Master Copy Science Class 8
Master Copy Science Class 8
Master Copy Science Class 8
1. Production of Crops
2. Microorganisms
3. Synthetic Fibres and Plastics
4. Metals and Non-metals
5. Combustion and Fuel
6. Cell Structure and Function
7. Reproduction
8. The Age of Adolescence
9. Force and Pressure
10. Friction
11. Sound
12. Chemical Effects of Current
13. Some Natural Phenomena
14. Light
1. Production of Crops
a. Ploughing : Ploughing /tilling is the first agricultural practice which involves loosening
of soil which is done by using wooden or iron ploughs that are pulled by animals.
Now a days tractors are used to make the process faster and cheaper.
b Harvesting : Cutting the matured crop in the fields is called harvesting, done manually
by sickle or by harvester.
d. Irrigation: The process by which adequate amount of water is supplied to the crops
is called irrigation, which is done using tube wells, wells, rivers, canals or naturally
through rains.
and pests.
Seeds are often dried in the sun and then stored in gunny bags or metal bins for small
scale storage.
For large scale storage of grains, Granaries or Silos are used.
In addition, the stored grains are sprayed with pesticides regularly to keep them safe
from attack by pests and disease causing microbes.
A. Rabi crops: The crops that are grown in the winter season during the months
of October-December and are harvested during March-April.
Eg. wheat, mustard, gram, barley.
Kharif crops: These are the crops that are grown during summer season from June-
July and are harvested during September-October. These crops need more water
to grow, hence they are grown in monsoon and are called as monsoon crops also.
Eg: paddy, groundnut
Q4. What are the advantages of ploughing the soil before sowing seeds in it?
A. Ploughing the soil has following advantages—
Loosened and well aerated soil, provides air to the roots of the plants to grow.
Loose soil helps the roots to penetrate deep in the soil
It allows the growth of earthworms and other useful microorganisms in the soil
Loosening and turning of soil provides fresh soil which is rich in nutrients to
the plants.
It also enables the soil to mix uniformly with the fertilizers.
A. Farmers these days use a combination of manures and fertilizers as depending entirely
on fertilizers might increase crop yield (as they are nutrient specific) but may
cause health hazards and in the long run the soil becomes acidic and unsuitable
for crops.
Hence to substantiate the use of fertilizers, manures are used which are organic and
have no health risks nor they harm the soil.
Q7. Why are pea plants sown after a crop of wheat is harvested?
A. Pea plant is a legume crop is sown between two successive cereal crops such as wheat.
Legume plants harbor rhizobium bacteria in their root nodules. These bacteria
help the plants in Nitrogen fixation in soil. Thus, growing a legume crop helps
in replenishment of nutrients in the soil.
Q 8. Give reasons.
a. Some crops are harvested in summers and others in winters.
Kharif crops are grown during rainy season as they need more water and are
completely matured by summer season during which they are harvested.
Eg. paddy,groundnut
Rabi crops are those which require less water, hence they are grown in winter season
and are harvested in March-April.
Eg: wheat
b. Manures are added to the soil.
Manures are added to the soil to make it rich in organic nutrients, increase
water holding capacity of the soil, no risk of making the soil toxic.
i.e it is ecofriendly.
In drip irrigation, water is allowed to drip directly near the roots of the crops which
prevents wastage of water, hence economical compared to sprinkler in which water is
sprinkled all over the field.
A. N2 + O2 - 2 NO
2 NO + O2 -- 2NO2
HOTS
A. Slash and burn is a method of farming that is adopted by farmers. Find out where
these methods are used. Explain the method.
A. Slash and burn farming is a form of shifting agriculture where the natural vegetation
is cut down and burned as a method of clearing the land for cultivation, and then
when the plot becomes infertile, the farmer moves to a new fresh plot and does the
same again. This process is repeated over and over.
The resulting layer of ash provides the newly-cleared land with a nutrient-rich layer
to help in the growth of the crops.
B. Mantra wants to grow some vegetables in her garden. Which kind of manure will
you suggest her to use – chemical fertilizer or farm manure, or both.
A. Mantra needs to go for farm manure as she is growing vegetables on a small scale
but occasionally she has to add NPK fertilizer to maintain the balance of all the
nutrients for better yield.
Pg3
Q Why is paddy not grown in the winter season?
A. Paddy is a kharif crop which is sown in rainy season. It requires lots of water for
its growth. During winter large amount of water is not available. Hence due to
unavailability of sufficient water and unfavourable climatic conditions (temperature,
humidity) paddy should not be grown in the winter season.
Pg 5
Q Why is ‘depth’ an important criteria while sowing seeds?
A. Seeds come in a variety of sizes and each size needs to be planted at different depth
for maximum germination.
Shallow planting can cause the plant's main root system to form right at the soil
surface and shallow roots do a poor job of taking up water and nutrients
and anchoring the plant.Hence depth is an important criteria while sowing seeds.
Pg 8
Q Which method of irrigation must be used in the areas which are prone to drought?
A. Drip irrigation system
Pg10
Buffer stock of crops is also maintained while storing crops. What do
you understand by buffer stock? Why do you think it is maintained?
A. While fixing procurement targets, need for renewing and building buffer stocks is kept
in mind. Buffer stocks are surplus stocks of food materials, which are stored in
Go downs. Buffer stocks serve the following purposes:
the country.
ii. These stocks can be used to satisfy the food requirements in the event of all
in production due to adverse climatic conditions such as floods and droughts or due
to plant diseases.
iv. These stocks make it convenient to send food supplies to distinct areas without undue
delay.
2. Microorganisms
Q2 What are vaccines? How do they help in the prevention of the disease?
Vaccines are introduced into the body through injections or in the form of oral
drops.
Our body produces Antibodies in response to the injected Antigens.
These Antibodies then remain in the body, to fight the Antigen and protect the
body from any future infections and the body becomes immune to that particular
disease for which the vaccine is given.
Any disease causing microbe or agent that enters our body is referred to as
Antigen.
The substances (proteins) that our body produces to help fight off the Antigens
are referred to as Antibodies.
