ncert science textbook for class 8th chapters 1 to...
TRANSCRIPT
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 1
PART ONE FOR CAPABILITY BUILDING OF TEACHERS
This section for teachers is merely an attempt to bridge the ‘facility
availability divide’ that exists in many part of India. It is well known that all
Teachers do not have equal access to a good library for preparing good quality
teaching content. Bridging this divide is the only intent here. The objective is to
inform the students to the extent possible.
CONTENT
The Textbook already gives Keywords and Summary of main aspects at
the end of each chapter under ‘What You Have Learnt’. These are very good
for revision and help in understanding the main points. Therefore, these are
to be studied from the Book and are not repeated here.
This document only gives some additional material as may further help in
clarifying the subject matter. Essay type questions are answered here to serve
as a cross-check for student’s own answers. Do not try to learn these by
heart, but to understand and link the concepts to what has already been
studied in Class 7th
. Also see power point presentation on Introduction to
Science.
This Section begins by listing the 18 chapters in this Science Book, as
pertaining to ‘living matter’ or ‘non-living matter’. The third column
indicates the subject area for that chapter, to be studied in higher classes.
Nos.
of
the
Chap
ter
Title of the Chapter Specialized
subject area of
the chapter
Later on
to be
studies in
advance
study of
Other
remarks/
Higher
studies
subject matter
or
professional
skills
LIVING MATTER
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 2
RELATED
1. 2. 3
1 Crop Production and
Management
Plant life Botany
and
Agricultur
al
Sciences
Agricultural
Sciences
7 Conservation of Plants
and Animals
Eco-system Botany
and
Zoology
Environment
al Studies
8 Cell – Structure and
Functions
Basis of all
life- Plant,
Animal and
Human
Botany,
Biology,
Zoology
Bio-
chemistry
Studied by
humans for
creation of
new materials
such as
Plastics and
Synthetic
fibres
9 Reproduction in
Animals
Animal / Bird
life
Zoology Zoology
10 Reaching the Age of
Adolescence
Human
anatomy and
physiology
Biology All Medicinal
studies
18 Pollution of Air and
Water
Eco-systems,
environment
and geography
Environm
ental
studies,
Geograph
y
And
environmenta
l studies
NON-LIVING
MATTER RELATED
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 3
3 Synthetic Fibres and
Plastics
Chemistry Inorganic
Chemistry
Inorganic
Chemistry,
Chemical
engineering
6 Combustion and Flame Energy Physics Physics and
Environment
al Science
4 Materials: Metals and
Non-Metals
Geography Geograph
y
Geography
and Mining
Engineering
5 Coal and Petroleum Geography Mineralog
y
Mining
Engineering
11 Force and Pressure Mechanics Physics Mechanical
Engineering
12 Friction Mechanics Physics Mechanical
engineering
14 Chemical Effects of
Electric Current
Physics and
Chemistry
Electricity
and Power
Electronics,
Electrical
Energy,
Advanced
Studies in
Matter and
Energy
15 Some Natural
Phenomena
Geography Geograph
y
Geography
and
Hydrology,
Meteorology
13 Sound Sound energy Physics Physics,
Astronomy
and Radiant
Energy
16 Light Physics Physics Physics,
Astronomy
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 4
CHAPTER 1 Crop Production and Management
INTRODUCTORY
1. The modern systems of irrigation, for the purpose of agriculture, were
developed about 200 years ago. Historically, agriculture has been
dependent upon manual work and seasonal rains. Traditional systems of
irrigation were all dependent on rain. As rainfall depends on climatic
conditions and cannot be controlled by humans, agricultural activity had
high uncertainty.
