Class-8
Subject-Science
CH- 10 To 18
CH- 10 Reaching the Age of Adolescence
Exercise : Solutions of Questions on Page Number : 124
Q1 :
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.
Q2 :
Define adolescence.
Answer :
Adolescence is the time period between the beginning of puberty and adulthood. During this period, the body undergoes several
changes alongside reproductive maturity. It begins around the age of 11 and lasts till 18 or 19 years of age. The period of
adolescence may vary from person to person.
Q3 :
What is menstruation? Explain.
Answer :
Menstruation is the process of the shedding of the uterine lining on a regular monthly basis. It begins at puberty and is the
reproductive cycle of the female body. Every month, the uterus prepares itself to receive a fertilized egg. Therefore, the inner lining
of the uterus becomes thick and is supplied with blood to nourish the embryo. If the egg is not fertilized, then the lining of the uterus
breaks down and gets released in the form of blood through the vagina. This lasts for about two to eight days. This cycle occurs
every month and is known as the menstrual cycle.
Q4 :
List changes in the body that take place at puberty.
Answer :
Changes at puberty:
(i) Sudden increase in height and weight.
(ii) Broadening of shoulders and widening of chest in boys. In girls, the region below waist becomes wider.
(iii) In boys, under the influence of hormones, the larynx becomes prominent, the vocal cords become longer and thicker. These
changes cause the voice to become hoarse.
(iv) Appearance of hair in areas such as underarms, face, hands, and legs.
(v) Appearance of acne as a result of excessive secretion of oil from skin.
(iv) Testis grows and starts producing sperms in males, whereas in females, the ovary enlarges and starts producing matured eggs.
Q5 :
Prepare a table having two columns depicting names of endocrine glands and hormones secreted by them.
Answer :
Endocrine gland Hormones
Testis Testosterone
Ovary Oestrogen
Thyroid Thyroxin
Adrenal Adrenalin
Pancreas Insulin
Pituitary Growth hormone
Q6 :
What are sex hormones? Why are they named so? State their function.
Answer :
Sex hormones are chemical substances produced by sex organs. For example, testosterone is the male sex hormone produced by
the testis, and oestrogen is the female sex hormone produced by the ovary. These hormones affect the sexual features of an
organism. Hence, they are known as sex hormones.
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, etc.
Oestrogen: This hormone is responsible for the development of secondary sexual characters in females such as the enlargement
of breasts, development of female reproductive organs, etc.
Q7 :
Choose the correct option.
(a) Adolescents should be careful about what they eat, because
(i) proper diet develops their brains.
(ii) proper diet is needed for the rapid growth taking place in their body.
(iii) adolescents feel hungry all the time.
(iv) taste buds are well developed in teenagers.
(b) Reproductive age in women starts when their
(i) menstruation starts.
(ii) breasts start developing.
(iii) body weight increases.
(iv) height increases.
(c) The right meal for adolescents consists of
(i) chips, noodles, coke.
(ii) chapatti, dal, vegetables.
(iii) rice, noodles and burger.
(iv) vegetable cutlets, chips and lemon drink.
Answer :
(a) (ii) Adolescents should be careful about what they eat, because proper diet is needed for the rapid growth taking place in their
body.
(b) (i) Reproductive age in women starts when their menstruation starts.
(c) (ii) The right meal for adolescents consists of chapatti, dal, vegetables.
Q8 :
Write notes on -
(a) Adam's apple.
(b) Secondary sexual characters.
(c) Sex determination in the unborn baby.
Answer :
(a) Adam's apple:
In human males, the larynx grows larger during puberty and can be seen as a protruding part of the throat. This protrusion is known
as the Adam's apple. In boys, under the influence of sex hormones, the larynx becomes prominent. As a result, the vocal cords
become longer and thicker, causing the voice to become hoarse. However, in females, the larynx is of a small size and is hardly
visible. Therefore, girls have a high pitched voice, while the voice of boys is low pitched.
(b) Secondary sexual characters are those features that help in distinguishing the male and the female body from each other.
They are physical or behavioural characteristics that appear in humans at the time of puberty.
Secondary sexual characters in boys:
(i) Appearance of moustache and beard.
(ii) Appearance of chest hair.
(iii) Growth of hair in genital area and other parts of the skin.
Secondary sexual character in girls:
(i) Increase in breast size and darkening of the skin of nipples present at the tip of the breasts.
(ii) Growth of hair in genital area and other body parts.
(c) Sex determination in an unborn baby:
The sex of a baby is determined by the type of male gamete that fuses with the female gamete.
All human beings have 23 pairs of chromosomes in their nuclei. Out of these 23 pairs, the last pair is known as the sex
chromosome.The human males have 23 pairs of chromosomes including XY sex chromosomes. Therefore, the male gamete has 22
chromosomes and either an X or Y sex chromosome.
Male gametes can be of two types: 22+X or 22+Y
Females have 23 pairs of chromosomes including XX sex chromosomes. Therefore, their gametes can only have 22 chromosomes
and one X sex chromosome.
Type of female gametes: 22+X
Thus, as the mother provides only X chromosome, the sex of the baby is determined by the type of male gamete (X or Y) that fuses
with the X chromosome of the female.
Q9 :
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
Answer :
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
Q10 :
The table below shows the data on likely heights of boys and girls as they grow in age. Draw graphs showing height and
age for both boys and girls on the same graph paper. What conclusions can be drawn from these graphs?
Age (Years)
Height (cm)
Boys Girls
0 53 53
4 96 92
8 114 110
12 129 133
16 150 150
20 173 165
Answer :
The graph depicts the relation between the age and height of both boys and girls. During puberty, there is a sudden increase in
height of both boys and girls. On the basis of the above graph, it can be observed that during the age of 4-8 years, girls have less
height as compared to boys. However, as soon as girls reach 12- 13 years, their height shows a sudden increase and becomes
more than boys. In later years, growth in both sexes becomes stable. Growth during puberty is under the control of hormones
CH-11 Force and Pressure
Exercise : Solutions of Questions on Page Number : 142
Q1 :
Give two examples each of situations in which you push or pull to change the state of motion of objects.
