containing gas. Now, cover the test tube
and shake it well After shaking, test the solution with blue litmus and red litmus. I will
tum blue litmus red, Thus, the gas is acidic in nature.

(b) Charcoal reacts with oxygen to form carbon dioxide gas.
O +0, -+ CO:
(Carbon from Charcoal) + Oxygen — Carbon Dioxide

(Carbon dioxide reacts with water o form carbonic acid, which tums blue itmus paper
red

CO: + HO — HaCO>

(Carbon Doixide) + Water ~ (Carbonic Acid - turns blue Itmus red)

11. One day Reeta went to a jewellers shop with her mother. Her mother gave an old
{gold jewellery to the goldsmith to polish, Next day when they brought the Jewellery back,
they found that there was a sight loss in its weight. Can you suggest a reason for the
loss in weight?

Answer
To polish a gold omament, itis cipped in a liquid called aqua regia (a mixture of
hydrochloric acid and nitric acid). On getting the environment of aqua regia, the outer
layer of gold dissolves and the inner shiny layer appears. The dssoiving of the layer
causes a reduction In the weight ofthe Jewellery.

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NCERT Solutions for Class 8th: Ch 5 Coal and
Petroleum Science

NCERT Solutions for Class 8th: Ch 5 Coal and Petroleum Science

Excor
1. What are the advantages of using CNG and LPG as fuels?

Answer
‘The advantages of using compressed natural gas (CNG) and Iqufied petroleum gas
(LPG) as fuels are:

— They can be burt directly

— They can be transported easily through pipe Ines.

— They are clean fuels and do not give smoke when bumt.

— They give a lot of heat energy when burnt.

2. Name the petroleum product used for surfacing of roads.

Answer
Bitumen, a petroleum product, is used for surfacing roads.

3. Describe how coal is formed from dead vegetation. What is this process called?

Answer
Milions of years ago, dense forests got buried under the soll due to natural processes
like stoms, floods, and earthquakes. These got compressed as more and more soll got
deposited over them. When they got buried deep in the soil, they were exposed to very
high pressure and temperature, Under these conditions, these slowly got converted into
Coa. This process of formation of coal from dead vegetation is called carbonization,

4. Fill in the banks.
(a) Fossil fuels are and,

(b) Process of separation of diferent constituents from petroleum Is called
(c) Least potting fue for vehicles is

Answer
(a) Fossil fuels are coal, petroleum and natural gas.

(b) Process of separation of diferent constituents from petroleum Is called refining.
(c) Least polluting fuel for vehicles is CNG.

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5. Tick TruolFalseagainot the following statements.
(8) Foss fuels can be made in the laboratory. (T/F)
> Falso

(BJ ENG Is more polig fuel han perl. (T/F)

» Face

(6) Coke is almost pure fom of caon.(T/F)

> True

(0) Coal taris a mur of various substances (T/F)
> True

(e) Kerosene isnot fosi fuel. (T/F)

> False
6. Explain why fossil fuels are exhaustible natural resources.

Answer
Fossil fuels require milions of years to form from the dead vegetation and animals that
get buried deep inside the Earth. They require high temperature and pressure for their
formation, which cannot be provided in the laboratory. Thus, foss are limited.
‘Therefore, the use of fossi fuels at this rate wil lead to their exhaustion.

7. Describe characteristics and uses of coke.

Answer
Characteristics of coke are:
= Tough

Porous

= Black in colour

Uses of coke:
= In manufacture of steel
— In the extraction of metals (as a reducing agent).

8. Explain the process of formation of petroleum.

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Answer
Petroleum was formed from dead organisms that got buried in the sea milions of years
ago. These dead bodies got covered with layers of sand and clay. Lack of air, high
temperature, and high pressure transformed these dead organisms into petroleum and
natural gas,

9. The following Table shows the total power shortage in India from 1991 - 1997. Show
the data in he form of a graph, Plot shortage percentage for the years on the Y-axis and
the year on the X-axis.

‘S.No. | Year | Shortage (%)
1 [ia] 7e
2 || 78
3 fio] ss
a [100] 74
5 |ies| 74
e |ios| 92
7 feel ns
Answer
i »
mue sa

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NCERT Solutions for Class 8th: Ch 6 Combustion and
Flame Science

NCERT Solutions for Class 8th: Ch 6 Combustion and Flame Science

Excor

1. List conditions under which combustion can take place.

Answer
‘The conditions required for combustion to take place are:

— Presence of a fuel

— Air (or oxygen)

— Igilion temperature (minimum temperature at which a substance catches fe)

2. Fill in the blanks:

(a) Burning of wood and coal causes, of air.
(b) A liquid fuel used in homes is
() Fuel must be heated tots before it starts buming.

(6) Fire produced by ail cannot be controlled by.

Answer
(a) Buming of wood and coal causes pallution of ar

(b) A liquid fuel used in homes is LEG.

(€) Fuel must be heated to its gnition temperature before it starts buming,
(a) Fire produced by oil cannot be controled by water.

3. Explain how the use of CNG in automobiles has reduced pollution in our cites.

Answer
‘Combustion of fuels lke petroleum causes formation of un-burnt carbon particles along
with carbon monoxide gas. These harmful pollutants enter the air and cause respiratory
diseases. Compressed Natural Gas (CNG) produces these harmful products in very less
quantity. Itis a comparatively cleaner fuel. Therefore, the use of CNG has reduced
Pollution in our its.

4. Compare LPG and wood as fuels.

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Answer

LPG Wood

thas more caloric value about 55000 ithastess calorific value about 17000 to
Kung 22000 Kung,

On burning wood release unbumt carbon

It does not cause any environmental ee

problem.

asthma
tis smokeless fuel I gives out ot of smoke

is easy to store and can be easily | itneeds a lot of space to store also very
trensportedin cyinders and pipelines. | cut to transport

Low ignition temprature High ignition temperature

5. Give reasons.
(a) Water is not used to control res involving electrical equipment.

(b) LPG is a better domestic fuel than wood.

(c) Paper by itself catches fre easily whereas a piece of paper wrapped around an
‘aluminium pipe does not

Answer
(a) Water is a conductor of electricity, so it can easily conduct electric current and cause
danger of electric shocks or short-cicuits. Therefore, water can not be used to control
the fre involving electrical equipment.

(b) LPG is a better domestic fuel as it does not produce smoke and un-burnt carbon,
Particles, which cause respiratory problems:

(€) Paper by itself catches fire easily because it has low ignition temperature but when
Wrapped around an aluminium pipe its temperature is lowered due to aluminium metal
absorbing the heat supplied to paper. Soit does not catch fre.

6. Make a labeled diagram of a candie fame.

Answer

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outerzonc of

hottest complete
par Combustion ie)
moderate”
hot middle zone of
paral combustion
least yellow)
hot
Anermest zone of
sax candle abr was
vapours (lack)

7. Name the unit in which the calorific value of a fuel is expressed,

Answer

‘The calorific value of a fuel is expressed in Kiljoules per kilogram (kg)
8. Explain how CO; is able to control fes.

Answer
Being heavier than oxygen, CO; covers the fre Ike a blanket. Since the contact between,
the fuel and

‘oxygen is cutoff, the fire is controled.

9. Itis dificut to bum a heap of green leaves but dry leaves catch fre easily. Explain,
Answer

Green leaves have a lot of moisture in them. This moisture does not allow them to catch
fire easily. However, dry leaves have no moisturein them. Therefore, they catch fre
easiy.

10. Which zone of a flame does a goldsmith use for meting gold and silver and why?
Answer

A goldsmith uses the outer pat ofthe candle flame for melting gold and silver because
in this zone the temperature is the highest which helps to melt tiese metals easly.

11. In an experiment 45 kg of a fuel was completely bumt. The heat produced was
measured to be 180,000 kJ. Calculate the calorif value of the fuel

Answer

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‘The calorific value of fuel is the amount of heat produced by the complete combustion of
1 kg of ful.

Now,

Heat produced by 4.5 kg of fuel = 180000 k/Therefore, heat produced by 1Kg of fuel =
180000/4.5= 1KIKg

= 40,000 kJkg

Hence, the calorific value of the fuel is 40,000 kung

12. Can the process of rusting be called combustion? Discuss,

Answer
‘Combustion is a chemical process in which a substance reacts with oxygen and gives.
out energy during the process in the form of either heat cr light or both. Rusting of ron is
an exothermic process as heat is released during rusting. Hence, itis a kind of slow
combustion.

3. Abida and Ramesh were doing an experiment in which water was to be heated in a
beaker. Abida kept the beaker near the wick in the yellow part of the candle flame.
Ramesh kept the beaker in the

‘outermost part of the lame, Whose water will get heated in a shorter time?

Answer
‘The water in the Ramesh's beaker will heat up in a shorter me. This is because the
‘outermost zone of aflame is the hottest zone, while the yellow zone (In which Abida had
kept the beaker) is less hot.