2. Along with this Chapter, please see Chapter 4 of NCERT Textbook in
Geography for Class 8th
. Geography Chapter 4 tells about the different
Farm Systems and Crops cultivated in different countries today. The
Crop Production in India is on a much smaller size farm than the crop
production in the USA. This is partly due to the difference in the soil,
climate, expanse of plain land available and the technology used in these
two regions of the world. The difference is also due to historical
development of the two societies and their different social and economic
structures.
and Radiant
Energy
17 Stars and the Solar
System
Our Universe
and Astronomy
Astronom
y
Astronomy
and Quantum
Mechanics
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 5
QUESTIONS
3. Q1. Fill in the blanks:
a) The same kind of plants grown and cultivated on a large scale
at a place is called crop.
b) The first step before growing crops is preparation of the soil.
c) Damaged seeds would float on the top.
d) For growing a crop, sufficient sunlight , water and nutrients
from the soil are essential
4. Match A with correct answer in B in the following:
a) Kharif crop (e) paddy and maize
b) Rabi crop (d) wheat, gram, pea
c) Chemical fertilizers (b) Urea and super phosphate
d) Organic manure (c ) Animal excreta, cow dung, urine, plant
waste.
5. Q3 Give two examples each of (a) Kharif crop and (b) Rabi crop
a) Kharif crops: Paddy, maize, soyabean, groundnut and cotton are a
few of the Kharif crop that are grown in the rainy season from June
to September each year
b) Rabi crop: Wheat, mustard, gram, peas, and linseed are examples
of Rabi crop that are grown in winter from October to March each
year
6. Q 4 Write one paragraph each in your own words on the following:
a) Preparation of soil is the first activity in the process of growing any
crop whether a kharif crop or a rabi crop. During preparation, the
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 6
soil is ploughed to turn and loosen it. The loosening of the soil allows
the roots of the crop plants to reach deep down the soil. It also helps
to trap air between the soil particles. The roots use this air for
breathing. The loose and turned soil also helps earthworms and other
organisms to grow. Earthworms are ‘friends’ of plants as the tiny
burrowing in soil helps to trap more air between the soil particles for
the roots to breathe. As soil contains minerals, water, air and humus,
preparation of soil helps in mixing these up properly. It also brings up
the more nutrient rich soil from the bottom layers to the top. (116
words)
b) Sowing is the second activity in growing of a crop. For this, clean
and healthy, good quality seeds are selected. These are then soaked in
water for a few hours or overnight. The next morning these are filled
in a funnel like tool that allows only one or two seeds to pass through
it at a time. This is important because some space is needed between
two plants. A seed drill is also used for the purpose. In this way
sowing of seeds is completed without causing wastage or damage to
the seeds. (93 words)
Picture of Sowing activity
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 7
c) Weeding is an important part of growing of a crop. Weeds are the
unwanted plants that grow on their own because their roots or seeds
are already spread in the soil. The separation and removal of weeds
from the crop plants is called weeding. The weeds are harmful to the
crop plants because they use up the nutrients that should in fact be
used by the crop plants. Some weeds are poisonous and therefore,
harmful both to humans and animals. So when the crop plants are still
small, the weeds are to be identified and uprooted by hand or with the
help of a tool called ‘khurpi’. They are then removed from the field
and disposed off at a distance or burnt after drying them in a separate
place. Certain chemicals called ‘weedicides’ are also used to control
weeds in place of manual weeding. These chemicals are dissolved in
water and sprayed. They destroy the weeds without harming the crop
plants.
(Note: (c) also covers Q 9 on what are weeds and how can they be
controlled?)
d) Threshing is the activity of separating the grain seed from the chaff
that covers it. The crop is first harvested, either by cutting the plant
stalk just above the ground, manually with a sickle, or
automatically by a harvester machine. The cut out plants are then
beaten softly, to rub off the outermost covering of the grain called the
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 8
‘chaff’. Then through winnowing the grain is separated from the
chaff.
Winnowing is mostly done by hand by women. A rectangular
base with raised borders along its three sides, is made of straw and is
very light. This tool, in Hindi, is called the ‘soop’. About one
kilogram of grain covered with chaff, is lifted up in the soop by
holding its two sides and raising it to above the head level. From there
the grain is and poured down on to the ground. The wind blows away
the light chaff, allowing the heavier grain to collect as a heap.
7. Q 5 Explain how fertilizers are different from manure
Fertilizers are chemical substances that are rich in certain nutrients
needed by the crop plants. These are produced in factories with the help
of machines. Urea, Super phosphate, Ammonium sulphate, and
potassium are examples of fertilizers. They help to give a higher yield
from a crop but they also make the soil less fertile. They have also
become a major source of water pollution.