Answer :
Two examples of push force are as follows:
(i) A heavy box at rest is pushed to move it from one room to another. This changes the state of motion of the box.
(ii) A player pushes a football using his foot. This changes the state of motion of the ball.
Two examples of pull force are as follows:
(i) Rope is pulled to draw water from a well. This changes the state of motion of the water bucket.
(ii) A drawer is pulled to open it. This changes the state of motion of the drawer.
Q2 :
Give two examples 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 follows:
(i) Squeezing of a plastic bottle changes the shape of the bottle.
(ii) Deformation of clay by pressing it between the hands.
Q3 :
Fill in 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.
(c) To move a loaded trolley we have to _________it.
(d) The north pole of a magnet ________the north pole of another magnet.
Answer :
(a) To draw water from a well we have to __pull__at the rope.
(b) A charged body __attracts__an uncharged body towards it.
(c) To move a loaded trolley we have to __either push or pull__ it.
(d) The north pole of a magnet __repels__the north pole of another magnet.
Q4 :
An archer stretches her 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 the gaps in the following statements using the following terms.
muscular, contact, non-contact, gravity, friction, shape, attraction
(a) To stretch the bow, the archer applies a force that causes a change in its _______.
(b) The force applied by the archer to stretch the bow is an example of ________force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a _______force.
(d) While the arrow moves towards its target, the forces acting on it are due to ________and that due to________of air.
Answer :
(a) To stretch the bow, the archer applies a force that causes a change in its __shape__.
(b) 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.
(d) While the arrow moves towards its target, the forces acting on it are due to __gravity__and that due to __friction__of air.
Q5 :
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 its juice.
(b) Taking out paste from a toothpaste tube.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete making a high jump to clear the bar at a certain height.
Answer :
(a) In squeezing a piece of lemon, we make use of muscular force to extract its juice. This muscular force acts on the lemon. As a
result, the shape of the lemon gets changed.
(b) We use our muscular force to take out paste from a toothpaste tube. The muscular force acts on the toothpaste tube. As a result,
the shape of the tube gets changed.
(c) Here, the suspended load exerts a force on the spring and pushes the spring downwards. As a result, the spring gets stretched.
Hence, its shape gets changed.
(d) An athlete pushes the ground with his feet. His feet exert a muscular force on the ground. This allows him to jump over the bar.
As a result, his state of motion gets changed.
Q6 :
A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of
iron?
Answer :
When a blacksmith hammers a hot piece of iron, he uses his muscular force. This muscular force changes the shape of the iron so
that it can be given a desired shape.
Q7 :
An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the
balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?
Answer :
On rubbing an inflated balloon with a piece of synthetic cloth, it becomes charged. A charged body attracts an uncharged body.
When this charged balloon is pressed against a wall, it sticks to the wall. The force acting between the charged balloon and the wall
is the electrostatic force.
Q8 :
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 :
We make use of muscular force to hold a bucket of water above the ground. This muscular force acts against the force of gravity
that pulls the bucket towards the ground. The two forces are equal in magnitude but opposite in direction. Therefore, the net force
on the bucket is zero. Hence, there is no change in its state of motion.
Q9 :
A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately
after leaving the launching pad.
Answer :
The two forces acting on 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.
Q10 :
When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of
bubbles. Once we release the pressure on the bulb, water gets filled 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
(d) atmospheric pressure
Answer :
(d) The rise of water in the dropper is due to atmospheric pressure.
The rise of water in a dropper is due to atmospheric pressure. When all the air escapes from the nozzle, the atmospheric pressure,
which is acting on the water, forces the water to fill the nozzle of the dropper.
Ch-12 Friction
Exercise : Solutions of Questions on Page Number : 155
Q1 :
Fill in the blanks.
(a) Friction opposes the ________between the surfaces in contact with each other.
(b) Friction depends on the _________of surfaces.
(c) Friction produces _________.
(d) Sprinkling of powder on the carrom board ________friction.
(e) Sliding friction is ________than the static friction.
Answer :
(a) Friction opposes the __motion__between the surfaces in contact with each other.
(b) Friction depends on the __nature__of surfaces.
(c) Friction produces __heat__.
(d) Sprinkling of powder on the carrom board __reduces__friction.
(e) Sliding friction is __less__than the static friction.
Q2 :
Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their
arrangements are given below. Choose the correct arrangement.
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling
Answer :
(c) static, sliding, rolling
Friction comes into play when irregularities present in the surfaces of two objects in contact get interlocked with each other. Static
friction comes into play when we try to move an object which is at rest. Sliding friction comes into play when an object slides over
the surface of another object. In sliding friction, the time given for interlocking is very small. Hence, interlocking is not strong.
Therefore, less force is required to overcome this interlocking. Because of this reason, sliding friction is less than static friction.
Similarly, the area of contact in case of rolling friction is smallest as compared to static or sliding friction. This area of contact
changes gradually because of rolling. Hence, rolling friction is lesser than both static and sliding friction. Thus, the correct sequence
is - static, sliding, rolling.
Q3 :
Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction
acting on the car on different surfaces in increasing orderwill be
(a) wet marble floor, dry marble floor, newspaper and towel.
(b) newspaper, towel, dry marble floor, wet marble floor.
(c) towel, newspaper, dry marble floor, wet marble floor
(d) wet marble floor, dry marble floor, towel, newspaper
Answer :
(a) wet marble floor, dry marble floor, newspaper and towel
Force of friction depends on the nature of surfaces in contact. The rougher the surface, the more is the friction between the surfaces
in contact and vice-versa. Roughness present in the given surfaces can be arranged in an ascending order as wet marble floor, dry
marble floor, newspaper and towel. Hence, the correct sequence of these surfaces when arranged according to the increase in the
force of friction acting on the car is - wet marble floor, dry marble floor, newspaper and towel.
Q4 :
Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force
acting on it.