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NCERT Solutions for Class 8th: Ch 7 Conservation of

Plants and Animals Science

NCE}

Excercise
1. Fill in the blanks:

Solutions for Class 8th: Ch 7 Conservation of Plants and Animals Science

(8) A place where animals are protected in ther natural habitats called a _

(b) Species found only in a particular area are known as.

(6) Migratory birds fy to far away places because of _ changes.

Answer

(8) A place where animals are protected in their natural habitat is called a sanctuary.

(b) Species found only in a particular area are known as endemic.

() Migratory birds fy to far away places because of cimatic changes,

2. Difforentiato between the following.

(a) Wie sanctuary and biosphere reserve

Wildlife sanctuary

Biosphere reserve

itis an area within which animals are protected
from possible dangers such as hunting. Ther
habitat is also conserved inthis area,

lis a large protected area
constructed for the conservation of

Itprovides protection and suitable Ing
Conditions to wild animals

Ithelps in the conservation of various
life forms such as plants, animals, and
mmicto-orgenisms.

(0) Zoo and wildlife sanctuary

Zoo

Wild life sanctuary

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Zoos are smaller places having some
‘animals protected for the public view.

Wildlife Sanctuary are larger places having
wld animals in thei natural habitat

tis an artificial habitat

It conserves the natural habitat of animals.

provides protection oniy.

(c) Endangered and extinct species:

Tt provides protection and suitable ing
condition to wi animals

Endangered species

Extinct species

His a populaton of species that is on the
verge of becoming extinct,

Itis a population of species that no longer
‘exists. Hence, itis extinct.

Blue whale, tiger, leopard, etc. are
‘examples of endangered species.

Dodo, passenger pigeon, etc. are
‘examples of extinct species,

(a) Flora and fauna

Flora

Fauna

refers to all IMng plants in a particular
area,

It refers to all animals iting in a particular
area.

‘Sal, teak, mango, ee. form the flora of
Pachmarhi biosphere reserve,

Leopard, wolf, wild dog, etc. form the fauna
of Pachmarhi biosphere reserve

3. Discus

the effects of deforestation on the following,

(a) Wi animals: When forests are cut, the natural habitat of the animals is destroyed
Without the natural habitat the animals are left with no place to live and breed. As à
result ofthis many animals are at the verge of extinction.

(b) Environment: Environment Is adversely affected by deforestation as ecological
balance is disturbed, Carbon dioxide isnot absorbed by plants and it gets accumulated

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in he atmosphere to cause global warming which increases the temprature of earth and
disturb water cycle.

(c) Vilages (Ruaral Areas): Deforestation leads to sol erosion and results in flash floods,
This hampers agricultural activities and thus disturbs the people in rural areas.

(6) Cities (Urban Areas): Deforestation in cities can increase the risk of many natural
calamities such as floods and droughts in that area.

(€) Earth: The whole earth is ful of various ecosystems which together make the
biosphere. Deforestation disturbs the balance in the biosphere. It disturbs dimatc
pattems,

(N The next generation: They will not be able to see wide variety of fora and fauna also
not get clean and cool environment. They will have to breathe paisonaus gas. Life would
be very difcuï for the next generation,

4. What will happen if:
(a) we g0 on cutting tees.

(©) te habitat of an animal is disturbed.
(6) the top layer of sol is exposed.

Answer
(a) If we go on cutting trees then:

— Earth will loose is top fer sal layer and get conevrted into desert,

= The ecological balance will get disturbed and and floods and drought vil become
more frequent

— Willie will also be affected

(b) The habitat of an animal provides it with necessities such as shelter, food, and
protection. Ifthe habitat of an animal is disturbed, then it will be forced to goto other
places in search of food and shelter. The animal could get killed by other animals inthis
process.

(©) tthe top layer of sol is exposed, then it will gradually expose the lower layer of soll,
which is hard and rocky in nature. This type of sol less fertle as it contains less
humus, Continued sal erosion will make the land barren or infer,

5. Answer in brief:

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(a) Wny should we conserve biodiversity?

Answer

We should conserve biodiversity because it is very important for al ving organisms and
for the environment. We should conserve biodiversity to save it rom becoming extinct.

(b) Protected forests are also not completely safe for wild animals. Why?
Answer

Protected forests are not completely safe for wild animals because people who live near
or adjacent to forests use resources from forests to fui theie own requirements. In this
process, wild animals are killed and sold for lucrative amounts of money.

(c) Some tribals depend on the jungle. How?

Answer

‘Tribals gather food, fodder, and allen branches of trees rom forests. Hence, they
depend on forests for their daily requirements.

(a) What are the causes and consequences of deforestation?

Answer

“The main cause of deforestation is growing urbanization and industialization. Need for
more land and resources have lead tothe cutting down of forests
Consequences of deforestation are as follows:

= Sol erosion

— Depleton of groundwater

= Flash flood

+ Global warming

(e) Vinatis Red Data Book?
Answer

Red Data Book is a source book that maintains an international ist of all endangered
animal and plant species.

(0 What do you understand by the term migration?

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Answer
‘Migration refers to the movement of an organism or a group of organisms from ts
natural habitat to another place at a particular time every year.

6. In order to meet the everincreasing demand in factories and for shelter, tees are
being continually cut. I justi to cut trees for such projects? Discuss and prepare a
brief report.

Answer
"No, Itis not at al justified to cut trees to meet the ever increasing demands of human
Population. Forests are the habitat of several organisms including wild animals. They
provide us with good quality ar as they give out Os and absorb the harmful CO, gas
from the atmosphere. In he process, they prevent the excessive heating of the
atmosphere, They prevent sol erosion and natural calamities such as floods and
droughts. They increase the frlity ofthe soil and help conserve biodiversity. The
cutting of forests to meet the demands of growing human population will lead to global
warming, sol erosion, greenhouse effect, droughts, floods, and many more problems.
‘The destruction of forests wil disturb the balance of nature. Hence, forests must be
conserved.

7. How can you contribute t the maintenance of green wealth of your locality? Make a
lst of actions to be taken by you.

Answer
We can contibute tothe maintenance of green wealth by taking following actions:

+ We can grow new plants in our locality.

— Water them regularly and care of them.

— The existing plants should be property nourished,

+ Encouraging awareness among the people inthe locality about the importance of
growing trees.

8. Explain how deforestation leads to reduced rainfall
Answer

Plants absorb water from the soll and evaporates it to form clouds. Deforestation leads
Lo cuting down of trees. As a result less waters absorbed from the soll and it disturbs

the water cycle. The formation of clouds become difficult which leads to reduced rainfall

10, Why should paper be saved? Prepare a ist of ways by which you can save paper.

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Answer
Paper should be saved because i takes around seventeen fll grown trees to make one
tonne of paper. Trees, as we know, are important to maintain a balance of nature.
‘Therefore, in order to save trees and prevent the impact of their loss on living organisms,
we need to save paper.

Ways by which paper can be saved:

— Collect used paper and recycte it.

+ Use both sides of a paper for writing

— Spread awareness about the importance of paper.

— Use paper inteligenty.

11. Complete the world puzzle.
Down

1. Species on the verge of extinction

2. A book carrying information about endangered species,
5. Consequence of deforestation.

Across
1. Species which have vanished,

3. Species found only in a particular habitat.

4. Variety of plants, animals and microorganisms found in an area,

eT TTT Tel

SEE EE |

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Answer

Down
1. ENDANGERED
2. RED DATA BOOK

5. DESERTS
Across
4. EXTINCT
3. ENDEMIC
4. BIODIVERSITY
GEGEN]
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Rao!
lll
lol Mm
[Tool TefrR str Tr Ty)
lof Te]
lo] Is
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NCERT Solutions for Class 8th: Ch 8 Cell - Structure

and Functions Science
N

Solutions for Class 8th: Ch $ Cell - Structure and Functions Science

Excercise
4. Indicate whether the following statomonts aro True (T) or Falso (F).

(a) Unicelluar organisms have one-celled body. (T/F)
> True

(b) Muscle cells are branched. (T/F)
> Flase

(c) The basic living unit of an organism is an organ. (T/F)
> False

(4) Amoeba has iregular shape. (T/F)
> True
2. Make a sketch of the human nerve cal. What function do nerve cells pertom?