Microbes like Bacteria and Fungi are used to make Antibiotics that are used to
cure many microbial diseases. Eg: Penicillin and Streptomycin are the Antibiotics
prepared from bacteria.
Vaccines are prepared from dead or weakened germs such as Polio vaccine.
Bacteria like Rhizobium and Blue green algae help in fixing atmospheric
Nitrogen and convert them to usable Nitrogen compounds in Leguminous
plants.
Microbes like Bacteria, Fungi and Protozoa help in decomposing the organic
waste into manure.
Many bacteria are used in Tanning and Leather Industry.
A. Heating the food items such as milk to high temperatures (70 0 C to 100 0 C) for 15 to
30 minutes followed by immediate chilling to kill microbes is called Pasteurization.
This technique is used to make it safe to consume and to increase the shelf life of milk
Q9 “Viruses are considered to be on the borderline of living and non living”. Justify.
A. Viruses are capable, of multiplying rapidly using the cellular machinery of the host
Cells (other living organisms) in which they grow.
A virus is like a non-living thing outside the body of the organisms, therefore they are
considered to be on the borderline of living and non-living.
Q10 Name any two diseases caused by microbes and their mode of transmission in
plants, animals and humans each.
Organism Microbe Disease Mode of
transmission
Plants 1.Bacterium 1.Citrus canker 1. Air
2.Virus 2. Tobacco mosaic 2. Insects
Animals 1. Bacterium 1. 1.Anthrax 4. 1.contact with
infected animals
2. Virus 2. 2.Foot and mouth
5. 2. Contaminated
disease 6. air and water
3.
Humans 1.Bacterium 1. Cholera 1. contaminated
food, water and
2.Virus 2. Chickenpox air.
2.Infected clothes
A. Different methods can be adapted to kill the germs in the food and to preserve it-
Drying or dehydration : Removing moisture or extra water content from food
items is called dehydration. This prevents the spoilage of food as moisture is one
of the pre- requisite conditions for the growth of microbes.
Sweetening and Salting : Addition of sugar or salt to food items forces microbes
lose water from their bodies by a process known as Osmosis,thus preventing
them from growth and reproduction in the food material and the food stays
fresh.
Heating : Heating food items to high temperatures by Pasteurization techniques
helps in killing germs present in the food. Eg: Dairy products.
Using chemical preservatives : Chemicals like Sodium benzoate and Potassium
metabisulphite when added to the food removes the oxygen content from the
food items thus preventing the growth of microbes.
Canning : Once the food items are sterilized, they are stored in air tight
containers and cans to prevent the growth of microbes, used in preservation of
jams, squashes, ketchups.
Freezing : Storing the food items in the freezer does not allow microbial growth
and keep the food items preserved for a very long time.
Eg: vegetables, fish, meat, fruits etc.
Q2 Microbes are both bane and boon. Justify the statement with examples.
A. Microbes are considered both boon and bane for their advantages and disadvantages :
Microbes are harmful to human, animal and plant life and to the overall environment
in general as they cause serious diseases in them which are sometimes fatal.
On the other hand, some microbes are actually helpful in maintaining the ecological
balance in the environment by their important role in breaking down the dead organic
matter to their basic constituents, thus returning it to earth’s crust for possible
recycling.
e. Viruses : These are the smallest of all microbes and cane be seen only with the
Electron Microscope.
They do not have any cellular structure
They remain inactive until they enter a living organism and hence are
considered to be the borderline of living and non-living
Eg: HIV virus, Tobacco mosaic virus etc.
A. Kneading a mixture of flour, salt, sugar, yeast cells and water into dough is an
important step in bread production.
The process responsible for the fluffiness of bread is the decomposition of sugar
by yeast cells to produce CO2 gas and alcohol and the process is called
fermentation.
As more and more gas is produced, the dough rises in volume making the bread
porous and spongy.
(Baking the expanded dough at 1800C kills the yeast and stops fermentation, the
alcohol evaporates to form special compounds which impart taste to the bread.)
A. Bacteria play important roles in the Global Ecosystem. Decomposition is the breakdown
of organic compounds by these organisms, and the release of nutrients back into the
environment, which is one of the most important roles of the bacteria in nature.
The cycling of Nitrogen is another important activity of Bacteria. Plants rely on nitrogen
from the soil for their health and growth, and cannot acquire it from the gaseous
nitrogen in the atmosphere. The primary way in which Nitrogen becomes available to
them is through Biological Nitrogen Fixation by bacteria such as Rhizobium.
HOTS
Q1 Microbes cause diseases when they are present in our body, but this is not the case
with ruminants. Why?
A. Some microbes cause diseases in our body but ruminants need certain microbes in their
Rumen to survive.
This is because they feed on plants and the microbes present in their gut help in
digestion of cellulose present in plant cell walls, with the help of the enzyme cellulose.
Q2. Riya’s mother used to wrap curd and keep it inside a hot case during winters. But
now in summers, she keeps the container outside to set the curd. Give reasons.
A. Curd is formed by setting of milk by Lactic acid Bacteria which can multiply at 37–
42oC.Due to low temp in winter the desired bacterial activity doesn’t take place, so
curd formation becomes difficult.
Hence Riya’s mother kept the semi hot milk(40–50oC) with curd culture in a hot case,
so that curd can be made easily in winters too, as temp. in the hot case can be
maintained to activate
the bacterial actions.
Q3 Ruchita boiled milk. She allowed it to cool down and left it in the kitchen. The
next morning she observed that milk got spoiled. Why? How could she have saved
the milk from getting spoiled. Discuss.
A. The milk spoiled due to the growth of pathogenic microbes. So the milk should be
stored in the refrigerator at very low temperatures to slow down the growth of the
pathogens.
HOTS
pg 26
1. Shouvik was advised to take antibiotics…..
A. If one fails to complete a course of antibiotics, some of the bacteria causing the infection
may survive and become resistant to the Antibiotic. This will result in increase of the
bacterial population having a higher than normal resistance to that antibiotic and the disease
would not be treatable with the same antibiotic.