Answer :
When a book slides on the writing desk, a frictional force acts between the book and the surface of the desk. The direction of
frictional force on the book is opposite to the direction of its motion and acts in upward direction, as shown in the following figure.
Q5 :
You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on
the floor? Why?
Answer :
We are able to walk because of the friction present between our feet and the ground. In order to walk, we push the ground in the
backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction
between the ground and feet decreases when there is soapy water spilled on the floor. Hence, it becomes difficult to walk on the
soapy floor.
Q6 :
Explain why sportsmen use shoes with spikes.
Answer :
Sportsmen use shoes with spikes because these shoes give them a better grip while running. This is because the force of friction
between the shoes and the ground increases with the help of spikes.
Q7 :
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 :
Force of friction arises because of interlocking of irregularities on the two surfaces in contact. When a heavy object is placed on the
floor, the interlocking of irregularities on the surfaces of box and floor become strong. This is because the two surfaces in contact
are pressed harder. Hence, more force is required to overcome the interlocking. Thus, to push the heavier box, Seema has to apply
a greater force than Iqbal.
Q8 :
Explain why sliding friction is less than static friction.
Answer :
Friction comes into play when irregularities present in the surfaces of two objects in contact get interlocked with each other. In
sliding, the time given for interlocking is very small. Hence, interlocking is not strong. Therefore, less force is required to overcome
this interlocking. Because of this reason, sliding friction is less than static friction.
Q9 :
Give examples to show that friction is both a friend and a foe.
Answer :
Advantages of friction:
(i) We are able to walk because of friction.
(ii) Friction between the tip of the pen and a paper allows us to write.
Disadvantages of friction:
(i) Tyres and soles of shoes wear out because of friction.
(ii) Friction between the different parts of machines produces heat. This can damage the machines.
Q10 :
Explain why objects moving in fluids must have special shapes.
Answer :
When a body moves through a fluid, 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 fluid.
Ch-13 Sound
Exercise : Solutions of Questions on Page Number : 168
Q1 :
Choose the correct answer.
Sound can travel through
(a) gases only (b) solids only
(c) liquids only (d) solids, liquids and gases.
Answer :
(d) Sound can travel through solids, liquids, and gases.
Sound requires a medium to travel through. Solid, liquid and gas provide the medium for sound. Hence, sound can travel through
solids, liquids and gases.
Q2 :
Which of the following voices is likely to have minimum frequency?
(a) Baby girl (b) Baby boy
(c) A man (d) A woman
Answer :
(c) A man
The voice of an adult man is of lower pitch in comparison to the voices of a baby boy, a baby girl and a woman. Since frequency of a
sound is directly proportional to its pitch, man's voice is of minimum frequency in comparison to a boy, a girl, or a woman's voice.
Q3 :
In the following statements, tick 'T' against those which are true, and 'F' against those which are false.
(a) Sound cannot travel in vacuum. (T / F)
(b) The number of oscillations per second of a vibrating object is called its time period. (T / F)
(c) If the amplitude of vibration is large, sound is feeble. (T / F)
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T / F)
(e) The lower the frequency of vibration, the higher is the pitch. (T / F)
(f) Unwanted or unpleasant sound is termed as music. (T / F)
(g) Noise pollution may cause partial hearing impairment. (T/F)
Answer :
(a) True
Sound requires a medium to travel through. Since vacuum is devoid of any medium, sound cannot travel through it.
(b) False
The number of oscillations per second of a vibrating object is known as its frequency.Time period is the time required to complete
one oscillation.
(c) False
Loudness of a sound is proportional to the square of the amplitude of its vibration. When the amplitude of vibration of a sound is
large, the sound is very loud. The sound is feeble for small amplitude.
(d) True
Humans cannot hear sounds of all frequencies. Humans can hear a sound whose frequency falls in the range of 20 Hz-20,000 Hz.
The sound having frequency out of this range is inaudible to humans.
(e) False
The pitch of a sound is proportional to its frequency. As the frequency of vibration increases, the pitch of the sound also increases
and vice-versa. A sound is said to be high pitched if its frequency of vibration is high, and is low pitched for a small frequency of
vibration.
(f) False
Unwanted or unpleasant sounds are known as noise. Sounds that are melodious and pleasing to ear are known as music.
(g) True
Unwanted or unpleasant sounds are known as noise. If one is subjected to loud unpleasant sound continuously for a long time, then
it may cause temporary hearing impairment.
Q4 :
Fill in the blanks with suitable words.
(a) Time taken by an object to complete one oscillation is called _________.
(b) Loudness is determined by the________ of vibration.
(c) The unit of frequency is _________.
(d) Unwanted sound is called _________.
(e) Shrillness of a sound is determined by the _________of vibration.
Answer :
(a) Time taken by an object to complete one oscillation is called __time period__.
(b) Loudness is determined by the__amplitude__ of vibration.
(c) The unit of frequency is __hertz (Hz) __.
(d) Unwanted sound is called __noise__.
(e) Shrillness of a sound is determined by the __frequency__of vibration.
(Shrillness is also called pitch of the sound. Pitch is directly proportional to the frequency of vibration. Hence, shrillness is
determined by the frequency of vibration.)
Q5 :
A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer :
Frequency of oscillations is defined as the number of oscillations of a vibrating body per second. It is given by
Q6 :
The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is
the time period of the vibration?
Answer :
The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.
Q7 :
Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak (b) Sitar (c) Flute
Answer :
(a) Dholakis a musical instrument. It consists of a stretched membrane called its head. When the head is beaten gently, the
stretched membrane sets into vibration. Since sound is produced when an object vibrates, the dholakproduces a sound.
(b) Sitaris a musical instrument. It consists of stretched strings. When a string is plucked, it sets into vibration. Since sound is
produced when an object vibrates, the sitarproduces a sound.
(c) Fluteis a hollow pipe. When air is blown over its mouth, the air inside the pipe is set into vibration. As a result, a pleasant sound
is produced.
Q8 :
What is the difference between noise and music? Can music become noise sometimes?