Answer

nudes

‘The function of a nerve cel o transmit messages to the brain and also to take away
messages from the brain to the receptor organs,

3. Write short notes on the following.

(a) Oytoplasm: It isa fluid that fis the cel and occurs between the plasma membrane:
and the nucleus. Cell organelles such as mitochondria, ribosomes, Golgi bodies, etc. are
suspended in the cytoplasm. The cytoplasm helps in the exchange of materials between
cell organelles,

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(b) Nucleus of a cel: Nucleus is the master ofthe cell. It commands all the functioning of
the cel [tis generally located in the center of the cell and is spherical in shape. A
membrane called nuclear membrane separates i rom cytoplasm. It contains the genetic
‘material DNA and RNA init, This porous membrane allows the transfer of material in the
nudeus and cytoplasm, Nucleus contains a dense body called Nucleolus which actually
contains Chromosomes, the genetic material,

4. Which pat ofthe call contains organelles?
Answer

CCytoplasm isthe part of the cel that contains various organelles such as mitochondria,
ribosomes, Golgi bodies, etc.

5. Make sketches of animal and plant cells. State three differences between them

Answer

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‘Animal cell Plant call
They are generally small in size, They are usualy lege than
Cell wallis absent. Cell wal is present
Vacuoles are smallin size Vacuoles are larger in size
No other animal cel possesses plastids except for the | Plastids are present
Protozoan Euglena,

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6. State the difference between eukaryotes and prokaryotes,

Answer

Prokaryotes Eukaryotes

Most prokaryotes are unicellular. Most eukaryotes are multceluar.

The nucleus is poorly defined dueto the | The nucleus is well defined and is

absence of a nuclear membrane. surrounded by a nuclear membrane,

‘Nucleolus is absent Nucleolus is present

‘Call organelles such as plastids, ‘Gell organalles such as plastas,

mitochondria, golg bodies, etc. are absent. | mitochondria, galgi bodies, etc. are
present

Bacteria and blue-green algae are Fungi, plant, and animal cells are

prokaryotic cals. ‘eukaryotic cells,

7. Where are chromosomes found in a cell? State their function

Answer
(Chromosomes are found in the nucleus of a cell, Their function isto carry characteristic
features of parent cals to the daughter cell means, from parent to offspring

8.'Cells are the basic structural units of living organisms’. Explain,

Answer
In Biology, he basic unit of which all ving things are composed is known as ‘cell. The
‘call is the smallest structural unit of living matter thats capable of functioning
independently. single cell can be a complete organism in itself, as in bacteria and
protozoans. A unicellular organism also captures and digests food, respires, excretes,
‘grows, and reproduces, Similar functions in mult-cellula organisms are carried out by
groups of specialized cells which are organized into tissues and organs such as, the
higher plants and animals. Hence, “cell is known as the basic stuctural and funcional
Unit of if,

9. Explain why chloroplasts are found only in plant cells?

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run

Chloroplasts are found only in plant calls because they are required for phalosynihesis.

10. Complete the crossword with the help of clues given below.

Across:
1. This is necessary for photosynthesis.

3. Term for component present inthe cytoplasm.

6. The living substance in the cel.

8. Units ofinhentance present on the chromosomes.

Down:
1. Green plastids.

2. Formed by collection of issues.

4. It separates the contents of the cell from the surrounding medium
5. Empty structure in the cytoplasm,

7.A group of cals.

el T fe] TI TTT

37T

Las) aaa

I
C

ITT
IT

EET

Answer

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SUBIO UNO]
No
Elfo]
lalala]
ey Bi El
o ae
orcas}
E ago
u age
E lo] fal
E a] II
EEE

Across
1. CHLOROPHYLL

3, ORGANELLE
6. PROTOPLASM
8, GENES

Down
1, CHLOROPLASTS
2. ORGAN

4, MEMBRANE

5, VACUOLE
T.TISSUE

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NCERT Solutions for Class 8th: Ch 9 Reproduction in
Animals Science
NCERT Solutions for Class 8th: Ch 9 Reproduction in Animals Science

Excor
1. Explain the importance of reproduction in organisms.

Answer
Reproduction is very important for the organisms. The organisms reproduce to produce
young ones Ike them. helps to cary on their generation. reproduction does not take
place, no iting being wll survive on the earth, Had the process of reproduction not been
there then all the plants and animals would have become extinct, Secondly, special
characters of an organism are carried over to its next generation thrcugh reproduction
only.

2, Describe the process of fertlzation in human beings.

Answer
Fertilization involves the fusion of the male and the female gamete, The male and the
female gametes are released from the male and the female reproductive organs.
Sperms or male gametes are released from the male reproductive organ ie, the penis,
‘These sperms then enter the female body through the vagina, Then, they travel through
the fallopian tubes where they meet the eggs. Hence, the process of fertlization takes
place inthe fallopian tubes. During fertiization, the haploid nucleus of the sperm and
that of the ovum fuse with each other to form the zygote. This zygote divides to form an.
‘embryo which in tum develops ino a foetus.

3. Choose the most appropriate answer.

(a) Internal fertization occurs
{in female body.

(i outside female body.

(li in male body.

(iv) outside male body.

> ()in female body,

(b) A tadpole develops into an adult frog by the process of
() fertization

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sra a]

(i) metamorphosis
(i) embedaing
(iv) budding

> (i) metamorphosis

() The number of nuclei present in a zygote is
(none

(one

(iy two

(iv) four

> Gone
4. Indicate whether the following statements are True (T) or False (F).

(2) Oviparous animals give bir to young ones. ()
> False

(©) Each spem is a single cel ( }

> True

(c) External fertilization takes place in frogs. ( )

> Tue

(a) A new human individual develops from a cal called gamete. ( )

> Flase

(e) Egg laid after fertilization is made up of a single cal. ()

» True

(0) Amoeba reproduces by budding, ()
> False

(0) Feriization is necessary even in asexual reproduction. ( )

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> False

(h) Binary fission is a method of asexual reproduction. ()

> True

() A zygote is formed as a result of fertilization. ()

> Tue

() An embryo is made up of a single cel ()
> False

5. Give two differences between a zygote and a foetus,

Answer

zygote Foetus

tis a fertlized egg formed after the | is a stage of the embryo that shows all the main
fusion ofthe sperm with the egg... | recognizable body parts of a mature organism,

The zygote divides several times to | An embryo gradually develops into a foetus.
form an embryo

6. Define asexual reproduction. Describe two methods of asexual reproduction In
animals.

Answer
‘Asexual reproduction is that type of reproduction in which only single parent is involved.
‘Asexual reproduction takes place by diferent methods ike Budding, Binary fission etc.

— Budding: This type of reproduction takes place in Hydra and Bacteria. A part ofthe
‘organism starts bulging out Slowly it grows and develops into a separate individual

— Binary fission — In this ype of reproduction, a single organism gets divided into two,
‘This type of reproduction takes place in Amoeba. The nucteus of the Amoeba gets
divided into two followed by division of ther bodies, each part getting one nucleus and
developing into a separate individual

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7. in which female reproductive organ does the embryo get embedded?

Answer

‘The embryo gets embedded in the wall of the uterus

8. What is metamorphosis? Give examples.

Answer

‘The transformation of the larva into adult through drastic changes is called
‘metamorphosis. Frogs and insects are examples of organisms showing metamorphosis

9. Differentiate between intemal feílizaion and extemal fertilization.

Answer

Internal fertilization

Extornal fertilization

involves the fusion of the male and the
female gamete inside the female body.

It involves the fusion of the male and the
female gamete outside the female body.

Chances of the survival ofthe offspring are
more, Therefore, a small number of eggs
are produced.

[chances of survival of the offspring are
less. Therefore, a large number of eggs
are produced,

‘Humans, cows, hens are organisms

Fish, frog, startsh are organisms showing

showing internal fertitzation, extemal ferlizaton.

10. Complete the crossword puzzle using the hints given below.

Across
1. The process of the fusion of the gametes,

6. The type of fertilization in hen,

7. The term used for bulges observed on the sides of the body of Hydra
8. Eggs are produced here.

Down
2. Spems are produced in these male reproductive organs.
3. Another term forthe fertilized egg.

4, These animals lay eggs.

5. À type of fission in amoeba,

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Answer

Across
1. FERTILIZATION
6. INTERNAL

7. BUDS

8, OVARY

Down
2. TESTIS

3, ZYGOTE

4, OVIPAROUS
5. BINARY

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NCERT Solutions for Class 8th: Ch 10 Reaching the Age
of Adolescence Science

NCERT Solutions for Class 8th: Ch 10 Reaching the Age of Adolescence Science

1. What is the term used for secretions of endocrine glands responsible for changes
taking place in the body?

Answer
Hormones are chemical substances which are secreted by endocrine glands. They are
responsible for changes taking place in the body.

2. Define adolescence.

Answer
Adolescence is the period af fe, when the body undergoes changes, leading to
reproductive maturity. Itbegins around the age of 11 and lasts til 18 or 19 years of age.
‘The period of adolescence may vary from person to person.

3. What is menstruation? Explain,

Answer
Menstruation is the process of the shedding of the uterine lining on a regular monthly
basis in woman. During menstruation the uterus wall along with blood vessels break of.
“his causes blood to come out and a new wall of uterus is formed. Menstruation lasts
general lasts for 410 5 days.