Manure is the natural decomposed mixture of cow-dung, dead plants
and humus. Traditionally only manure was used in agriculture. Mixing
of manure with the soil increases the capacity of the soil to retain water.
It makes the soil more porous and due to this more air is trapped in the
soil. Manure also increases the friendly microbes in the soil and
improves the soil texture.
8. Q6 What is irrigation? Describe two methods of irrigation that conserve
water.
The supply of water to the crop plants at different intervals is called
irrigation. Water is necessary for agriculture because 90 per cent of
the content of all plants is water. Irrigation is the activity through which
water is supplied to the crop plants. Irrigation is needed for the sprouting
of the seeds, for proper development of the plants, flowers and fruits.
Nutrients from the soil get dissolved in water and reach the plants. To
retain the moisture in the soil the fields have to be irrigated regularly.
Wells, tube-wells, ponds, lakes, rivers, dams, canals are sources of water
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 9
for irrigation. Drip system and Sprinkler system are two methods of
irrigation that conserve water. Under the Drip System water falls drop
by drop very near the roots and therefore water is not wasted at all. In
the Sprinkler system pipes with nozzles at regular intervals are used. As
the water passes through the pipes the nozzles rotate to sprinkle the
water as if during the rainfall. This system also conserves water and is
very useful for uneven land and sandy soil.
9. Q 7 If wheat is sown in the kharif season what would happen?
The wheat crop needs less water and a long sunshine season for
maturing. If wheat is sown in kharif months of June to September, the
rains would destroy the crop. Therefore, wheat is sown as a rabi crop in
October after the end of the rainy season, to prevent it from getting
destroyed before harvest.
10. Q 8. Explain how soil gets affected by the continuous plantation of crops
in a field
11. Q 9. What are weeds? How can we control them?
Please see answer to Question No. 4 (d) above for this question.
12. Q. 10 Arrange the following boxes in proper order to make a flow chart
of sugarcane crop production.
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 10
13. Crossword: Down
a) Irrigation
b) Storage
c) Crop
Across
d) Harvester
e) Gram
f) Winnowing
CHAPTER 2. MICRO-ORGANISM
Questions 1 to 4 can easily be done by the student by reading the Chapter two
or three times.
1. Ploughing the field
2. Preparation of soil
3.Sowing
4.Manuring5. Irrigation6.Harvesting
7. Sending crop to sugar
factory
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 11
Q5 What are the major groups of micro-organisms?
Answer. Micro-organisms or microbes are classified into four major groups.
These are called (1) bacteria (single cell) (2) fungi ( multi-cell)(3) protozoa
(single cell) and (4) algae (single and multi-cell). These four groups have the
common characteristic of growing externally on all living matter. In addition
there is a fifth group called viruses but a virus can only grow inside another
living cell. It cannot grow externally like the four major groups.
Q6 Name the micro-organisms which can fix atmospheric nitrogen in the
soil?
Answer. Certain bacteria and blue-green-algae can fix atmospheric nitrogen
into the soil by converting it in to useable compounds.
Q7 Write ten lines on the usefulness of micro-organisms in our lives.
Answer. Micro-organisms are very useful in our lives as they help in
breaking down and converting more complex substances in to compounds that
are more easily used by plant, animals and human. In our daily lives curds,
bread, cheese and pickle are made with the help of useful and human-friendly
micro-organisms. In industry, the micro-organism called ‘yeast’ is used on a
large scale to produce alcohol, wine and vinegar by converting sugar in to
alcohol. Several medicines, such as antibiotics are also produced with the help
of microorganisms on a commercial scale. Antibiotics made from fungi and
algae are also mixed in the feed given to livestock to prevent infection. They are
also used to prevent many plant diseases. In this way we find that micro-
organisms are useful in our lives.
Q8. Write a short paragraph on the harms caused by micro-organisms.