Answer :
The sound that is pleasing to the ear is called music. For example, the sound produced by violins, pianos, flutes, pungs, etc.
The sound that is unpleasing to the ear is called noise.
Some examples of noise are as follows:
(i) Sound produced by horns of buses and trucks
(ii) Sound of electrical generators
(iii) Sound of a gun shot
(iv) Sound produced by jackhammers
Yes. Music can become noise when played at high volumes.
Q9 :
List sources of noise pollution in your surroundings.
Answer :
Some sources of noise pollution are as follows:
(i) Televisions and transistors running at high volumes
(ii) Loudspeakers and crackers
(iii) Horns of buses, cars and trucks
(iv) Home appliances such as mixer, desert cooler, etc.
Q10 :
Explain in what way noise pollution is harmful to humans.
Answer :
Noise pollution can lead to a number of health-related problems. Some of them are as follows:
(i) Hearing loss
(ii) Insomnia; inability to sleep
(iii) Hypertension
(iv) Severe headache
(v) Stress
Q11 :
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 parents should buy? Explain your answer.
Answer :
There will be more noise in the house which is along the roadside. This is because noise produced by transportation vehicles may
cause trouble to the residents. The intensity of noise decreases with the distance between the source and the listener. Hence, it is
better to take the house that is three lanes away from the roadside.
Q12 :
Sketch larynx and explain its function in your own words.
Answer :
Larynx is a part of the throat. It is responsible for production of sound. A sketch of a human larynx is shown in the following figure.
Larynx moves when we swallow something. Inside the larynx, there are two vocal cords. There is a small gap between them. This
small gap allows air to pass through. When we speak, air is forced into this small gap by the lungs. This prompts vocal cords to
vibrate. Since vibrating objects produce sound, sound is produced due to the vibration of vocal cords.
Q13 :
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. Hence, light reaches us before the sound during a lightning, which is
accompanied by thundering.
Ch-14 Chemical Effects of Electric Current
Exercise : Solutions of Questions on Page Number : 180
Q1 :
Fill in the blanks.
(a) Most liquids that conduct electricity are solutions of ________, _______ and ________.
(b) The passage of an electric current through a solution causes _________ effects.
(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to
the__________ terminal of the battery.
(d) The process of depositing 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 __salts__.
(The solutions of acids, bases or salts are conducting in nature. They allow the current to pass through.)
(b) The passage of an electric current through a solution causes __chemical__ effects.
(When an electric current passes through a solution, the solution decomposes into its positive and negative ions. This process of
decomposition of the solution is a chemical effect.)
(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the__negative__ terminal
of the battery.
(When an electric current passes through a copper sulphate solution, the solution decomposes into positively charged copper ions
and negatively charged sulphate ions. These positively charged copper ions get attracted towards the plate which is connected to
the negative terminal of a battery.)
(d) The process of depositing a layer of any desired metal on another material by means of electricity is called__electroplating__.
Q2 :
When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the
reason?
Answer :
The deflection in the compass needle shows that current is flowing through the wounded wire and hence, through the circuit. The
circuit is complete since free ends of the tester are dipped in a solution. The solution is certainly a conducting solution. This is the
reason why the compass needle shows a deflection.
Q3 :
Name three liquids, which when tested in the manner shown in Fig. 14.9, may cause the magnetic needle to deflect.
Answer :
Liquids like lemon juice, salt water and vegetable oil allow electricity to pass through them. Hence, these liquids can be used as in
the beaker to show the given effect.
Q4 :
The bulb does not glow in the setup shown in Fig. 14.10. List the possible reasons. Explain your answer.
Answer :
The bulb may not glow because of the following reasons:
(i) Liquid in the beaker is non-conducting. In such case, the electric current would not be able to pass through the liquid. Hence, the
circuit is not complete.
(ii) Electric current in the circuit is very weak. This can happen if the material used for making the circuit is not a good conductor of
electricity or the battery does not have sufficient energy to generate electricity.
Q5 :
A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the
tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that
(i) liquid A is a better conductor than liquid B.
(ii) liquid B is a better conductor than liquid A.
(iii) both liquids are equally conducting.
(iv) conducting properties of liquid cannot be compared in this manner.
Answer :
(i) Liquid A is a better conductor than liquid B.
The amount of current flowing through a conducting solution depends on the conductivity of the solution. With more conductivity,
more current passes through the solution and vice-versa. Hence, the conductivity of liquid A is more than the conductivity of liquid B.
Q6 :
Does pure water conduct electricity? If not, what can we do to make it conducting?
Answer :
No. Pure water does not conduct electricity. This is because pure water is devoid of any salts. Pure water can conduct electricity
when a pinch of common salt is added to it, as salt solution is conducting in nature.
Q7 :
In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why
they do this.
Answer :
Water may conduct electricity. If the electrical supply for the area is not shut off and water is poured over electrical appliances, then
electricity may pass through water and harm the firemen. That is why, in case of a fire, the firemen shut off the main electrical supply
for the area before they use the water hoses.
Q8 :
In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why
they do this.
Answer :
Water may conduct electricity. If the electrical supply for the area is not shut off and water is poured over electrical appliances, then
electricity may pass through water and harm the firemen. That is why, in case of a fire, the firemen shut off the main electrical supply
for the area before they use the water hoses.
Q9 :
A child staying in a coastal region tests the drinking water and also the seawater with 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, it is more conducting than the drinking water. Because of
this reason, the compass needle deflects more in seawater than in the drinking water.
Q10 :
Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.
Answer :
No. It is not safe to repair electrical appliances outdoors during heavy downpour. This is because rain water contains dissolved
salts. Therefore, rain water can conduct electricity. The electrician may get electrical shocks while working outdoors during rain.
Q11 :
Paheli had heard that rain water is as good as distilled water. So, she collected some rain water in a clean glass tumbler
and tested it using 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 solution. There are no dissolved salts present in the distilled water.
Hence, rain water can allow electricity to pass through it while distilled water cannot.
Q12 :
Prepare a list of objects around you that are electroplated.