4, List changes In the body that take place at puberty,

Answer
Changes at puberty

— Sudden increase in height and weight

+ Broadening of shoulders and widening of chest in boys. In grs, the region below
waist becomes wider.

= Inbboys, under the influence of hormones, the larynx becomes prominent, the vocal
‘cords become longer and thicker. These changes cause the voice to become hoarse.
— Appearance of hair in areas such as underams, face, hands, and legs.

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+ Appearance of acne as a result of excessive secretion of oll from skin.
— Tests grows and starts producing sperms in males, whereas in females, the ovary
enlarges and starts producing matured eggs.

5. Prepare a table having two columns depicting names of endocrine glands and
hormones secreted by them.

‘Answer
Endocrine gland | Hormones
Testis Testosterone
ovary estrogen
Thyroid Thyroxin
Arenal Adrenalin
Pancreas insutn
Ptutery [Growth nomone

$. What are sex hormones? Why are they named so? State their function,

Answer
‘Sex hormones are the hormones that contra the development of secondary sexual
characteristics and also regulate the proper functioning of the sex organs. They are
named so because they are produced in both males and females by the respective sex
organs under the influence of the pitultary gland,

Functions of sex hormones:

— Testosterone: This hormone brings about secondary sex characters in boys such as

the growth of a beard, the voice becoming hoarse, development of reproductive organs,

ex.
+ Oestrogen: This hormone is responsible forthe development of secondary sexual
characters in females such as the enlargement of breests, development of female
reproductive organs, ec.

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7. Choose the correct option.

(a) Adolescents should be careful about what they eat, because
(1) proper diet develops their brains.

(i) proper diet is needed for the rapid growth taking place in their body.
(li) adolescents feet hungry all the time.

(iv) taste buds are well developed in teenagers.

> (i) proper diet is needed for the rapid growth taking place in their body,

(b) Reproductive age in women starts when their
(i) menstruation stats.

(i) breasts start developing.

(ii) body weight increases.

(iv) height increases.

> () menstruation starts

(c) The right meal for adolescents consists of
(i) chips, noodles, coke.

(i chapatt, dal, vegetables.

(ii ice, noodles and burger.

(iv) vegetable cutlets, chips and lemon drink.

> (i) chapat dal, vegetables
8. Write notes on -

(8) Adam's apple: The protruding part of the throat is called Adam s apple. Itis the
enlarged voice box or larynx which gets enlarged and visible from outside in boys at the
‘onset of puberty. This makes the voice of boys hoarse,

(©) Secondary sexual characters: These are features features that help in distinguishing
the male and the female body trom each other. They are physical or behavioural
characteristics that appear in humans at the time of puberty,

‘Secondary sexual characters in boys:

— Appearance of moustache and beard

— Appearance of chest hair

Growth of hair in genital area and other parts of the sin,

‘Secondary sexual character in gts

— Increase in breast size and derkening of the skin of nipples present at the tip of the

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breasts.
= Growth of hair in genital area and other body parts
— Sex determination in the unborn baby.

() Sex determination in an unborn baby: Sex of a unbom baby depends upon the pair of
‘sex chromasomes that are present in te cells of the baby. The two sex chromosomes
are X and Y. The unfertiised egg always has one X chromosome.But sperms are of two
kinds. One kind has an X chromosome, and the other kind has a Y chromosome

both the chromosmes are X then the baby is gi.

IM one of the chromosomes are X and the other is Y Men the baby is boy.

9. Word game: Use the clues to work out the words.

Across
3. Protruding voice box in boys

4, Glands without ducts

7. Endocrine gland attached to brain
8. Secretion of endocrine glands

9, Pancreatic hormone

10, Female hormone

Down
1. Male hormone

2. Secretes thyroxin

3, Another term for teenage

5. Hormone reaches here through blood stream
6. Voice box

7. Term for changes at adolescence

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Across
3, ADAM'S APPLE
4, ENDOCRINE
7. PITUITARY

8, HORMONE

9, INSULIN

10, ESTROGEN

Down
1. TESTOSTERONE,
2. THYROID

3, ADOLESCENCE
5. TARGET SITE

6. LARYNX

7. PUBERTY

10, The table below shows the data on likely heights of boys and gifs as they grow in
age. Draw graphs showing height and age for both boys and gris on the same graph
paper. What conclusions can be drawn from these graphs?

Height (cm)
‘Age (Years)

Boys Girls
E [ss
a | 96 [e
se [ta] v0
2 [1m9]1
16 | 180 | 150
a [173 | 165

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Answer
‘The graph depicts the relation between the age and height of both boys and grs, During
puberty, there Is a sudden increase in height of both boys and gis. On the basis of the
above graph, it can be observed that during the age of 4-8 years, gris have less height
as compared to boys. However, as soon as gi reach 12- 13 years, their height shows
a sudden increase and becomes mare than boys. In later years, growth in both sexes
becomes stable. Growth during puberty Is under the control of hormones.

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NCERT Solutions for Class 8th: Ch 11 Force and
Pressure Science

NCERT Solutions for Class 8th: Ch 11 Force and Pressure Science
Excercise

1. Give two examples each of situations in which you push or ui to change the state of
‘motion of ebjects.

Answer
Two examples of push force are as follows:

=> A heavy box at rest ls pushed to move it from one room to another. This changes the
state of motion ofthe box.

— A player pushes a football using his foot, This changes the state of motion of the bal,

‘Two examples of pul force are as follows:
+ Rope is pulled to draw water from a well This changes the state of motion of the
water bucket

=> A drawer is pulled to open. This changes the state of motion of the drawer.

2. Give two exemples of situations in which applied force causes a change in the shape
of an object

Answer
‘Two examples of forces that cause a change in the shape of an object are as folows:
—+ Making of bread from dough

+ Deformation of clay by pressing it between the hands.

3. Fill n the blanks in the following statements.
(a) To draw water from a well we have to at the rope.

(b)A charged body _an uncharged body towards it.

(©) To move a loaded trolley we have to it

(@) The north pole of a magnet the north pole of another magnet

Answer
(a) To draw water rom a well we have to pul at the rope,

(b) A charged body attracts an uncharged body towards it.

(€) To move a loaded trolley we have to push or pul it

(4) The north pole of a magnet repels the north pole of another magnet.

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4. An archer stretches hor bow while taking aim at the target. She then releases
the arrow, which begins to move towards the target. Based on this information fill
up tho gaps in the following statements using the following terms.

‘muscular, contact, non-contact, gravity, fiction, shape, atraction

(a) To stretch the bow, the archer applies a force that causes a change in Is
(b) The force applied by the archer to stretch the bow is an example of ore
(c) The type of force responsible for a change in the state of motion of the arrow is an
example of a, force.
(4) While the arrow moves towards ts target the forces acting on it are due to

and that due to o ar.

Answer
(a) To stretch the bow, the archer applies a force that causes a change in its shape,

(©) The force applied by the archer to stretch the bow is an example of muscular force,

(c) The type of force responsible for a change in the state of motion of the arrow is an
example of a contact force.

(4) While the arrow moves towards its target the forces acting on it are due to
‘gravity and that due to fiction of air.

5. In the following situations identify the agent exerting the force and the object on
which it acts. State the effect of the force in each case.

(a) Squeezing a piece of lemon between the fingers to extract it juice.

Answer

Agent exerting the force - Fingers
(Object on which force acts - Lemon
Effect of force ~ Change in shape of lemon

(b) Taking out paste from a toothpaste tube,

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Answer

Agent exerting the force - Fingers
‘Object on which force acts — toothpaste tube
Effect of force ~ Change in shape of toothpaste tube.

(€) Aload suspended from a spring while its other end is on a hook fixed to a wall

Answer
Agent exerting the force - Load

Object on which force acts - Spring

Effect of force ~ Change in shape of spring

(@) An athlete making a high jump to clear the bar at a certain height.

Answer
Agent exerting the force - Muscles of Athiete

Object on which force acts - Athlete

Effect of force - Change of state of mation of athlete

6. A blacksmith hammers a hot piece of ron while making a tool. How does the force
due to hammering affect the piece of ron?

Answer
Wen a blacksmith hammers a hot piece of iron, he uses his muscular force, This
‘muscular force changes the shape of the iron so that can be given a desired shape.

7. Aninflated balloon was pressed against a wall after it has been rubbed with a piece of
synthetic cloth. tt was found that the balloon sticks to the wall. What force might be
responsible forthe atraction between the balloon and the wall?

Answer
Electrostatic force is responsible or the attraction between the balloon and the wall

8, Name the forces acting on a plastic bucket containing water held above ground level
in your hand, Discuss why the forces acting on the bucket do not bring a change in its
state of motion.