Answer. Microorganisms can also cause harm. Several micro-organisms
called ‘pathogens’ cause many plant, animal, and human diseases. Other micro-
organisms spoil food articles because convert the nutrient in to toxic or
poisonous substance. Spoilt food emits a different smell, develops a different
colour and is also bad to taste. These changes are due to growth of fungus that
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 12
make the food toxic. Consuming of such spoilt food can result in food poisoning
and diseases. Another harmful effect is that medicines made from
microorganisms should be used strictly with in the date of expiry. After the date
of expiry the microorganisms stop being useful in sickness and become harmful.
Q9. What are antibiotics? What precautions must be taken while using
Antibiotics?
Answer. Antibiotics are special medicines produced commercially with the
help of microorganisms such as bacteria and fungi. They can cure several
diseases. But antibiotics should be used strictly before the date of expiry
indicated on the medicine. After the date of expiry the microorganism base in
the medicine becomes harmful and will result in causing poisoning instead of
curing the disease.
CHAPTER 3 SYNTHETIC FIBRES AND PLASTICS
Q. 1 Explain why some fibres are called synthetic.
Ans. Some fibres are made from chemicals by man. They are not made from
natural primary products such as cotton and jute. Therefore, in contrast to
natural fibres these man-made fibres are called ‘synthetic fibres’. The word
‘synthetic’ means artificial because it is ‘synthesized together or blended
together through a chemical /mechanical procedure’ by human beings. It is
opposite of ‘natural’
Q. 4 Give examples which indicate that nylon fibres are very strong
Ans Nylon fibres are used for making parachutes, ropes for mountain
climbing, ropes for hauling of heavy luggage in ships and shipyards, tents for
camping in forests and for the army, car seat belts, socks for winter and cold
regions only because it is a very warm fibre, bags for army that need to be tough
under all conditions, sleeping bags for travel as they can be rolled in to a small
space, very light curtains, umbrellas.
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 13
Nylon thread is prepared from coal water and air. It is tested to be stronger
than steel thread. It is also very long-lasting and does not wear out easily.
Therefore, many articles used by the soldiers in war are prepared from nylon.
However, nylon is highly combustible, meaning that it catches fire very quickly.
Q.5 Explain why plastic containers are favoured for storing food
Plastic containers are very lighter and lower priced than metal
containers, are unbreakable, give a clean look and are easy to handle. In
addition they are non reactive do not get affected by rust or bacteria and do not
corrode like the metals.
Q6 Explain the difference between thermoplastic and thermosetting plastics
Thermoplastics get deformed and loose their original shape easily if
kept in places with high temperature or when heated. This is because they
conduct heat. Therefore, they cannot be used for making cooking utensils.
Thermosetting plastics, such as bakelite and melamine, are poor conductors of
heat and resist fire. Therefore, they are used for making handles of cooking
utensils such as pressure cookers, kitchenware, electric switches and plugs,
plug-points, and also floor tiles that do not wear out easily.
Q 7 Explain why the following are made of thermosetting plastics:
(a) Saucepan handles
Saucepans are used for cooking over fire. Its handle is made from
thermosetting plastics as such plastics can tolerate heat better than other
plastics. They are also a poor conductor of heat and therefore, the handle does
not get warm or hot while cooking.
(b) Electric plugs / switches / plug boards
Electric plugs / switches / plug boards are made from thermo-setting plastics
because these are points in the electric system that are touched by human beings
to switch on or switch off the current. To prevent damage through electric shock
these articles need to be made of material that is a poor conductor of heat and
electricity. Thermo-setting plastics are very poor conductors of heat and electric
current. As such they are used for making electric plugs and switches and any
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 14
part of the electricity flow system that comes in to contact with the human hand.
Also thermosetting plastics are very long lasting.
Q8 Categorize the materials of the following products into ‘can be recycled’
and ‘cannot be re-cycled’
Telephone instruments, plastic toys, cooker handles, carry bags, ball point
pens, plastic bowls, plastic covering on electrical wires, plastic chairs, electric
switches.
Can be recycled - because are made
of thermoplastics
Cannot be re-cycled -because are
made of thermosetting plastic
Plastic toys, carry bags, ball point
pens, plastic bowls, plastic chairs
Cooker handles, telephone
instruments, electric switches,
covering on electrical wires
Q9 Rana wants to buy shirts for summer. Should he buy cotton shirts or
shirts made from synthetic material? Advice Rana, giving your reason.