Pg 27
2. Anil did not cook chicken properly….
A. If improperly cooked food (chicken) is consumed, many microbes(pathogens) may
linger on the food, which may cause illness in the person such as Food Poisoning
(Salmonella Poisoning), Vomitting, Diarrhoea, Headache, Fever etc.
3. SYNTHETIC FIBRES AND PLASTIC
I. A.MCQ’s
1(c) acrylic 2(c) polythene 3(a) acrylic 4(a)jute 5(d)PET 6(c)ester
B. True or false:
C.
1. Cotton 2.polystyrene 3.renew 4.expensive
D. 1.rayon 2.nylon 3.acrylic 4.bakelite 5.PET 6.spandex
HOTS:
1. Which materials are parachutes and ropes made of? What is the basic quality that a fibre should
possess to make parachute and rope?
A. Nylon is used to make parachutes and ropes as nylon is strong, elastic, light and absorbs less
water.
2. What kind of fibre we should not wear while working in a lab? Why or why not?
A. We should not wear fabrics made of synthetic fibre in the lab as they may catch fire easily.
3. Why do we prefer to store materials and chemicals in a plastic bottle rather than a steel, iron, or
glass bottle?
A. Plastic does not react with chemicals and does not corrode. Hence, it is used to store chemicals.
4. Nalini wants to learn swimming. She goes to a store to buy swimming costume and a cap. What
kind of material should these be made of and why?
A. Nalini should buy swimming costumes made of spandex as it snugs to the body , absorbs less
water and dries fast.
5. The MCH has kept two bins namely green and blue bin. What kind of garbage should be thrown
in these bins?
A. Wet waste like vegetable, fruit peels and organic matter should be thrown in a green bin. Dry
waste such as plastics, newspapers, card boards etc should be thrown in a blue bin.
Page 32
1. Polyester is blended with cotton to make polycot or polyester is blended with wool to make
polywool.
2. a. Sodium b. Sodium
Page 60
b. 2K + 2H2O 2KOH + H2
c. 4Na + O2 2Na2O
Page 63
3, 4 true
6. More reactive metals react with water to give metal oxides , hydroxides and hydrogen.
Page 64
I. A
1.b. red-brown 2.c. malleability 3.b. potassium 4.c.steel
5.c.graphite 6.b.sulphur
B
1.Silicon, Germanium, Arsenic
2.Platinum, Copper , Lead
3.Mercury 4. Alloy 5. Galvanization 6. Diamond
C
1. Iron is in solid state at room temperature.(Gallium has low mp, melts on palm)
2. Zinc is more reactive than mercury.
3. Non-metals form acidic oxides.
4. Gold is a lustrous metal.
5. Potassium is at the top of the reactivity series.
F.
4. The property of metals which says that they can be beaten into sheets.
1. Define;
a. Corrosion: It is the process by which metals get eaten up when they come in contact with air
and moisture present in the atmosphere.
b. Metallurgy: It is the branch of science that deals with the extraction of metals.
C. Displacement reaction: A reaction in which a more reactive metal displaces a lesser reactive
metal from the aqueous solution of its salt is known as displacement reaction.
D. Sonority: It is the property of metals by virtue of which metals produce a ringing sound when
struck by another object.
A. Potassium and sodium are the most reactive metals that react with oxygen and moisture even
at room temperature and catch fire. Hence, to avoid this they are stored under kerosene.
3. Name one metal and one non-metal that exist in liquid state at room temperature.
A. Mercury – metal
Bromine – non-metal
4. Name any two ores along with their metals from which they are extracted.
b. Zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen
Zn + H2SO4 ZnSO4 + H2
A. Potassium is the most reactive metal whereas platinum is the least reactive metal.
A. Plastic is an insulator and hence heat does not pass through it. To prevent burning of hands,
handles are made of plastic.
4. Define rusting of iron and write any three methods to prevent rusting of iron.
A. When an iron object reacts with oxygen and water, its surface gets covered with a reddish
brown substance called rust and the process is called rusting.
B .galvanization
A. Pickles and curd contain organic acids that react with metals to form metal salts that are
toxic.
6. What is reactivity series? Why cannot copper displace iron from iron sulphate solution?
A. The arrangement of metals in the order of decreasing reactivity is called reactivity series of
metals. Copper being less reactive than iron, cannot displace iron from iron sulphate solution.
A. Metals react with acids to replace hydrogen to form metal salts. As non-metals do not react
with acids hence don’t replace hydrogen.
HOTS:
1 .What is the choice of material for building bridges? Why?
A. Steel is used for building bridges due to its high tensile strength (resistance of material to
break under tension ability to bear heavy weights)
2. Helium gas is used for filling hot air balloons. Which property of He is used here?
A.Helium is lighter than air and moreover it is the least reactive gas and hence it is used for
filling hot air balloons.
3.A material appears dull and brittle and does not conduct electricity. Is it a metal or a non-
metal?
A. These are the physical properties of non-metals hence non-metal.
4. Mercury is a metal, but appears unusual. Why?
A. Mercury is a only metal which is liquid at room temperature hence appears unusual.
5. Why are cooking utensils, knives and surgical instruments made of stainless steel?
A. Stainless steel is strong, durable and does not corrode and hence it has a wide application.
Page 76
E. Concept map
1.combustible – eg petrol, kerosene
2.Non-combustible- eg stone, glass
Types of combustion : complete-eg LPG ; Incomplete- eg forest fire
F.crossword
1.calorific value 2.combustion 3.fuel 4.flame 5.non-renewable 6.renewable
7.acid rain 8.fire extinguisher 9.global warming
II.Short answers:
b.Exhaustible resources: The resources that get exhausted on using are called exhaustible
or non-renewable resources. Eg. Coal, petrol
Inexhaustible resources: The resources that do not get depleted and can be used over a
long period of time are called inexhaustible or renewable resources. Eg. Sun, wind
To generate electricity
In motor vehicles
A. The efficiency of a fuel is determined by its calorific value. Higher the calorific value, more
efficient is the fuel.