Answer :
Examples of electroplated objects are as follows:
(i) Chromium plating is done on different parts of cars, buses and motor cycles to give them shiny appearance.
ii. A fine layer of gold is deposited on the silver ornaments and they are called gold-plated ornaments.
ii. Iron used in constructing a building is coated with a layer of zinc. This protects iron from corrosion and rusting.
Q13 :
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 to the thin copper plate. 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 terminal of the battery. As
copper ions are transferred to the thin copper plate, this thin pure copper plate must be connected to the negative terminal of the
battery. Consequently, impure copper rod is connected to the positive terminal of the batteries.
Ch-15 Some Natural Phenomena
Exercise : Solutions of Questions on Page Number : 197
Q1 :
Which of the following cannot be charged easily by friction?
(a) A plastic scale
(b) A copper rod
(c) An inflated balloon
(d) A woollen cloth
Answer :
(b) A copper rod
Only non-conducting materials can be easily charged by friction. Copper is a highly conducting materials. Therefore, a copper rod
cannot be charged easily by friction.
Q2 :
When a glass rod is rubbed with a piece of silk cloth the rod
(a) and the cloth both acquire positive charge.
(b) becomes positively charged while the cloth has a negative charge.
(c) and the cloth both acquire negative charge.
(d) becomes negatively charged while the cloth has a positive charge.
Answer :
(b) The rod becomes positively charged, while the cloth has a negative charge.
When an object is charged by rubbing it against another object, the two objects get oppositely charged. By convention, it is
considered that the charged acquired by the glass rod is positive and charged acquired by the cloth is negative. Therefore, the rod
becomes positively charged and the cloth becomes negatively charged.
Q3 :
Write T against true and F against false in the following statements.
(a) Like charges attract each other. (T / F)
(b) A charged glass rod attracts a charged plastic straw. (T / F)
(c) Lightning conductor cannot protect a building from lightning. (T/F)
(d) Earthquakes can be predicted in advance. (T / F)
Answer :
(a) False
Like charges repel each other. It is the unlike charges that attract each other.
(b) True
A charged glass rod has positive charges on its surface while a charged plastic straw has negative charges on its surface. Since
unlike charges attract each other, a charged glass rod attracts a charged plastic straw.
(c) False
During a lightning, the lightning conductor conducts all the atmospheric charges to the Earth directly, leaving the building safe.
Hence, lightning conductors protect a building from lightning.
(d) False
Although the causes of earthquakes is known, but no instrument could be invented to detect it till now. Hence, earthquakes cannot
be predicted in advance.
Q4 :
Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.
Answer :
Whena sweater is taken off, the woollen sweater gets charged because of the friction between the sweater and the body. Hence,
one can hear a crackling sound during the given process.
Q5 :
Explain why a charged body loses its charge if we touch it with our hand.
Answer :
When we touch a charged object, our body conducts its charges to the earth. That is why a charged body loses its charge, if we
touch it with our hand. This phenomenon is known as electric discharge.
Q6 :
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? Is it likely to cause much damage?
Answer :
The destructive energy of an earthquake is measured by the Richter scale. This scale has the readings from 1 to 10.
The reading of magnitude 3 on the Richter scale would be recorded by a seismograph.
If the Richter scale gives a reading of magnitude 3, then the earthquake is not likely to cause much damage. Generally, earthquake
of magnitudes higher than 5 is considered destructive in nature.
Q7 :
Suggest three measures to protect ourselves from lightning.
Answer :
Protective measures against lightning are as follows:
(i) Stay in a completely closed place. If you are moving in a car, then remain there until the lightning is over. Close the windows of
the car immediately.
(ii) Do not touch any electrical wires, telephone cables, metal pipes, etc.
(iii) Do not bath in running water. This may cause an electric shock.
Q8 :
Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by
another charged balloon?
Answer :
The nature of charges present on the surface of charged balloons are similar. Since like charges repel each other, two charged
balloons repel each other. When a charged body is brought near an uncharged body, the uncharged body acquires charges on its
surface caused by the induction of charges. The charges are of opposite nature in relation to the charged body. Since unlike
charges attract each other, a charged body always attracts an uncharged body. Hence, an uncharged balloon is attracted by
another charged balloon.
Q9 :
Describe with the 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.
It consists of a metal rod. At one end of the rod, two leaves of aluminium foil are fixed and at the other end, there is a metal disc.
The leaves of aluminium foil are placed inside a conical flask and the flask is corked to isolate the leaves from air.
When the metal disc is touched with a charged body, the aluminium strips move away from each other. This happens because
some of the charges of the body are transferred to the strips through the metal rod. This method of charging a body is called
charging by conduction. The nature of charges on both the leaves and the charged body are the similar. Hence, both the leaves of
the aluminium foil will move away from each other. If the body was not charged, then the leaves of the foil would remain as they
were before. They would not repel each
other.
Q10 :
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.
Q11 :
Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?
Answer :
Some of the precautions are as follows:
(i) Try to find an open field away from tall buildings, installations, tall trees, and electric wires and poles.
(ii) If travelling in a bus or a car, then do not come out when an earthquake strikes. Ask the driver to drive in an open field.
Q12 :
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 accompanied with lightning, electric
discharge from the clouds can travel through the metallic rod of the umbrella. This may give an electric shock to the person who is
carrying it. Hence, it is not safe to carry an umbrella during lightning.
____________________________________________________________________________________________
Ch-16 Light
Exercise : Solutions of Questions on Page Number : 212
Q1 :
Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Answer :
If we are in a dark room, then it is not possible for us to see objects in the room. However, objects outside the room are visible to us.
An object becomes visible when light reaches our eye after being reflected from the object. If there is no light in the room, then the
objects inside the room cannot reflect any light. Hence, we cannot see in a dark room. If there is light present outside the room, then
we can see the objects outside the room.
Q2 :
Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Answer :
Regular reflection takes place from a smooth or a regular surface. In regular reflection, all reflected rays are parallel to each other
for parallel incident rays. Irregular or diffused reflection takes place from an irregular surface. In diffused reflection, the reflected rays
are not parallel to each other for parallel incident rays. This happens because of the presence of irregular microscopic surfaces.