Answer
In this case, muscular force is acting in upward direction and the force of gravity is acting
in downward direction. Both forces are acting in opposite directions to each other and
hence nulity he effect of each other. Due to this, there is no change inthe state of

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‘motion of the bucket

8. A rocket has been fired upwards to launch a satelite in its orbit. Name the two forces
acting on the rocket immediately after leaving the launching pad.

Answer
‘The two forces acting an the rocket are the force of gravity, which pulls the rocket
towards the ground, and the force of friction due to earth's atmosphere, which opposes
its motion.

10, When we press the bulb of a dropper with ts nozzie kept in water, ar inthe dropper
Is seen to escape in the form of bubbles. Once we release the pressure on the bulb,
Water gets lied in the dropper. The rise of water in the dropper is due to

(a) pressure of water

(b) gravity of the earth.

(c) shape of rubber bulb

(a) atmospheric pressure

Answer
(6) atmospheric pressure

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NCERT Solutions for Class 8th: Ch 12 Fi

NCERT Solutions for Class 8th: Ch 12 Friction Science

(a) Friction opposes the. between the surfaces in contact with each other.
(b) Friction depends on the of surfaces.

(6) Fricton produces.

(@) Sprinking of powder on the carrom board friction

(e) Sliding tition is than the static tition

Answer

(a) Friction opposes the motion between the surfaces in contact with each other.
(b) Friction depends on the nature of surfaces,

(€) Friction produces heat

(@) Sprinking of powder on the carrom board reduces friction.

(e) Sliding fiction is less than the static fiction.

2. Four children were asked to arrange forces due to rallng, static and slicing frictions in
adecreasing order. Their arrangements are given below. Choose the correct
arrangement.

(a)roling, static, sting

(0) roling. siding, static.

(c) static, slicing, rating

(a) siding, stati, roting

> (0) static, siding, roling,

3. Alda runs her toy car on dry marble floor, wet marble foor, newspaper and towel
‘spread on the floor. The force of fiction acting on the car on diferent surfaces in
increasing orderwill be

(a) wet marble floor, cry marble floor, newspaper and towel.

(b) newspaper, towel, dry marble floor, wet marble flor.

(c) towel, newspaper. dry marble floor. wet marble floor

(6) wet marble floor, cry marble floor. towel, newspaper

> (a) wet marble floor, dry marble flor, newspaper and towel,

4. Suppose your witing desks ited a lite. A book kept on it starts siding down. Show
the direction of frictional force acting on it

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Answer
Frictional force is acting opposite to the movement ofthe booki.e. upwards.

5. You spill a bucket of soapy water on a marble floor accidentally. Would it make it
easier or more dificult for you to walk on the floor? Why?

Answer
It woukl make it ore difficult for us to walk on a soapy floor because layer of soap
makes floor smooth, The coating of soapy water reduces the fiction and the foot can not
make a proper grip en the floor and it stats geting to slip on the floor.

6. Explain why sportsmen use shoes with spikes.

Answer
‘Sportsman use shoes with spike to increase the friction so thatthe shoes do not slip
while they run or play.

7. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the
‘same floor. Who will have to apply a larger force and why?

Answer
‘Seema will have fo apply larger force since she will experience more frictonal force
because heavy object willbe pressed hard against the opposite surface and produces
more friction

8. Explain why siding friction is less than static fiction,

Answer
Sliding ficton is always less than static fichon because two sliding objects find less time
Lo get interlocked against each others regularities of surfaces as a result of which they
experience less friction

9. Give examples to show that friction is both a friend and a foe,

Answer
Friction as rend

— We are able to walk because of eo.

— Friction between the tip ofthe pen and a paper allows us to write,

Friction as foe

— Tyres and soles of shoes wear out because of fiction.

— Friction between the diferent parts of machines produces heat This can damage the
machines.

10, Explain why objects moving in Auids must have special shapes.

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Answer
Wen a body moves through a fui, it experiences an opposing force which tries to
‘oppose its motion through the fluid. This opposing force is known as the drag force. This
frictional force depends on the shape of the body. By giving objects a special shape, the
force of friction acting on it can be minimised. Hence, it becomes easier for the body to
move through the fui,

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sra]

NCERT Solutions for Class 8th: Ch 13 Sound Science

NCERT Solutions for Class 8th: Ch 13 Sound Science

Exorcl

4. Choose the correct answer.

‘Sound can travel through
(a) gases only

(0) solids only
(6) liquids only

(4) solids, iquids and gases.

> (6) solids, liquids and gases,

2, Which of the following voices is Ikely to have minimum frequency?
(a) Baby git

(6) Baby boy
(Aman

(4) A woman

> (c)Aman

3.1n the following statements, tick"
those which aro f

inst those which aro true, and 'F' against

(a) Sound cannot travel in vacuum, (T/F)
> True

(b) The number of oscillations per second of a vibrating object is called its time period. (T
IF)

> False
(€) Ifthe amplitude of vibration is large, sound is feeble. (T /F)
> Falso

(6) For human ears, the audible range is 20 Hz to 20,000 He. (T/F)

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> True

(e) The lower the frequency of vibration, the higher isthe pitch. (T/F)
> False

(f) Unwanted or unpleasant sound is termed as music. (T/F)

> False

(9) Noise pollution may cause partal hearing impairment. (T/F)

» True

4. Fil inthe blanks with suitable words.
(a) Time taken by an object to complete one oscilan is called

(b) Lougness is determined by the of vibration

(c) The unit of frequency is.

(6) Unwanted sound ls calle

(e) Shrilness of a sound is determined by the _of vibration.

Answer
(a) Time taken by an object to complete one oscilañon is called ime period
(b) Loudness is determined by the amplitude of vibration

(€) The unit of frequency is hertz (Hz).

(a) Unwanted sound is called noise.

(e) Shrilmess of a sound is determined by the frequency of vibration,

5. A pendulum oscllates 40 times in 4 seconds. Find its ime period and frequency.
Answer

Frequency of oscillations is the number of oscilaions of a vibrating object per second.
‘Therefore frequency Is = 40 vibrations 4 seconds= 10 Hertz

Time period is the time required to complete one oseilation. Or itis the inverse of time
period. Therefore time period = 1/10 = 0.1 seconds.

6. The sound from a mosquito is produced when it vibrates its wings at an average rate
‘of 600 vibrations per second. What is the ime period of the vibration?

Answer

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‘Time Period given by the inverse ofthe frequency.
Time Periode Frequency of oscillation = 1/500 = 0.002 sec.

7. Identity the part which vibrates to produce sound in the following instuments.
(a) Dhotak (9) Sitar (c) Flute

Answer
(a) Diaphragm (stretched membrane)
(b) Sting

(6) Ar column

8. What isthe difference between noise and music? Can music become noise
sometimes?

Answer
‘Unwanted sound that is unpleasant to ears called noise. A pleasant sound is called
music.

‘Music can become noise at many instances. When someone tries to enjoy very loud
music it can be noise for someone else. When loud music is played during religious
Celebrations or martages, it can be annoying for many people.

9. List sources of noise pallution in your surroundings.

Answer
Some sources of noise pollution are as follows:
Televisions and transistors running at high volumes.
— Loudspeakers and crackers

— Hors of buses, cars and trucks

— Home appliances such as mixer, desert cooler, etc

10, Explain in what way noise pollution is harmful to humans.

Answer
[Noise pollution is harmful to humans in many ways. Constant exposure to noise pallution
can create many health related problems; ke insomnia, hypertension and may even
lead to loss of hearing

11. Your parents are going to buy a house. They have been offered one on the roadside
and another three lanes away from the roadside, Which house would you suggest your

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parents should buy? Explain your answer.
Answer
‘There will be more noise in the house whichis along the roadside due to noise produced
by transportation vehicles may cause trouble tothe residents. On the other hand, the
house which is three lanes away from the roadside would be quieter. Therefore itis
better to take the house that three lanes away from the roadside.
12. Sketch larynx and explain its function in your own words.

Answer

e
Trace SO
(Wind pipe)

Larynx is a part of the threat. It is responsible for production of sound.

13, Lightning and thunder take place in the sky at the same time and at the same
distance from us. Lightning is seen earlier and thunder is heard later. Can you explain?

Answer
‘The speed of sound is less than the speed of light. Due to this, light reaches to us faster
than sound. Hence, during lightning we see the streak of light earlier than hearing the
sound of thunder.

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NCERT Solutions for Class 8th: Ch 14 Chemical Effects
of Electric Current Science

NCERT Solutions for Class 8th: Ch 14 Chemical Effects of Electric Current Science

Excorcis
1. Filin the blanks.

(a) Most liquids that conduct electricity are solutions of and
(©) The passage of an electric current through a solution causes effects,

(€) you pass current through copper suphate solution, copper gets deposited on the
Plate connected tothe terminal of the battery,

a) The process of deposling a layer of any desired metal on another material by means
of electricity is called

Answer
(a) Most liquids that conduct electricity are solutions of acids, bases and sale
(b) The passage of an electric current through a solution causes chemical effects.