A. The increase in average temperature of the earth due to increased levels of green house
gases like CO2 produced as a result burning of fuels in huge quantities is called global
warming. The carbon dioxide traps the infrared radiation from sun , not allowing it to escape
from earth’s surface.This increases the temperature of earth resulting in global warming.
It causes melting of polar ice caps that increases the sea levels which results in floods in
coastal areas.
A.A soda acid fire extinguisher consists of a glass bottle filled with sulphuric acid. This bottle
is placed over sodium bicarbonate solution, placed in a closed container. When the knob of the
container strikes the acid bottle, the bottle breaks and thus the acid reacts with baking soda
and liberates carbon dioxide that is forced out with the liquid through the nozzle to extinguish
the fire.
HOTS:
1.How is the spilling of crude oil in the sea harmful for animal and plant life?
A. Spilling of crude oil is harmful especially to sea birds and sea otters because these animals
lose the ability to keep them warm when the oil gets smudged on their body. Plants on the
other hand cannot synthesise food due to lack of sunlight that is blocked due to oil that floats.
A. LPG is an odourless gas. To detect the leakage of this gas to prevent accidents, a typically
smelling chemical called ethyl mercaptan is mixed with LPG.
A. Steam coal which is intermediate between anthracite and bituminous is used for power
generation as it is energy efficient and causes less pollution.
4. Why do goldsmiths blow the outermost zone of a flame to get metallic gold?
A. Outermost zone is the zone of complete combustion and it is the hottest zone.
5. What will happen if the ignition temperature of the substance is lower than room temp?
Q2. Draw a well labelled diagram of a generalized cell—pg 100 figure 7.8
b. Centrosomes :
These are found only in animal cell and not in plant cells.
They are not membrane bound and help in cell division
They contain two granules like structures called centrioles which are hollow and
cylindrical and are made of microtubules.
c. Cell membrane :
It forms boundary of the cell which is about 7 to 10 nm thick.
It gives shape and protection to the cell. It is made up of Phospholipids and
Proteins.
Being selectively permeable, cell membrane regulates the movement of materials
in and out of the cell.
Q1 With the help of a neat and well labelled diagram, differentiate between plant
and animal cells.
A. Table 7.2-pg104/105
Synthesis of complex
polysaccharides, processing,
Golgi Apparatus
packaging and distribution of
materials.
Centrosome(only
Helps in cell division
in animal cells)
A. Lysosomes are called suicidal bags of the cell as they have the ability to digest t
the dead or damaged cell in which they are present. In the process they themselves are
also digested by the enzymes they produce.
HOTS
A. Which part of the plant cell that we eat acts as roughage? Why we cannot get
roughage from animal food?
A. The term "roughage" is used for the different types of fibre, one of which is cellulose
present in cell wall of plant cells only, and not in the animal food.
A. The brain, located in the head and spinal cord in the vertebral column are vital to
keep the body alive and functioning. Everything we do, depends on the messages
(nerve impulses) that are sent from the brain, along with the spinal cord to the rest of
the body. Hence any injury to either of these might prove fatal.
A. Air bubbles interfere with resolving power of lens and we might get unclear images due
to the air bubbles. Secondly, bubbles sometimes appear as part of the specimen
(Eg. cocci) and interfere with our observations.
A. Children inherit pairs of genes from their parents. A child gets one set of genes from
the father and one set from the mother. These genes can match up in many ways to
make different combinations. This is why many family members look a lot alike and
others don't look like each other at all.
Pg 10
Q 4. Do you know cells also kill themselves? Find out more about this killing? What is
it called?
5.What will happen when the ignition temperature is lower than the room temperature?
A. Combustible substances catch fire without the supply of heat if their ignition temperature is
less than the room temperature.
A. Flame is less denser than the surrounding air. The surrounding air being denser pushes the
flame upwards.
7. Reproduction
A. The organisms in which both male and female reproductive organs are
present within the same individual are called hermaphrodites.
Eg: Earthworm, Leech
Q2 Briefly describe the process of reproduction in Amoeba.
A.
In a human body, there are 23 pairs of chromosomes out of which 22
pairs of chromosomes are responsible for the physical characters and
the 23 rd pair of chromosome also called the Sex Chromosome is what
determines the gender of a baby.
A female has 22+ XX chromosomes, whereas a male has 22+XY
chromosomes.
When an egg is fertilized by a sperm that contains Y chromosome,
the zygote will develop into a male child.
When the egg is fertilized by a sperm that contains X chromosome, the
zygote will develop into a female child.
Which means that the chromosome of the father determines the sex of
the child and not the chromosome of the mother.
Male gametes are introduced into the female reproductive tract by the
penis.
The male gametes i.e. the sperms swim through the female genital tract
and reach the Fallopian tube.
The sperm fuses with the egg in the Oviduct/Fallopian thbe which
results in the formation of the Fertilized egg or Zygote.
Q6 With reference to Gestation period, explain the transition of embryo to an
adult.
Ans. The period of development of zygote into foetus after which the child birth
occurs is called the gestation period.
After the zygote is formed it divides many times and forms a ball of
cells called embryo which gets embedded in the inner lining of the
uterus wall where further development occurs.
Once implanted, the embryo divides further and starts differentiating
into different cells and perform specialized functions. This is called cell
differentiation or cell maturation.
The cells then fprm tissues and organs.
At about 6 to 8 weeks,the embryo reaches the stage where it can be
recogised and is called foetus.
The foetus develops completely in about 40weeks.
This period of complete development of the foetus is called Pregnancy
or Gestation period.
After the completion of Gestation Period of 40 weeks the child birth
occurs which is called Parturition.
Q7 Differentiate between :
A. a.
Asexual reproduction Sexual reproduction
In this type of reproduction , a In this method of reproduction,
single parent gives rise to a new both the male and female
individual. organisms are involved.
It does not involve the fusion of Male and female gametes fuse to
male and female gametes. form a zygote.
The offspring produced by this The offspring produced by this
method are the exact copies of method are not exact copies of
their parent as there is no genetic their parents as they show genetic
variation. variation.
Lower group of organisms Higher group of organisms
reproduce by this method reproduce by this method.
Eg: Amoeba, Hydra Eg:Humans,Dogs
b.