Hence, parallel incident rays reflect in different directions. However, each ray obeys the laws of reflection. Therefore, laws of
reflections are not violated in diffused or irregular reflections.
Q3 :
Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes.
Justify your answer in each case.
(a) Polished wooden table (b) Chalk powder
(c) Cardboard surface (d) Marble floor with water spread over it
(e) Mirror (f) Piece of paper
Answer :
(a) Polished wooden table → 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) Chalk powder → Diffused reflection
Chalk power spread on a surface is an example of an irregular surface. It is not smooth. Therefore, diffused reflection will take place
from chalk powder.
(c) Cardboard surface → Diffused reflection
Cardboard surface is also an example of an irregular surface. Hence, diffused reflection will take place from a cardboard surface.
(d) Marble floor with water spread over it → Regular reflection
Marble floor with water spread over it is 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) Mirror → Regular reflection
Mirror has a smooth surface. Therefore, it will give a regular reflection.
(f) Piece of paper → Diffused reflection
Although a piece of paper may look smooth, but it has many irregularities on its surface. Due to this reason, it will give a diffused
reflection.
Q4 :
State the laws of reflection.
Answer :
Laws of reflection:
(i) The angle of reflection is always equal to the angle of incidence.
(ii) The incident ray, the reflected ray and the normal to the reflective surface at the point of incidence all lie in the same plane.
Q5 :
Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same
plane.
Answer :
Place a plane mirror on the table. Take a paper sheet and make a small hole in its centre. Make sure that the light in the room is not
bright. Hold the sheet normal to the table. Take another sheet and place it on the table in contact with the vertical mirror. Draw a
normal line on the second sheet from the mirror. Now, light a torch on the mirror through the small hole such that the ray of light falls
on the normal at the bottom of the mirror. When the ray from this hole 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 mirror at the point of incidence on the sheet placed on
the table. This shows that the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the
same plane.
Q6 :
Fill in the blanks in the following.
(a) A person 1 m in front of a plane mirror seems to be ____ __ m away from his image.
(b) If you touch your ________ ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right
ear is touched with your _________.
(c) The size of the pupil becomes ________ when you see in dim light.
(d) Night birds have _________ cones than rods in their eyes.
Answer :
(a) A person 1 m in front of a plane mirror seems to be __2 __ m away from his image.
(Object distance and image distance are the same from a plane mirror. The image of a person 1 m in front of a mirror is 1 m back to
the mirror. Hence, the image is 1 + 1 = 2 m away from the person.)
(b) If you touch your __left__ ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is
touched with your _left hand_.
(This is because of lateral inversion of images formed in a plane mirror.)
(c) The size of the pupil becomes __large__ when you see in dim light.
(In dim light, the amount of light entering the eye is very little. To increase the amount of light, the pupil expands.)
(d) Night birds have __less__ cones than rods in their eyes.
(Night birds can see in the night, but not in the day. They have on their retina a large number of rod cells and only a few cones.)
Q7 :
Angle of incidence is equal to the angle of reflection.
(a) Always (b) Sometimes
(c) Under special conditions (d) Never
Answer :
(a) The angle of incidence is always equal to the angle of reflection. This is the first law of reflection.
Q8 :
Image formed by a plane mirror is
(a) virtual, behind the mirror and enlarged.
(b) virtual, behind the mirror and of the same size as the object.
(c) real at the surface of the mirror and enlarged.
(d) real, behind the mirror and of the same size as the object.
Answer :
(b) Image formed by a plane mirror is virtual, behind the mirror and of the same size as the object.
The image formed by a plane mirror is of the same size as the object. The image is formed behind the mirror. The image cannot be
obtained on a screen. Hence, it is a virtual image.
Q9 :
Describe the construction of a kaleidoscope.
Answer :
Construction of a kaleidoscope:
Three rectangular mirror strips of dimensions 15cm x 4cm (l x b) are joined together to form a prism (fig a). This prism is fixed into a
circular cardboard tube. The circular cardboard tube should be slightly longer that the prism (fig b). This circular tube is now closed
at one end with a cardboard disc. This disc has a hole in it through which we can see (fig c) .At the other end of the circular tube, a
plane glass plate is fixed. It is important that this glass plate touches the prism mirrors. 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 (fig d).
Q10 :
Draw a labelled sketch of the human eye.
Answer :
Q11 :
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 light is harmful for the human eyes, because its intensity is very high. It can cause damage to the retina and lead to blindness.
Hence, it is advisable not to look at a laser beam directly.
Q12 :
Explain how you can take care of your eyes.
Answer :
To protect our eyes, the given points should be taken into account:
(i) Visit an eye specialist regularly.
(ii) Avoid reading in dim light and very bright light.
(iii) Avoid direct exposure of sunlight to the eye.
(iv) Clean your eyes with cold water quickly if dust particles or small insects enter your eye. Do not rub your eyes.
(v) Maintain a distance of at least 25 cm between the book and your eyes while reading.
Q13 :
What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Answer :
If the reflected ray is at an angle of 90° to the incident ray, then the angle of incidence is 45 °. According to the law of reflection, the
angle of incidence is equal to the angle of reflection. Therefore, the angle of incidence and the angle of reflection both are =
45°.
Q14 :
How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Answer :
Infinite or multiple images of the candle will be formed because of multiple reflections between the mirrors. When two mirrors are
placed parallel to each other, then infinite numbers of images are formed.
Q15 :
Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the
reflected ray from the second mirror.
Answer :
The first law of reflection is used to obtain the path of reflected light.
It can be observed that the given ray of light will reflect from the second mirror at an angle 60°.
Q16 :
Boojho 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?
Answer :
A plane mirror forms a virtual image behind the mirror. The image is as far behind the mirror as the object is in front of it. A cannot
see his image because the length of the mirror 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).
Answer :
A plane mirror forms a virtual image behind the mirror. The image is as far behind the mirror as the object is in front of it. A cannot
see his image because the length of the mirror 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)
Q17 :
(a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
(b) Can Paheli at B see this image?