(c) Ifyou pass current through copper sulphate solution, copper gets deposited on te
Plate connected to the negative terminal ofthe battery.

(6) The process of depositing a layer of any desired metal on another material by means
of electricity is called electroplating

2. Wen the free ends of a tester are dipped into a solution, the magnetic needle shows,
deflection. Can you explain the reason?

Answer
Electric current produces magnetic effect which causes deflection of the magnetic
needle of a compass. So, when the free ends ofa tester are dipped into a solution which
is a good conductor of electricity, the needle shows deflection.

3. Name three liquids, which when tested in the manner shown in Fig. 14.9, may cause
the magnetic needle to deflect.

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Answer

Liquids like lemon juice, alt water and vegetable ol allow electricity to pass through
them, Hence, these liquids can be used as in the beaker to show the given effect

A. The bulb does not glow in the setup shown in Fig, 14.10. List he possible reasons.
Explain your answer.

Fig. 14.10

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Answer

‘There could be several possible reasons for not glowing of the bulb:
— The bub may be fused

— Even after replacing the bulb fit does not glow, then the connection may be loose or
faut.

— Even after checking and repairing the connections ifthe bulb does not glow that
means the solution does not conduct electnity

5. A tester is used to check the conduction of electricity through two liquids, labelled A
and B. It is found that the bulb ofthe tester glows brightly for liquid A while it lows very
dimly for quid B. You would conclude that

(0 quid As a better conductor than liquid B.

(i) liquid 8 isa better conductor than liquid À.

(i) both liquids are equally conducting,

(iv) conducting properties of liquid cannot be compared inthis manner.

> (liquid A is a better conductor than liquid B.

6. Does pure water conduct electricity? I not, what can we do to make it conducting?

Answer
|No. Pure water does not conduct electricity. Pure water can conduct electricity when a
pinch of common salt is added tot, as salt sluticn is conducting in nature

7. n case of a fre, before the fremen use the water hoses, they shut off the main
electrical supply for the area. Explain why they do this.

Answer
Water may conduct electric. Ifthe electrical supply for the area is not shut off and
‘water is poured over electrical appliances, then electricity may pass through water and
ham the firemen. That is why, in case of fre, the fremen shut of the main electrical
Supply for the area before they use the water hoses.

8. A child staying in a coastal region tests the drinking water and also the seawater wth
his tester. He finds that the compass needle deflects more in the case of seawater. Can
you explain the reason?

Answer
‘Sea water contains more dissolved salts than the drinking water. Hence, itis more
‘conducting than the drinking water. Because of his reason, the compass needle deflects
more in seawater than in the drinking water.

9. Isit safe for the electrician to carry out electrical repairs outdoors during heavy

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downpour? Explain

Answer
"No, itis very risky and unsafe far the electrician to carry out electrical repairs outdoors
during heavy downpour because rain water contains dissolved salts. Therefore, rain
water can conduct electricity, The electician may get electrical shocks while working
‘outdoors during rain

10. Paheli had heard that rain water Is as good as disiled water. So, she collected some
rain water in a clean glass tumbler and tested itusing a tester. To her surprise, she
found that the compass needle showed deflection. What could be the reasons?

Answer
Rain water contains dissolved salts. This makes it a conducting soluton. There are no
dissolved sats present in the cistiled water. Hence, rain water can allow electricity to
pass through it while distlled water cannot.

11. Prepare alist of objects around you that are electroplated

Answer
=: Cars bumpers and cycles handles are chromium plated to give them shiny
appearance,

Artificial jewellery items are gold or silver plate

— Iron used in constructing a bullding is coated with a layer of zinc to protect rom
rusting,

12, The process that you saw in Activity 14.7 is used for purification of copper. A thin
plate of pure copper and a thick rod of impure copper are used as electrodes. Copper
from impure rod is sought to be transferred tothe thin copperplate, Which electrode
should be attached to the positive terminal of the battery and why?

Answer
Copper ion is positively charged. It is attracted towards the plate which is connected to
the negative teminal ofthe battery. As copper ions are transferred to the thin copper
Plate, this thin pure copper plate must be connected tothe negative terminal ofthe
battery. Consequently, impure copper rod is connected to the postive terminal of the
battery

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NCERT Solutions for Class 8th: Ch 15 Some Natural
Phenomena Science
NCERT Solutions for Class 8th: Ch 15 Some Natural Phenomena Science

Exerc!
1. Which of the folowing cannot be charged easly by fiction?
(a) À plastic scale

(D) A copper rod

(c) An infated balloon

(a) A woallen doth

> (b)A copperrod

2. When a glass rod is rubbed with a piece of sik cloth the rod
(a) and the cloth both acquire positive charge,

(©) becomes positvely charged while the cloth has a negative charge.
(c) and the cloth both acquire negative charge.

(a) becomes negatively charged while the cloth has a positive charge.

> (b) becomes positively charged while the cloth has a negative charge.

3. Write T against true and F against falso in the following statements.

(a) Like charges attract each other. (T/F)
> False

(©) A charged glass rod attracts a charged plastic straw. (T/F)
» True

(c) Lightning conductor cannot protect a building from lightning. (T/F)
> False

(6) Earthquakes can be predicted in advance. (T/F)
> False

4. Sometimes, a cracking sound is heard while taking off a sweater during winters.
Explain

Answer

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‘Sweater is made of wool and generally shirt we wear is made of cotton blended with
‘some synthetc fibers, Constant fiction between the shirt and sweeter lets transfer of
electrons from one material to the other. This results in building of electric potential
nen enough potential has been accumulated, while taking of the sweater it discharges
and transfer of electrons take place. Passage of electrons through air in the forms of
‘spark let out heat and sound energy. Thus we hear the crackling sound and in dark
rooms we may see the spark lights as wall

5. Explain why a charged body loses its charge fe touch it with our hand,

Answer
When we touch a charged body, with our hand, the excess of accumulated charge or
static charges on it, gets transfer to ground through our body. Thus the charged body
losesits charge, and becomes neutral

6. Name the scale on which the destructive energy of an earthquake is measured. An
‘earthquake measures 3 on this scale. Would it be recorded by a seismograph? I tIkely
to cause much damage?

Answer
‘The destructive energy of an earthquake is measured on Richter Scale.

‘The reading of magnitude 3 on the Richter scale would be recorded by a seismograph
Ifthe Richter scale gives a reading of magnitude 3, then the earthquake isnot likely to
cause much damage. Generally earthquake of magnitudes higher than 5 is considered
destructive in nature.

7. Suggest three measures to protect ourselves from lightning,

Answer
Protective measures against lightning are: Stay indoor or under covered area.
_ Do not take bath curing lightning

+ Do not use any electrical appliances during lighting.

8. Explain why a charged balloon is repelled by another charged balloon whereas an
uncharged balloon is attracted by another charged balloon?

Answer
A charged balloon is repelled by another charged balloon because both carry same
charges and we know same charges repel each other whereas an uncharged balloon is
attracted by charged balloon because unlike charges attract each other

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9, Describe withthe help of a diagram an instrument which can be used to detect a
charged body.

Answer
An electroscope can be used to detect whether a body is charged or not. The following
figure shows a simple electroscope.

Cee

O oper

sacar

samen

peo onen

In an Electroscope, two metal foils are are hanged withthe help of a conductor in a glass
ar with ald, When a charged body touches this conductor, its charged gets transferred
to the metal fol and they move apart due to Ike charges. This shows that an electrocope
detects charges,

10, List three states in India where earthquakes are more likely to strike.

Answer
‘The three states in India where earthquakes are more likely to strike are Jammu and
Kashmir Gujrat, and Assam,

11. Suppose you are outside your home and an earthquake strkes, What precaution
would you take to protect yourself?

Answer
‘Some of the precautions are:—+ Try to move to an open space.

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+ We should not take shelter under trees, buildings or overhead power lines.
—+ lftravelling in a bus or a car. then do not come out when an earthquake stikes. Ask
the driver to drive in an open fed

12, The weather department has predicted that a thunderstorm is likely to occur on a
certain day. Suppose you have to go out on that day. Would you carry an umbrella?
Explain.

Answer
"No, We should not carry an umbrella in a thunderstorm. During thunderstorms, which
are accompenied with lightning, electric discharge from the clouds can travel through the
metal rod of the umbrella. This may give an electric shock tothe person who is
camping it. Hence, its not safe to carry an umbrella during lightning

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NCERT Solutions for Class 8th: Ch 16 Light Science

NCERT Solutions for Class 8th: Ch 16 Light Science

Excorcisos:
1. Suppose you are in a dark room. Can you see objects in the room? Can you see
‘objects outside the room? Explain,

Answer
We can see any object, when light reflected by that object reaches our eyes. Butin the
(dark room, no lights reflected by the object so we are unable to see objects in dark
roam. If here is light present outside the room, then we can see the objects outside the
room.