Zygote Foetus
It is a stage of the embryo that
It is a fertilized egg formed after shows all the main recognizable
the fusion of the sperm with the body parts of a mature organism.
egg.
The zygote divides several times to An embryo gradually develops
form an embryo. into a foetus.
c.
Internal fertilization External fertilization
Fertilisation which takes place External fertilization is a form of
inside the body of the female fertilization in which a sperm cell
organism is called internal fuses with an egg cell external to
fertilization. the body of the female.
HOTS
A. How does malarial parasite reproduce?
The female anopheles mosquito infects humans with plasmodium parasite
that causes malaria.
There are two stages in the life cycle of Plasmodium-the parasite
reproduces asexually in the RBCs of the humans.
i. After fertilisation takes place inside the body of the hen, the fertilized egg
or zygote divides repeatedly to form embryo which travels down the
oviduct.
ii. As it travels down the oviduct, many protective layers are formed around
the embryo. The hard shell that we see in a hen’s egg is the outermost
protective layer.
iii.Complete development of the embryo takes place inside the egg.
iv. When the eggs are laid they hatch to release the chicks.
Pg 114
1. How are twins born? What are Siamese twins?
Ans. Identical twins come from a single fertilized egg, or zygote, that splits into
two. Both halves contain the same DNA and eventually form two foetuses.
Conjoined twins or Siamese twins develop when an early embryo partially
separates to form two individuals. Most conjoined twins are still born or die
shortly after birth. The babies are often attached at the chest, pelvis or buttocks.
The twins may share organs.
Surviving twins may be separated surgically. Success depends on where they're
joined and which organs they share.
Pg 118
2. A mother has given birth to a female child. What is the sex chromosome
given by both the parents?
A. Mother – X chromosome and Father --- X chromosome
3. Can human cloning be referred to as a natural process when identical twins
are delivered?
A. Cloning is a process of creating an exact genetic copy of an organism.
Birth of identical twins when a single zygote splits into two which results in the
formation of two embryos in the same pregnancy can be referred to as a
natural process of cloning.
4. Do you think there can be ethical issues regarding cloning? If yes, what are
those issues?
A. Yes, some of the ethical issues regarding human cloning are-
It interferes with nature.
It might decrease the overall value of human life.
It might be used for exploitation.
8. THE AGE OF ADOLESCENCE
HOTS Page 126
Q. Who needs more Calcium adults or children?
A. The Calcium is an important constituent of the bones and majority of growth occurs in
the bones in children and adolescents in bones, hence this group needs it more. (Daily
requirement 1300 mg/day).
Endocrine
System
1. Discuss the Abnormalities caused because of over and under secretion of hormones.
2. Differentiate and compare between the changes that occur in both females and males
at the time of puberty.
A.
Parameters Boys Girls
Age Puberty sets in boys between Puberty sets in girls between ages 8-
9-14 years old (11 ½, on 13 (age 10, on average)
average)
Physical Chest widens and shoulders Hips broaden and the breasts start
Appearance broadens. Attainment of adult developing. Attainment of adult
height. height.
Voice Increased size of larynx and Smaller size of larynx and vocal
vocal chords cause voice to be chords cause voice to be shrill and
of low pitch. Adam’s apple high pitched.
develop.
Sexual Growth of facial, armpit and Growth of armpit and pubic hair.
characteristics pubic hair. Menstruation starts.
Sweet glands Starts secreting more sweat Starts secreting more sweat and hence
and hence cause increased cause increased body odour
body odour
Sebaceous Pimples and acne erupt Pimples and acne erupt
glands
Behaviour Emotional and psychological Emotional and psychological changes
changes occur. occur.
6. What are the functions of the pituitary gland? Name two hormones it produces, along
with their functions?
A. Pituitary Gland
The hormones of the pituitary gland help regulate the functions of other endocrine
glands.
The pituitary gland has two parts—the Anterior lobe and Posterior lobe—that have two
very separate functions.
The pituitary gland is called the “master gland” because its hormones control other parts
of the Endocrine system, namely the Thyroid gland, Adrenal glands, Ovaries, and Testes.
Hormones of the Pituitary Gland-
Growth hormone (GH): GH is essential in early years to maintain a healthy body
composition and for growth in children. In adults, it aids healthy bone and muscle mass
and affects fat distribution.
Luteinizing hormone (LH): LH works along with FSH to ensure normal functioning of
the ovaries and testes.
4. If the force exerted by an object on an area of 20 m2 is 20N. What will be the pressure
applied by the object?
A. A= 20 m2
F =20 N
P= F/A
P= 20 / 20
P = 1 Pa
5. Why does an inflated balloon burst even with a small prick of a pin?
A. Pressure is inversely proportion to area. If area decreases pressure increases and vice
versa. Pointed tip of a pencil has less surface area so the pressure applied on inflated
balloon is more so it bursts.
6. Describe in brief the composition of the air?
A. Air is a mixture of gases. It consists of 78% nitrogen, 21 % oxygen and the rest 1% is a
combination of gases like carbon di oxide, water vapour, helium.
7. Why do iron fillings get stick to a magnet?
A. Iron fillings stick to the magnet because of magnetic force.
8. Describe in brief the working of spring balance.
A. Text book Pg.No. 138
9. What causes nose bleeding at higher altitudes?
A. At higher attitudes, the external pressure is lower than the pressure within our body. As
a result, the blood tends to ooze out by breaking the blood vessels. As the capillaries in
the nose are thin, bleeding generally happens from the nose.
HOTS
1. Deep sea divers wear a special suit to prevent their bodies from getting crushed.
Why?
A. With the increase in depth, the pressure exerted by the liquid also increases. This huge
pressure deep inside the sea might crush the bodies.
2. Activity based question: Stem expels air out. Pouring cold water will make the steam
condense and become water. This reduces air inside the can. Since the pressure of air
inside can becomes lower than air outside, the can’s shape will become distorted.
3. If two objects of larger and smaller mass are thrown from the same height. Which
one will exerts greater force on the ground. Why?