(c) Can Boojho at C see this image?
(d) When Paheli moves from B to C, where does the image of A move?
Answer :
(a) Image of the object placed at A is formed behind the mirror. The distance of the image from the mirror is equal to the distance of
A from the mirror. Image of A is shown in the given figure.
(b) Yes. Paheli at B can see this image.
(c) Yes. Boojho at C can see this image.
(d) Image of the object at A will not move. It will remain at the same position when Paheli moves from B to C.
____________________________________________________________________________________________
CH-17 Stars and the Solar System
Exercise : Solutions of Questions on Page Number : 234
Q1 :
Which of the following is NOT a member of the solar system?
(a) An asteroid
(b) A satellite
(c) A constellation
(d) A comet
Answer :
(c) Constellation
A constellation is not a member of the solar system.Constellations are groups of stars that form recognisable shapes.
Q2 :
Which of the following is NOT a planet of the sun?
(a) Sirius
(b) Mercury
(c) Saturn
(d) Earth
Answer :
(a) Sirius
Sirius is a star and not a planet of the sun.
Q3 :
Phases of the moon occur because
(a) we can see only that part of the moon which reflects light towards us.
(b) our distance from the moon keeps changing.
(c) the shadow of the Earth covers only a part of the moon's surface.
(d) the thickness of the moon's atmosphere is not constant.
Answer :
(a) Phases of the moon occur because we can see only that part of the moon which reflects light towards us.
Moon does not produce its own light. We are able to see the moon because the sunlight falling on it gets reflected towards us. Thus,
we see only that part of the moon from which the light of the sun gets reflected towards us.
Q4 :
Fill in the blanks:
(a) The planet which is farthest from the sun is _________.
(b) The planet which appears reddish in colour is _________.
(c) A group of stars that appear to form a pattern in the sky is known as a __ _________.
(d) A celestial body that revolves around a planet is known as a_________.
(e) Shooting stars are actually not _________.
(f) Asteroids are found between the orbits of ________ __ and _________.
Answer :
(a) The planet which is farthest from the sun is __Neptune__.
(b) The planet which appears reddish in colour is __Mars__.
(c) A group of stars that appear to form a pattern in the sky is known as a __constellation__.
(d) A celestial body that revolves around a planet is known as asatellite_.
(e) Shooting stars are actually not stars __.
(Shooting stars are not stars, they are meteors)
(f) Asteroids are found between the orbits of __Mars __ and __Jupiter.
(Asteroids occupy a large gap between the orbits of Mars and Jupiter)
Q5 :
Mark the following statement as true (T) or false (F).
(a) Pole star is a member of the solar system. ( )
(b) Mercury is the smallest planet of the solar system. ( )
(c) Uranus is the farthest planet in the solar system. ( )
(d) INSAT is an artificial satellite. ( )
(e) There are nine planets in the solar system. ( )
(f) Constellation Orion can be seen only with a telescope. ( )
Answer :
(a) False
Stars are not a member of the solar system. The sun and the celestial bodies revolving around it form the solar system.
(b) True
Mercury is the smallest planet of the solar system.
(c) False
Neptune is the farthest planet in the solar system.
(d) True
INSAT is an artificial satellite.
(e) False
There are eight planets in the solar system.
They are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
(f) False
Constellation Orion can be seen during winters around late evenings. It is one of the most magnificent constellations in the sky,
visible to the naked eyes.
Q6 :
Match items in column A with one or more items in column B.
A B
(i) Inner planets (a) Saturn
(ii) Outer planets (b) Pole star
(iii) Constellation (c) Great Bear
(iv) Satellite of the Earth (d) Moon
(e) Earth
(f) Orion
(g) Mars
Answer :
A B
(i) Inner planets (g), (e) Mars, Earth
(ii) Outer planets (a) Saturn
(iii) Constellation (c), (f) Great Bear, Orion
(iv) Satellite of the Earth (d) Moon
Q7 :
In which part of the sky can you find Venus if it is visible as an evening star?
Answer :
Venus appears in the western sky after sunset and is called the evening star.
Q8 :
Name the largest planet of the solar system.
Answer :
The largest planet of the solar system is Jupiter.
Q9 :
What is a constellation? Name any two constellations.
Answer :
A constellation is a group of stars that form a recognisable pattern in the sky.
The two well known constellations are Ursa Major and Orion.
Q10 :
Draw sketches to show the relative position of prominent stars in (a) Ursa Major and (b) Orion
Answer :
(a) Ursa Major appears like a big dipper. There are three bright stars in the handle and four stars in the bowl of the dipper (as shown
in the given figure).
(b) Orion appears like a hunter. Three bright stars appear in the belt, while five bright stars are arranged in the form of a
quadrilateral (as shown in the given figure).
Q11 :
Name two objects other than planets which are members of the solar system.
Answer :
(i) Asteroids
A collection of a large number of small objects, gases and dust are revolving around the sun. They occupy a large gap between the
orbits of Mars and Jupiter. However, these are not planets. These celestial objects are known as asteroids.
(ii) Meteors
Meteors are small celestial objects that are seen as bright streaks of light in the sky. They brunt out on entering the Earth's
atmosphere because of the heat produced by friction. This results in bright streaks in the sky. They are not planets.
Q12 :
Explain how you can locate the Pole Star with the help of Ursa Major.
Answer :
In order to locate the Pole star in the sky, first of all Ursa Major or Big Dipper constellation must be located. The bowl of the Big
Dipper consists of four bright stars (as shown in the given figure).
Consider two stars at the end of this bowl. Now, draw an imaginary straight line towards the Northern direction connecting these two
stars (as shown in the given figure).
This imaginary line meets a star called the Pole Star. The length of the imaginary line from the bowl is about five times the distance
between the two stars of the bowl.
Q13 :
Do all the stars in the sky move? Explain.