2. Differentiate between regular and difused reflection. Does difused reflection mean
the failure of the laws of reflection?

Answer

gines | Diffused rellection

en all the parallel rays reflected rom When al he parallel rays reflected from a
plane surface are para, the plane surface are not parallel, the reflection
lecton is known as regular reflection }sknown as diffused or regular reflection

Simages are fered by regular reflection, Images are notfomed by regular reflection,

‘The diftused reflection is not due tothe failure of laws of reflecton but caused by
irregularites in the reflecting surface,

3. Mention against each of the folowing whether regular or diflused reflection wil take
place when a beam of ight strikes. Justiy your answer in each case.

(a) Polished wooden table

(b) Chalk powder

(c) Cardboard surface

(4) Marble Moor with water spread over it
(e) Mirror

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(N Piece of paper

Answer
(a) Regular reflection

A polished surface is an example of a smooth surface. A polished wooden table has a
smooth surface. Hence, reflections from the polished table will be regular.

(b) Diffused reflection
(Chalk power spread on a surface is an example of an irregular surface. Its not smooth
‘Therefore, difused reflection will ake place from chalk powder,

(c) Diftused reflection
Cardboard surface is also an example of an iregular surface. Hence, difused reflection
will take place from a cardboard surface.

(a) Regular reflection
Marble floor with water spread over its an example of a regular surface. This is because
‘water makes the marble surface smooth. Hence, regular reflection will take place from
this surface,

(e) Regular reflection
Mirror has a smooth surface. Therefore, it wil give a regular reflection

(1) Diffused reflection
Although a piece of paper may lock smooth, butt has many Imeguarties on its surface,
Due to this reason, itwil give a diffused reflection,

4, State the laws of reflection,

Answer
Laws of reflection are:

— The angle of reflection is always equal tothe angle of incidence.

= The incident ray, the reflected ray and the normal to the reflective surface atthe point
of incidence alien the same plane.

5, Describe an activity to show thatthe incident ray, the reflected ray and the normal at
the point of incidence le in the same plane.

Answer

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Place a plane mirror on the table, Take a paper sheet and make a small hole in ts
(centre. Make sure thatthe light inthe room is not bright. Hold the sheet normal tothe
table. Take another sheet and place ¡ton the table in contact withthe vertical mirror.
Draw a normal line on the second sheet from the mirror. Now, ight a torch on the mirror
through the small hole such thatthe ray of ight falls on the normal atthe bottom of the
‘mirror. When the ray from this hale is incident on the mirror, it gets reflected in a certain
direction, You can easily observe the incident ray, reflected ray and the normal to the
miror at the point of incidence on the sheet placed on the table. This shows thatthe
incident ray, the reflected ray, and the normal to the surface atthe point of incidence al
lie in he same plane.

normal
Incident reflected ray

6. Fill in the blanks in the following.
(8) A person 1 m in front of a plane mirror seems to be m away from his
image.

(0) Ifyou touch your ‘car with your right hand in font of a plane mirar, it wil
be seen in the mirror that your right ear is touched with your

(c) The size ofthe pupil becomes, ‘when you see in dim ight.

(@) Night birds have ‘cones than rods in their eyes.

Answer

(8) A person 1m in front ofa plane mirror seems to be 2m away from his image.
(b) Ifyou touch your left ear with your right hand in front ofa plane minor, it wil be seen
in the mirror that your right ea is touched with your lett hand.

(c) The size ofthe pupil becomes large when you see in dim light.

(@) Night birds have less cones than rods in ter eyes.

7. Angle of incidence is equal to the angle of reflection,
(a) Always

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(b) Sometimes.
(c) Under special conditions

(a) Never
> (2) Always

8. Image formed by a plane mirror
(a) vitual, behind the mirror and enlarged.
(b) virtual, behind the mirror and of the same size as the object

() real atthe surface of the mirror and enlarged
(4) real, behind the mirror and of te same size as the object.

> (b) virtual, behind the mirror and of the same size as the object
8. Describe the construction of a kaleidoscope.
Answer

‘Three rectangular minor strips of dimensions 15cm x 4em (1x b) are joined together o
form a prism. This prism is fixed into a circular cardboard tube. The circular cardboard
tube should be slightly longer that the prism. This circular tube is now closed at one end
with a cardboard disc. This disc has a hole init through which we can see „At the other
end of the circular tube, a plane glass plate is fixed. It is important that this glass plate
teuches the prism mimors, On this glass plate, several small and broken pieces of
Coloured glass are placed. This end is now closed by a round glass plate allowing
enough space for the coloured glass pieces to move,

10. Draw a labelled sketch of the human eye.

Answer

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11. Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her
not to do so. Can you explain the basis of the teacher's advice?

Answer

Laser lights harmful for the human eyes, because its intensity is very high. It can cause
damage tothe retina and lead to bindness. Hence, it's advisable not to look at a laser
beam direct,

12. Explain how you can take care of your eyes.

Answer
We can take care of eyes by;

— Visit an eye specialist regulany.

+ Avoid reading in dim light and very bright light

— Avoid direct exposure of sunlight to the eye.

_ Clean your eyes with cold water quicky if dust particles or small insects enter your
eye. Do not rub your eyes.

— Maintain a distance of at least 25 cm between the book and your eyes while reading.

13, What isthe angle of incidence of a ray if the reflected ray is at an angle of 90° tothe
Incident ray?

Answer
As per the laws of reflection
‘The angle of incidence al

le of reflection zr

Here as given 4i + ar= 90°
+29"

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Hence angle of incidence of a ray = 45 degree

14. How many images of a candle will be formed if ti placed between two parallel
Plane mirrors separated by 40 cm?

Answer
Infinite mages of the candle willbe formed due to parallel mirrors,

15, Two mirrors meet at right angles. A ray of lights incident on one at an angle of 30°
as shown in Fig. 18.19. Draw the reflected ray from the second mirror.

e”
mers
Answer
The firstlaw of reflecton is used to obtain the path of
retlected light
he.
E le

If can be observed that the given ray of light il reflect from the second mirror at an
angle 60°.

16. Bodjho stands at A just on the side of a plane mirror as shown in Fig. 16.20. Can he
‘see himself in the mirror? Also can he see the image of objects situated at P, Q and R?

Awe * 2

SEGA
Fig 1620

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Answer
A plane mirror forms a vitual image behind the mirror. The image is as far behind the
miror as the objects in front of it. cannot see his image because the length of the
minor is too short on his side. However, he can see the objects placed at points P and
(Q. but cannot see the object placed at point R (as shown in the given figure).

4 Es

17. (a) Find out the postion of the image of an object situated at Ain the plane mirror
(Fig. 16:21),

(b) Can Panel at B see this mage?

(c) Can Boojho at C see this image?

(4) When Paheli moves from B to C, where does the mage of A move?

= Een

EE

Ce
Bite

Answer
(a) Image of the object placed at A is formed behind the mitrer. The distance of the
image from the mirrors equal to the distance of A from the mirror, Image of A is shown
Inthe given figure,

apo ai

(b) Yes. Paheli at B can see this image
(c) Yes. Boojno at C can see this image.

(a) Image of the object at A will not move. It wll remain atthe same position when Pahel
moves from B to ©.

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NCERT Solutions for Class 8th: Ch 17 Stars and the
Solar System Science
NCERT Solutions for Class 8th: Ch 17 Stars and the Solar System Science

Exerci
1. Which of he following is NOT a member ofthe solar system?
(a) An asteraid

(0) A satelite

(c) Aconstellation

(6) comet

> (c) A constellation

2. Which of the folowing is NOT a planet of the sun?
(a) Sirius

(b) Mercury

(©) Satum

(6) Earth

> (a) Sinus

3. Phases of the moon occur because
(a) we can see only that part of the moon which reflects light towards us.
(©) our distance from the mocn keeps changing.

(c) the shadow of the Earth covers only a part of the moon's surface.

(a) the thickness of the moon's atmosphere is not constant.

» (a) we can see only that part ofthe moon which reflects light towards us.

4. Fill in the blanks:
(a) The planet which is farthest from the sun is
(©) The planet which appears reddish in colour
(c) A group of stars that appear to form a pattem in the sky is known as a_
(@)A celestial body that revalves around a planet is known as a

(e) Shooting stars are actually not

(f) Asteroids are found between the orbits of

nd

Answer:
(a) The planet which is farthest rom the sun is Neptune.

(b) The planet which appears reddish in colour is Mars.

(©) À group of stars that appear to form a pattern in the sky is known as a constellation.

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(4) A celestial body ht revolves around a planet is known as a satel
(e) Shooting stars are actually not stars

(f) Asteroids are found between the orbits of Mars and Jupiter.