A. When you drop two objects on earth, they do fall together, but that doesn't mean that
they both exert the same force on the earth. The object with the larger mass attracts the
earth more strongly than the one with the smaller mass. Similarly, the earth attracts the
object with the larger mass more strongly than it attracts the one with the smaller mass-
the more massive object weighs more. The reason why they fall together is that while
the more massive object weighs more and is pulled downward harder, it is also more
massive and thus harder to accelerate. A stronger force is needed to accelerate it
downward so that it keeps pace with the smaller mass as the two falls. Its larger weight
perfectly compensates for its larger mass and the two objects fall at the same rate.
Whether on earth or in space, more massive objects exert stronger gravitational forces.
4. A bad posture can result in the pressure in muscles, joints and bones. Explain?
A. In a bad posture the amount of pressure distributed to the body parts are not equal. So
this results in pain due to lack of blood supply to that area.
A. Friction depends on the smoothness or roughness of the two surfaces which are in
contact with each other.
a. When the two surfaces are in contact are smooth, then the friction between them will
be small because the interlocking of smooth surfaces is less. As the degree of
roughness of the two surfaces in contact increases, the friction also increases.
b. When the two surfaces in contact are very rough, then the friction between them will
be very large because the interlocking of very rough surface is too much.
3. A ball on smooth surface is rolling faster than a ball on the rough surface. Why?
A. On smooth surface ball is rolling faster because the interlocking would not have taken
place. Whereas on rough surface interlocking takes place and hindered the free motion
of the ball.
4. Friction produces heat. Is this an advantage or a dis advantage? Explain.
A. When two surfaces rub against each other, they generate heat because of friction. This is
unwanted heat. If this unwanted heat produced in machines is increased it causes wear
and tear of machines.
5. Differentiate between static and sliding friction.
Ans: Static friction occurs when two objects are at rest with respect to each other, but
kinetic friction occurs when two are moving with respect to each other. Static
friction varies proportional to the external forces, while the sliding (dynamic) frictional
forces remain constant, independent of velocity and the acceleration (and the resultant
external force).
1. Show with the help of an activity that a box, on a floor when pushed, will move with
ease than a box has a carpet, underneath it.
2. List some advantages of friction.
A. Text book page no 152 any 5 points.
3. How can you reduce friction?
A. Text book page no 154 (3 points).
4. Why is sliding friction less than the static friction?
A. When something is static, a greater force is required to break the interlocking between
two surfaces. When something is in motion, there is no need to break the interlocking
and a smaller force is required the keep the object in motion. Hence, sliding friction is
less than static friction.
5. How is friction applied in our everyday life?
A. Text book page no 152 any 5 points.
6. Why friction is considered dis advantageous?
A. Text book page no 153 any 5 points.
1. Shoes has many treads which offer more friction because of more irregularities in the
surface. This gives better grip to sportsmen while running.
2. Treading in tyres increases friction because of more irregularities in the surface. This
gives better grip to tyres.
3. Teflon is the registered trade name of the highly useful plastic material
polytetrafluoroethylene (PTFE). It an excellent material for coating machine parts
which are subjected to heat, wear, and friction, for laboratory equipment which must
resist corrosive chemicals, and as a coating for cookware and utensils.
4. We cannot walk, write and drive, without, friction. In certain cases like the motion of
machines, wear and tear of machines, loss of energy friction is an ‘evil’.
5. Remains unchanged
6. Whenever an object moves through a fluid .It is called fluid friction. When solid bodies
move through liquids and gases, they also experience this opposing force of friction. For
example, a kite flying in the air, an aero plane moving through air and a submarine
moving through water.. Water is much more viscous than air, so there will be much more
frictional force or drag on an object when it moves through water than when it moves
through air.
7. Glass surface is so smooth that it does not provide friction to the tip of the chalk. Hence,
it is difficult to write on a glass surface.
11. SOUND
Answer.
The ear are the sense organs which help us in hearing sound.
1) Outer Ear: The outer part of the ear is called Pinna and is attached to a passage called ear
canal.
2) At the end of ear canal a thin, elastic and circular membrane called an eardrum or
tympanum, is stretched tightly.
3) There are three small and delicate bones called hammer, anvil and stirrup in the middle part
of the ear which are linked to one another.
4) One end of hammer touches the eardrum and its other end is connected to second bone
anvil.
5) The other end of anvil is connected to third bone called stirrup. And the free end of stirrup
touches the membrane over the oval window.
6) The inner part of ear has a coiled tube called cochlea. Cochlea is filled with liquid. The
liquid present in cochlea contains nerve cells which are sensitive to sound. The other end of
cochlea is connected to auditory nerve which goes into the brain.
Q2. Show with the help of an activity that sound cannot travel in vacuum whereas it can travel
in water.
Experiment A: Sound cannot travel in vacuum- An electric bell is suspended inside an airtight
glass bell jar connected to a vacuum pump. As the electric bell circuit is completed, the sound
is heard. Now if the air is slowly removed from the bell jar by using a vacuum pump, the
intensity of sound goes on decreasing and finally no sound is heard when all the air is drawn
out. We would be seeing the hammer striking the gong repeatedly. This clearly proves that
sound requires a material for its propagation.
Experiment B: Sound can travel in water - Take a bucket, fill it with water and take a small
bell in one hand. Shake the bell inside the water to produce sound. Make sure that the bell
does not touch the body of the bucket. Place your ear gently on the water surface. You can
hear the sound of the bell which indicates that sound can travel through liquids.
Q3. What are the some ways in which we can control noise pollution?
Answer. Refer page 169 (six points given to reduce sound pollution).
Q5. How does sound travel? Refer page 162 figure 12.4.
Answer. Sound travels through a medium in the form of waves. When vibrations take place,
they are transmitted through a medium and form alternate compression and rarefactions.
Compressions are regions in the medium where the particles are closer whereas rarefactions
are regions in the medium where the particles are spread out.
Q6. Differentiate between:
a)
String instruments Percussion instruments
Stringed instruments make use of a string Percussion instruments make use of
or wire to produce vibrations and sound. stretched animal hide to produce
vibrations and sound.