Answer :
No. The Earth rotates from West to East on its axis. Hence, all stars in the sky (except the Pole star) appear to move from East to
West. With reference to the Earth, the Pole star does not appear to move in the sky because it is located above the axis of rotation
of the Earth in the north direction. It appears to remain stationary at a point in the sky.
Q14 :
Why is 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. It is inconvenient to express these
distances in kilometer (km). Thus, these large distances are expressed in light years. One light year is the distance travelled by light
in one year. One light year is equal to 9.46 x 10
12km.
A star is located eight light years away from the Earth. This means that the distance between the star and the Earth is equivalent to
the distance travelled by light in eight years, i.e., a star is located 8 x (9.46 x 10
12
) = 7.6 x 10
13km away from the Earth.
Q15 :
The radius of Jupiter is 11 times the radius of the 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 radii Rand R'respectively. Given that the radius of Jupiter is 11 times the
radius of the Earth.
Thus, R' = 11 R
Hence, this ratio suggests that Jupiter can accommodate 1331 number of Earths within it.
Q16 :
Boojho made the following sketch (Fig. 17.29) of the solar system. Is the sketch correct? If not, correct it.
Answer :
Planets of the solar system in sequence of their distances from the sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
and Neptune. Therefore, Boojho's sketch of the solar system is not correct because he has interchanged the positions of Mars and
Venus and also the positions of Uranus and Neptune. Also, he has shown the Asteroid belt in the gap between the orbits of Jupiter
and Saturn. This is not correct. The asteroid belt is located between the orbits of Mars and Jupiter. The correct sketch of the solar
system is shown in the given figure.
CH-18 Pollution of air and water
Exercise : Solutions of Questions on Page Number : 252
Q1 :
What are the different ways in which water gets contaminated?
Answer :
Water gets contaminated by the addition of:
(i) Agricultural chemicals: Farmers use excessive amounts of pesticides and fertilizers to increase crop production. These
chemicals get carried away to the water bodies due to rains and floods which lead to water pollution.
(ii) Industrial wastes: Industries release harmful chemical wastes into water sources, thereby polluting them.
(iii) Sewage wastes: Waste materials from kitchens, toilets, and laundry sources are also responsible for contaminating water.
Q2 :
At an individual level, how can you help reduce air pollution?
Answer :
An individual can reduce air pollution by:
(i) Avoiding the use of cars as much as possible and by using public transport whenever possible.
(ii) By not using vehicles for short distances.
(iii) By using clean fuels such as LPG and CNG instead of diesel and petrol.
(iv) Always disposing the garbage properly and not burning it.
(v) Controlling the emissions from vehicles and household chimneys.
Q3 :
Clear, transparent water is always fit for drinking. Comment.
Answer :
No. Clear and transparent water is not always fit for drinking. Water might appear clean, but it may contain some disease causing
micro-organisms and other dissolved impurities. Hence, it is advised to purify water before drinking. Purification can be done by
water purifying systems or by boiling the water.
Q4 :
You are a member of the municipal body of your town.
Make a list of measures that would help your town to ensure the supply of clean water to all its residents.
Answer :
To ensure the supply of clean water to all residents the following steps must be taken:
(i) The main water source must be built in clean surroundings and should be maintained properly.
(ii) Chemical methods such as chlorination must be used for purifying water.
(iii) The area around water pipes must also be clean.
Q5 :
Explain the differences between pure air and polluted air.
Answer :
Pure air contains around 78% nitrogen, 21% oxygen, and 0.03% carbon dioxide. Other gases such as argon, methane, ozone, and
water vapours are also present in small quantities. When this composition of air is altered by the addition of harmful substances or
gases such as nitrogen dioxide, sulphur dioxide, carbon monoxide, and particulate matter, then the air is said to be polluted.
Q6 :
Explain circumstances leading to acid rain. How does acid rain affect us?
Answer :
Burning of fossil fuels such as coal and diesel releases a variety of pollutants such as sulphur dioxide and nitrogen dioxide into the
atmosphere. These pollutants react with water vapours present in the atmosphere to form sulphuric acid and nitric acid respectively.
These acids come down with the rain, thereby resulting in acid rain.
Effects of acid rain:
(i) Acid rains damage crops.
(ii) Acid rains corrode buildings and structures especially those made of marble such as Taj Mahal.
Q7 :
Which of the following is not a greenhouse gas?
(a) Carbon dioxide
(b) Sulphur dioxide
(c) Methane
(d) Nitrogen
Answer :
(d) Nitrogen
Q8 :
Describe the 'Greenhouse Effect' in your own words.
Answer :
Greenhouse effect may lead to global warming, i.e., an overall increase in the average temperature of the Earth. Greenhouse effect
is caused by greenhouse gases. Examples of greenhouse gases include carbon dioxide, methane, and water vapour. When solar
radiations reach the Earth, some of these radiations are absorbed by earth and then released back to the atmosphere. Greenhouse
gases present in the atmosphere trap these radiations and do not allow heat to leave. This helps in keeping our planet warm and
thus, helps in human survival. However, an indiscriminate increase in the amount of greenhouse gases can lead to excessive
increase in the Earth's temperature leading to global warming.
Q9 :
Prepare a brief speech on global warming. You have to deliver the speech in your class.
Answer :
Global warming is an increase in the average temperature of the Earth's surface. It occurs as a result of an increased concentration
of greenhouse gases in the atmosphere. The greenhouse gases include carbon dioxide, methane, and water vapour. 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.
Q10 :
Describe the threat to the beauty of the Taj Mahal.
Answer :
Acid rain is a major threat to the beauty of the Taj Mahal. When acid rains fall on the monument (that is completely made of marble),
they react with marble to form a powder-like substance that is then washed away by the rain. This phenomenon is known as marble
cancer. Also, the soot particles emitted from the Mathura oil refinery located near Agra is leading to the yellowing of the marble.
Q11 :
Why does the increased level of nutrients in the water affect the survival of aquatic organisms?
Answer :
An increase in the level of nutrients in a water body leads to an excessive increase in the population of algae in the water body.
When these algae die, they serve as food for decomposers. A lot of oxygen is utilised 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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