Rana should be advised to buy a cotton shirt for summer because it will
keep him cooler than a synthetic material shirt. This is because cotton shirt
weave has space between each fibre when it is woven into a fabric. The fibre of
a synthetic cloth is very tightly woven leaving no space for air to flow through
the cloth. This makes synthetic clothes warmer than cotton clothes. Therefore,
in summer cotton shirt is better while in winter a synthetic shirt is better. The
tighter weave of synthetic cloth is also the reason why it takes longer than
cotton to get soaked in water.
Q 10 Give examples to show that plastics are non-corrosive in nature.
In electric switches, plugs and plug points, we see that the outer frame that
is made of thermosetting plastics does not get effected by air, water, or heat.
Therefore these do not wear and tear easily. However, the metal screw with
which the switch is fixed of the metal parts of a plug are seen to get corroded or
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 15
rusted. Often they need to be replaced after a year or so. This is because the
metallic part is corrosive while the thermosetting plastic part is non-corrosive.
Q11 Should the handle and bristles of a tooth brush be made of the same
material? Explain your answer.
No. The bristles are made from thermoplastic that is more flexible and
therefore makes brushing of teeth possible without damaging the gums. The
handle of the tooth brush is made of a harder version of thermoplastic as it
needs to remain firm to grip while brushing, and also needs to hold the bristles
together for a longer period of use. Therefore, the handle and the bristles of a
tooth brush cannot be made of the same material.
Q 12. ‘Avoid plastics as far as possible’. Comment on this advice.
Plastics are non-bio-degradable and they continue to exist in garbage
for more than 500 years without decaying. For this reason they are a major
source of pollution. If they are burnt they emit (give off) poisonous fumes that
pollute the air.
As such plastics are a source of both environment pollution as well as air
pollution. Therefore, in our daily lives we should avoid plastics as far as
possible. Moreover, when cows and other animals eat garbage they eat up parts
of plastic and plastic bags also. The plastic bags do not get digested and cover
the lining of the animal’s stomach. This is known to cause their untimely death.
Therefore, plastic bags should not only be avoided, but they should not be
thrown around. Use of bio degradable bags needs to be encouraged.
In sum, the reasons that plastics cause air pollution, environmental pollution,
and are not bio-degradable they should be avoided as far as possible.
Q13 Match the terms in column A correctly with phrases in column B
1. Polyester - fabrics do not wrinkle easily
2. Teflon - used to make non-stick cookware
3. Rayon - prepared by using wood pulp
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 16
4. Nylon - used for making parachutes and stockings
Q 14 ‘Manufacturing syntheticfibres is actually helping conservation of
forests’. Comment
This is not a correct statement. Forests do not give an natural
fibre. Cotton and jute are the only two natural fibres and both are
cultivated. Jute grows almost naturally in the delta regions of rivers and
not in the forests. So forests in not way are conserved due to synthetic
fibres. In fact rayon is the only synthetic fibre that is made by treating
wood pulp artificially to make it stronger. So manufacturing of rayon
uses up a lot of wood from the forests, and cannot be said to be
conserving forests. If synthetic fibres are burnt they emit poisonous
gases that cause air pollution. For this reason also they can be said to
be harmful to forests. In sum, synthetic fibres cannot be said to be
conserving forests in any manner. The given statement is false and
misleading.
Q15 Describe an activity to show that thermoplastic is a poor conductor of
electricity.
Take a plastic bucket and fill it two third with water. Place an
‘immersion rod’* in to the water and connect it to the plug point. Switch
on the immersion rod.
Immediately, observe that small bubbles of water form along the
immersion rod as it starts to get heated. After 15 minutes it is
observed that water in the bucket is very warm and small fumes of
steam are forming on the surface. However, when the outside of the
plastic bucket is touched by hand no shock from the flow of electric
current is felt. This shows that the bucket made of thermoplastic is a
poor conductor of electricity. However, if the immersion rod is
allowed to remain ‘on’ for a longer period, the outside of the bucket
will also become slightly warm to touch. This shows that
thermoplastics are a poor conductor of electricity but they do
conduct some heat.