The frequency of sound is varied by The frequency of vibration can be
varying the length of the vibrating wire. increased by stretching the hide more.
In a sitar, the shorter the length of the In a drum the hollow body amplifies the
wire, the higher the pitch it produces. noise of a stretched membrane being hit.
Pitch is the quality of sound which is decided or governed by rate of vibrations. Pitch depends
on frequency. It is measured in hertz. Higher the frequency higher the pitch.
c)
Ultrasonic Infrasonic range
Ultrasonic is usually applied to sound Infrasonic, is sometimes referred to as
which is referred to anything above the low-frequency sound, this sound is lower
frequencies of audible sound, and in frequency than 20 Hz (hertz).
includes anything over 20,000 Hz.
Bats, dolphins produce ultrasonic sound. Avalanches, meteors, and earthquakes
produce infrasonic sounds.
B. Does noise have high frequency or low frequency? Are the vibrations regular and periodic
in case of noise?
A. Noise is unpleasant/unwanted sound caused by high frequency vibrations.
These vibrations are irregular and non- periodic in case of noise.
Sound is what we hear. Noise is unwanted sound. The difference between sound and noise
depends upon the listener and the circumstances. Rock music can be pleasurable sound to one
person and an annoying noise to another.
*********
12. Chemical Effects of Current
1. Define electrolysis.
A. The process by which the electrolytes get dissociated into ions when electric current is
passed through them.
2. Differentiate between electrolyte and non- electrolyte.
Electrolytes Non- electrolyte
Electrolytes are substance that Non - Electrolytes are substance
conduct electricity in the molten that do not conduct electricity in
state or dissolved in water. the molten state or dissolved in
water.
Electrolytes dissolve in water and Non - electrolytes cannot get
forms ions in solution ionized when dissolved in water.
They have molecules.
Ionic compounds and some Compounds with non-polar bonds
compounds with polar bonds can are mostly non electrolytes.
be electrolytes.
1. If a plastic rod is rubbed with wool it gains electrons and becomes negatively charged
body. It develops static electricity. It can neither attract nor repel magnet.
2. They repel each other.
3. Cloth acquires negative charge.
13. SOME NATURAL PHENOMENA
Page 189:
Page 193:
Page 196:
2, 5 are true
D. Crossword:
2. How can you find the nature of charge on a body using an electroscope?
A. The nature of charge on a body can be found out by touching the charged electroscope by
the object whose charge needs to be known. If the gold leaves repel, then the charge of the
object is the same as electroscope.
A. Lightning conductor protects the high rise buildings by allowing the lightning discharge
pass to the earth.
A. The flow of negatively charged free electrons through a conductor constitutes electric
current. Its SI unit is ampere.
1. Description of working of gold leaf electroscope is given on pages 189 and 190.
*Charging by induction: This can be done by bringing the object closer to the charged body
without touching it. The body acquires an opposite charge.
*Friction: This can be done by rubbing the two bodies against each other to acquire equal and
opposite charges.
*Conduction: This can be done by touching an already charged body. The object acquires the
same charge as the charged body.
A. The tectonic plates present in the continental crust move apart or towards each other. When
the plates accidentally slip against each other, a lot of heat is released and sends seismic waves
in all directions. The point of origin of seismic waves is called seismic focus and the place
above the point of focus where shock is felt is called epicentre of the earthquake.
A. Benjamin Franklin performed kite and key experiment to show that lightning occurs
because of charges between the clouds.
He made a kite of silk cloth and for the central spar, he used an iron wire. He tied silk thread
to the spar and to the free end, he tied a metal key. He tied insulating material to the key to let
the kite fly on a cloudy day. The kite soared very high. When it started raining, he observed
electric sparks from the key due to flow of electric charges in the sky.
6. Refer to page 193 for answer and diagram of structure of the earth.
HOTS:
1. Can a lightning strike a flying plane? How it might affect the plane? Find out any one
instance from the past when lightning struck.
A. commercial plane crash in the USA is attributed to lightning wayback in 1967 which caused
explosion of fuel tank. These days the planes receive lightning certifications which help the
passengers to fly safely without much damage.
2. What will be the charge of the electroscope when an ebonite rod rubbed with wool touches
the metal disc?
A. An ebonite rod acquires negative charge due to flow of electrons from the wool to the
ebonite rod.
14. LIGHT
When a parallel beam of light that When a parallel beam of light that
falls on smooth and well-polished falls on irregular surface, the ray
surface the rays gets reflected in gets scattered in all directions.
only one direction. This This is called irregular or diffused
phenomenon is known as regular reflection
reflection.
Regular reflection gives a clear Irregular reflection gives a blurred
image. image.
Regular reflection obeys the law of Irregular reflection does not obey
reflection the law of reflection
2. With the help of an activity, demonstrate laws of reflection.
A. Text book page no 200 activity 1.
3. Describe refraction through a glass slab.
A. Text book page no 204activity 3.
4. Describe the structure of the human eye, elaborating the function of each part.
A. Cornea - It is transparent part of eye. It is also known as the window or aperture of the
eye.
Functions – It allows the light to enter in the eye.
Iris - It is a circular diaphragm having a hole in its centre. This hole is called pupil. The
circular diaphragm has muscles and coloured pigments. The colour of an eye depends
upon the colour of these pigments.
Function of iris – it controls the amount of light enlarging the eye through the pupil.
Eye lens - The eye lens is a crystalline double convex lens and made of transparent and
flexible tissues. It is behind the pupil and held by the muscles called ciliary muscles.
Functions - It focuses the images of objects on the retina of the eye.
Retina - It acts as a screen to obtain the image of the object. It contains number of cells
in the form of rods and cones which are sensitive to light. These cells convert light
energy into nerve impulses or signals.
5. Distinguish between myopia and hypermetropia.
Myopia Hypermetropia.
Myopia, also known as short Hypermetropia also known as long
sightedness or near sightedness, is a sightedness is a defect of vision in
defect in which a human eye can which a human eye cannot see
see nearby objects clearly but nearby objects, clearly but can see
distant objects appear blurred and distant objects clearly.
unclear.