5. Mark the following statement as rue (T) or false (F)

(a) Pole stars a member of the solar system. ( )

» False

(b) Mercury is the smallest planet ofthe solar system. ()
> Tue

(c) Uranus is the farthest planet in the solar system. ( )
> False

(a) INSATis an artical satelite. ()

> True

(e) There are nine planets in the solar system. ()

> False
(0 Constellation Orion can be seen only with a telescope. ( }

> False
6. Match items in column A with one or more items in column B.

A 8
© | Inner planets (a) [satum
(m | Outer planets (0) [Pole star
iy | Constellation (c) | Great Bear
(1) | Satetite of the Earth | a) | Moon

(0) [Earth

€ [orion

(0) [Mars

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Bosc
A 5

I Timerieneis [een] mars, earn

Go Jouerpines | [sum

cuy [censetaron —— [ter [reat Bear, non

10 | Seno rin Earn a) [con

7. in which part of the sky can you find Venus ifitis visible as an evening star?

Answer
Venus appears in the western sky after sunset and is called the evening star.

8. Name the largest planet of the solar system.
> Jupiter

9. Whatis a constellation? Name any two constellations,

Answer
À constellation is a group of stars that form a recognisable patter inthe sky,
‘The two constellations name are Ursa Major and Orion.

10, Draw sketches to show the relative position of prominent stars in (a) Ursa Major and
(b) Orion.

Answer
(a) Ursa Major appears Ike a big dipper. There are three bright stars in the handle and
four stars in the boul of the dipper as shown in below figure,

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(b) Orion appears Ike a hunter. Three bright stars appear in the bel, while five bright
stars are arranged in the form of a quadnlateral as shown in he below figure.

11. Name two objects other than planets which are members ofthe solar system.
Answer

Asteroids and
system

eteors are objects other than planets which are members of the solar

12. Explain how you can locate the Pole Star with the help of Ursa Major.

Answer
‘The pole star can be
line towards the Northern direction connecting two stars pr
boul, This imaginary line meets Pole Star. The length of the imaginary line from the bow’
Is about five times the distance between the two stars of the bow

cated using the ursa major constellation, by imagining a
esnt atthe end of the

+ Pole Star

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3. Do all he stars in the sky move? Explain
Answer

No, Stars do not move they actually appear to move from east to west due to rotation of
earth on its axis from west to east.

14. Whyis the distance between stars expressed in light years? What do you
Understand by the statement that a star is eight light years away from the Earth?

Answer

‘The distance of the stars from the Earth and the distance between the stars are very
large. itis inconvenient to express these distances in kilometer (km). Thus, these large
distances are expressed in light years. One light year isthe distance travelled by light in
one year. One light year is equal to 9.45 x 10°km.

A star is located eightlight years away from the Earth. This means that the distance
between the star and the Earth is equivalent tothe distance travelied by light in eight
years, ie, a star is located 8 x (9.46 x 10") = 7.6 x 10"km away from the Earth

15. The radius of Jupiter is 11 times the radius ofthe Earth. Calculate the ratio of the
volumes of Jupiter and the Earth. How many Earths can Jupiter accommodate?

Answer
Earth and Jupiter can be considered as two spheres with radi Rand R'respectvely
{Given that the radius of Jupiter is 11 mes the radius ofthe Earth.
Thus, Re 11
4
Volume of a sphere of the radius ris given as= mr?
Volume of the earth= ¿701?

nr)?

‘And, volume of Jupiter= m(11R)? = 13316 71R?)

3
The ratio of the volumes of Jupiter and Earth

Volume of up
Volume of Earth

3331GnR?)

BEG a
al

Hence, this ratio suggests that Jupiter can accommodate 1331 number of Earths
within it

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16. Boojho made the folowing sketch (Fig. 17.29) of the solar system. Is the sketch
correct? ifnot, correct it.

Answer

No, the sketch is not correct as the planet because he has interchanged the positions of
‘Mars and Venus and also the postions of Uranus and Neptune. Also, he has shown the
‚Asteroid beltin the gap between the orbits of Jupiter and Saturn which is not

Correct. The asteroid belts located between the orbits of Mars and Jupiter. The correct
figis given below.

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NCERT Solutions for Class 8th: Ch 18 Pollution of Air
and Water Science

NCERT Solutions for Class 8th: Ch 18 Pollution of Air and Water Science
Excorcis
1. What are the different ways in which water gets contaminated?

Answer
Water gets contaminated by the addition of.
— Agricultural chemicals: Farmers use excessive amounts of pesticides and fertlizers to
increase crop production. These chemicals get cerried away tothe water bodies due to
rains and floods which lead to water pollution,
— Industial wastes: Industries release harmful chemical wastes into water sources,
thereby polluting them.

+ Sewage wastes: Waste materials from kitchens, toilets, and laundry sources are also
responsible for contaminating water.

2. Atan individual level, how can you help reduce air pollution?

Answer
An individual can reduce ar pollution by.

— Avoiding the use of cars as much as possible and by using public transport whenever
possible.

= By not using vehicles for short distances,

— By using clean fuels such as LPG and CNG instead of diese! and petro,

— Always disposing the garbage property and not buming i.

+ Controling the emissions from vehicles and household chimneys,

3. Clear, transparent water is always ft for drinking. Comment.

Answer
(Clear and transparent water isnot always ft for drinking. Water might appear clean, but
Itmay contain some disease causing micro-organisms and other dissolved impurtis.
Hence, itis advised to punfy water before drinking. Purification can be done by water
purifying systems or by baling the water.

4. You are a member of the municipal body of your town.
Make a lst of measures that would help your town to ensure the supply of clean water to
all ts residents.

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Answer
To ensure the supply of clean water to al residents the flowing steps must be taken:
— Leakages in pipaines of water should be repaired

— The main water source must be builtin clean suroundings and should be maintained
propery.

_ Open defection in water resources by slum dwellers should be stictly prohibited

_ Chemical methods such as chiornation must be used for purfying water.

5. Explain the dtferences between pure air and polluted ar.

Answer

Pure Air Polluted air

Pure air contains around 78% nitrogen, 21% | Polluted air nudes gases Ike sulfur
oxygen, and 0.03% carbon dioxide also small | dioxide, nitrogen dioxide, carbon
‘amounts of argon, methane, ozone, and water | cioxide, etc. and other particulate

vapours, matter
‘Smog is not present. ‘Smog can be present,
[Not Rarmful for human beings Harmful for human beings as cause

diseases Ike asthma

6. Explain circumstances leading to acid rain. How does acid rain affect us?

Answer
When harmful gases like sulphur dioxide and nitrogen cioxide, react with moisture
present in air. they form nitric acid and sulphuric acid. These acids when drop down on
earth along with rain, is called acid rain Effects of acid rain

+ Acid rains damage crops, decrease fertityof sal and affect aquatic fe

— Acid rains corrode buildings and structures,

7. Which of the folowing isnot a greenhouse gas?
(a) Carbon dioxide

(©) Sulphur dioxide

(©) Methane

(6) Nitrogen

> (6) Nitrogen

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8. Describe the ‘Greenhouse Effect in your own words.

Answer
‘The greenhouse effect is he rise in the temprature of the Earth's surface due

to increased concentration of greenhouse gases Ike carbon dioxide, methane, and
water vapour in the atmosphere, These gases trap solar radiations released back by the
Earth. This helps in keeping our planet warm and thus, helps in human survival
However, an increase in the amount of greenhouse gases can lead to an increase in the
Earth's temperature leading to global warming.

9. Prepare a brief speech on glcbal warming. You have to deliver the speech in your
class.

Answer
‘Giobal warming is one of he greatest challenges that our planetis facing, Itis ne
unequivocal and continuing rise in the average temperature of the Earth's climate
system. It occurs as a result of an increased concentration of greenhouse gases in the
almosphere, These gases trap sola raciations released back by the Earth. This helps in
keeping our planet warm and thus, helps in human survival. However, an increase in the
amount of greenhouse gases can lead to an increase in the Earth's temperature leading
to global warming.

10, Describe the threat to he beauty of the Taj Mahal.

Answer
‘Acid rain is a major threat tothe beauty of the Taj Mahal. When acid rains fall on the
monument made of marble, they react wth marble to form a powder-ike substance that
is then washed away by the rain. This phenomenon is known as marble cancer. Also,
the soot particles emitted from the Mathura ol refinery located near Agra is leading to
the yellowing of the marble.

11. Why does the increased level of nutrients in the water affect the survival of aquatic
organisms?

Answer
‘The increased in the level of nutrients in the water leads to an excessive Increase in the
Population of algae in the water body. hen these algae die, they serve as food for
decomposers. A ot of oxygen is utlised in this process, consequently leading to a
decrease in the level of oxygen dissolved in the water body. This in turn causes fishes
and other aquatic organisms to die

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