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 17
In case a metal bucket is used for warming water with an
immersion rod, the bucket will get as heated as the water and the
chances of getting a shock from the flow of current will be very
high. For this reason it is advisable to use an immersion rod with a
plastic bucket only. Also because thermoplastic is a poor conductor
of electricity, electric gyser is also made of thermoplastic or
thermosetting plastic.
*An Immersion rod is a simple electrical household appliance. It is
the cheapest device for warming water for bathing during winters as
it consumes very little electricity. It is available for about Rs 120/- to
Rs 150/- from any electrical goods shop and can be used for several
years.
CHAPTER 4 MATERIALS - METALS AND NON-METALS
In this Chapter the chemical equation and formulae are introduced. For this
the basis are the following formulae and valancy:
Radical Formula Valancy
Sulphate SO4 2
Sulphite SO3 2
Nitrate NO3 1
Nitrite NO2 1
Carbonate CO3 2
Phosphate PO4 3
Hydroxide OH 1
As you study the reactions and write the word equations the above table will
be very useful. For example in Question 9 and wherever sulphate / sulphuric
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 18
acid is involved the word equation will have SO4. Similarly for others in the list
The details of the formula for three acids is given below: (In the equations (=)
sign is used in place of ‘arrow’ sign in the book)
1) Sulphuric acid H2SO4 = 2H+ + SO4
-
2) Nitric acid HNO3 = H+ + NO3
3) Hydrochloric acid HCl = H+ + Cl
In the case of sulphuric acid the sulphate radical has a valency of 2. This means
that two hydrogen atoms are required to complete the stability of the equation.
Therefore H2 SO4. Displacement is also due to gaining or losing of ions by the
atoms.
Certain oxides and alkalis can neutralize acids. Bases include such oxides as
well as hydro-oxides
a)Caustic soda (sodium hydroxide) NaOH = Na + OH
b)Caustic potash (potassium hydroxide) KOH = K + OH
c)Slaked lime ( calcium hydroxide) Ca(OH)2 = Ca ++
+ 2OH
Basic oxides are always oxides of metals. Acidic oxides are compounds of
non-metals and oxygen with react with bases to form salts. An example is
Carbon dioxide gas (CO2) which reacts with sodium hydroxide (NaOH) to form
the salt sodium carbonate:
CO2 + 2NaOH = Na2CO3 + H2O
When an acid is neutralized by a base a salt is formed. An example is the
formation of the common salt or sodium chloride (NaCl) from sodium
hydroxide and hydrochloric acid = NaOH + HCl = NaCl+H2O. We know that
H2O is water.
FROM THE CHAPTER
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 19
Q6. Give reason for the following:
(a) Aluminum foils are used to wrap food items.
Aluminum foils are used to wrap food items because aluminum is a
metal and because of the characteristic of ‘malleability’. It can be beaten in to
thin sheets called foils. The foil retains the heat longer due to which the food
remains warm for a longer period. In addition the foil prevents air from
reacting with the wrapped food. This keeps it fresh for long. The foil is also
light and odourless. This makes it an ideal wrapping for food items.
(b) Immersion rods for heating liquids are made up of metallic
substances.
Metallic substances can be beaten into any shape. They are also very good
conductors of heat and electricity. Therefore, household appliances such as
Immersion rods that can warm water for bathing during winter are made of
metallic substances.
(c) Copper cannot displace zinc from its salt solution
Zinc is more reactive than copper. Therefore copper cannot displace
zinc from zinc sulphate. The rule is that a more reactive metal can replace a
less reactive metal, but a less reactive metal cannot replace a more reactive
one. For this reason, zinc can displace copper from copper sulphate.
(d) Sodium and potassium are stored in kerosene.
Sodium is a metal and is very reactive to oxygen in the air. As oxygen
is present in water, sodium reacts vigorously with water also. For this reason
sodium starts creating a lot of heat during its reaction with oxygen both in air
and in water. Therefore, it is stored in kerosene.