The eye ball is too long than The eye ball is shorter than normal.
normal.
The image is obtained in front of The image is obtained behind the
the retina. retina.
Myopia can be corrected by placing Hypermetropia can be corrected by
a suitable concave lens. placing a suitable convex lens.
6. How can you take care of your eyes?
A. Text book page no 209 (taking care of eyes). (any 6 points)
HOTS
1. An incident ray on a plane mirror is reflected in a direction such that the angle
between the incident and the reflected ray is 75o. What is the angle of incidence?
What is the angle of reflection?
A. The angle of incidence and the angle of reflection = 37.5o
LIGHT: Refraction of light
Introduction
The change of direction of light because of change of medium is known as Refraction or
Refraction of Light. The ray of light changes its direction or phenomenon of refraction takes
place because of difference in speed in different media.
The light travels at faster speed in rarer medium and at slower speed in denser medium.
The nature of media is taken as relative. For example air is a rarer medium than water or
glass.
When ray of light enters from a rarer medium into a denser medium, it bends towards
normal at the point of incidence. On the contrary, when ray of light enters into a rarer
medium from a denser medium it bends away from the normal.
Laws of Refraction:
a. The incident ray, refracted ray and normal to the interface of given two
transparent media, all lie in same plane.
b. For a given pair of media of different densities, the ratio of sine of angle of incidence to
the sine of angle of refraction is always a constant
The constant is called refractive index of the second medium in relation to the first medium.
This law was first stated by a Dutch astronomer, Willebrord Snellius, and is therefore,
known as Snell’s Law.
The value of constant is fixed for a set of two transparent media and depends upon their
nature. It is called refractive index of the second medium with respect to the first medium.
It is represented by the Greek letter µ (mu). It is the ratio of two similar quantities and thus
it has no units.
Refractive index is also defined as the ratio of the speed of light in vacuum to the speed
of light in the given medium.
𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑣𝑎𝑐𝑢𝑢𝑚
µ=
𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑎 𝑚𝑒𝑑𝑖𝑢𝑚
Table of Different Values Refractive Index of Different Media
1. Diamond 2.42
2. Rock salt 1.54
3. Glass 1.52
4. Water 1.33
5. Ice 1.31
6. Air 1.00
2. A straight stick which is immersed partly in water always looks to be bent at the surface of
water, because the light coming from the stick bends when it comes out at the surface due
to refraction of light.
3. A coin or stone lying at the bottom of a container filled with water appears to be raised
because of refraction of light.
4. A line or a spot of ink on a paper always appears to be raised when viewed through the glass
slab due to the refraction of light.
5. Optical illusions such as mirage and looming are also produced due to refraction of light.
RULES OF REFRACTION OF LIGHT
Following are the rules for getting the direction of bending of a ray of light.
Rule 1
When a ray of light travelling obliquely from an optically rarer medium to an optically denser
medium, it bends towards the normal at the separation of the two media. In this case the angle
of refraction will be smaller than the angle of incidence.
Rule 2
When a ray of light travelling obliquely from an optically denser medium to an optically rarer
medium, it bends away from the normal at the separation of the two media. In this case the
angle of refraction will be greater than the angle of incidence.
Rule 3
When a ray of light strikes the surface of separation of the two media at right angle, that is, the
incident ray strikes the surface normally; it travels along the same straight line path, without
any deviation.
Spherical Lens
Lens is an optical device which converges or diverges the rays of light before transmitting.
A lens has similar shape to lentils and genus of lentil is called Lens, thus a lens got its name
after the shape and name of genus of lentils. A lens is made by combining at least one part
of sphere made of transparent material, generally glass.
Spherical Lens: Most of the lenses are made by the combination of parts of transparent
sphere. Concave and Convex lens are most commonly use spherical lens.
Convex lens is the most commonly used lens in our day to day life.
Convex lens:
A lens having two spherical surface bulging outwards is called Convex Lens. It is also
known as biconvex lens because of two spherical surface bulging outwards.
A lens having two spherical surface bulging inwards is called Concave Lens. It is also
known as biconcave lens because of two spherical surface bulging inwards.
Important terms for spherical lens:
Aperture – the effective length or width of a lens available for refraction is called its aperture.
Optical centre: The geometrical centre of a lens is called its Optical Centre. It is denoted by
the letter O.
Principal Axis: A line passing through the optical Centre and perpendicular to both the faces
of the lens is called Principal axis.
Centre of curvature: The centre of the imaginary glass sphere of which a lens is a part is
called the centre of curvature of the lens. Since a lens has two surfaces it has two Centre’s of
curvature.
Principal Focus: On refraction through a lens, rays of light parallel rays of light parallel to the
principal axis converge or appear to diverge from a point on the principal axis. This point is
called Principal Focus and denoted by F.
Similar to centres of curvature; convex and concave lenses have two Foci. These are
represented as F1 and F2.
Principal Focus of convex lens: The focus of a convex lens is a point on its principal axis at
which all parallel rays coming from infinity converge after passing through the lens.
Principal Focus of concave lens: The focus of a concave lens is a point on its principal axis at
which all parallel rays coming from infinity appear to diverge after passing through the lens.
Focal Length: The distance between optical Centre and principal focus is called focal length
of a lens. Focal length of a lens is half of the radius of curvature.
This is the cause that the Centre of curvature is generally denoted by 2F for a lens instead of
C.
Rules for drawing images in concave and convex lens
2.Incident ray passing through focus 2.Incident ray passing through focus
A ray passing through principal focus A ray passing through the principal focus
emerges parallel to the principal axis after emerges parallel to the principal axis after
refraction from a convex lens. diverging from a concave lens.
3.Incident ray passing through optical 3.Incident ray passing through optical
centre centre
Ray passing through the optical centre of Ray passing through the optical centre of
convex emerges in same direction without concave lens emerges in same direction
any deviation. without any deviation.
There are six possible position of object in the case of convex lens:
a. Object at infinity
b. Object is placed beyond 2F1
c. Object is placed at 2 F1
d. Object is placed between F1 and 2 F1
e. Object is placed at F1
f. Object is placed between O and F1
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