Potassium is also a very soft metal that can be cut with a knife. It is very
reactive to oxygen both in air and water. Therefore, potassium also is stored in
kerosene
Q7 Can you store lemon pickle in an aluminum utensil? Explain.
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 20
Lemon is sour because its nature is acidic. Aluminum is a metal. All metals
react with acids because their oxides are basic by nature. Therefore, lemon
pickle cannot be stored in an aluminum utensil.
Q 9. What happens when -
(a) Dilute sulphuric acid is poured on a copper plate?
When dilute sulphuric acid is poured on a copper plate, copper sulphate
is formed. This is written in equation as copper (Cu) + sulphuric acid (SO4) =
Copper sulphate CuSO4
(b) Iron nails are placed in copper sulphate solution.
Write word equation of the reaction involved.
When iron nails (Fe) are placed in copper sulphate (CuSO4) solution the
colour of the solution changes from blue to green due to formation of a new
substance called ‘iron sulphate’. The word equation for this reaction is Copper
sulphate solution (blue) + Iron (Fe) sulphate solution (green) + copper deposit
on iron nails (brown deposit)
Q 10 Saloni took a piece of burning charcoal and collected the gas in a test
tube.
(a) How will she find the nature of the gas?
She will pour some limewater in to the test tube. The gas collected will
turn the limewater milky. This will indicate that the gas collected is carbon-
dioxide. When the limewater turns milky Calcium carbonate is formed
(b) Write down word equations of all the reactions taking place in this
process.
Charcoal + Oxygen (O) = Carbon-dioxide (CO2) + lime water Ca OH2 =
Calcium carbonate (CaCO2) + Water (H2O).
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 21
Q11 What is the reason for the loss in weight of old jewellery that has been
polished?
When old jewellery is polished the gold rust that had accumulated over it is
scrubbed off along with a very tiny part of the gold surface. This gives the
jewellery a new shine but also results in loss of weight.
COMPARATIVE TABLE OF ACID / BASE / METALS / NON METALS
FOR REVISION AND EASIER UNDERSTANDING
Acids (sour) Bases (bitter and
soapy)
Metals Non-metals
Nature is acidic Nature is basic All metals are
malleable, ductile and
sonorous. Malleability
means they have the
quality of being beaten
in to thin sheets and
cast into various
shapes. Ductile means
that they can be drawn
in to wires. Sonorous
means that when hit
they emit balanced
sound waves that cause
a ringing or a echo like
sound. For this reason
bells are made of
metal.
Non metals
are not
malleable, or
ductile or
sonorous.
All metals are very
good conductors of
heat and electricity.
Therefore, all electric
wires are made of
Non-metals
are not good
conductors of
heat or
electricity
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 22
various metals or
alloys of metals.
Alloys are new metals
created by mixing two
or more natural metals.
Many alloys are better
for making certain
kinds of machines
because they have the
good qualities of more
than one metal.
Some metals such as
sodium and potassium
react very vigorously
with oxygen in water
and air. Both are
therefore stored in
kerosene.
Others metals, such as
iron, react very slowly.
But all metals react
with water. They
become corrosive
(weak in strength and
worn out) due to this
reaction.
Generally,
non-metals
do not react
with oxygen
in the water,
but they do
react to
oxygen in the
air.
Phosphorous
is a very
intensively
reactive non-
metal that
catches fire if
exposed to
air. To
prevent this
reaction,
phosphorous
is stored in
water.
All Metals react with Generally
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 23
acids and produce
hydrogen gas that
burns with a ‘pop’
sound when a match is
brought near the test
tube. However, Copper
does not react with
hydrochloric acid even
after heating, but it
does react with
sulphuric acid.
non-metals
do not react
with acids
Metals react with
Bases such as sodium
hydroxide to produce
hydrogen gas.
Non-metals
react with
bases in a
complex
manner.
In an aqueous solution
one metal displaces
another metal from its
compound. This is
called ‘displacement
reaction’
The general rule for
this displacement is
that a more reactive
metal will replace the
less reactive metal, but
a less reactive metal
cannot replace a more
reactive metal. For this
reason Zinc which is
more reactive will
replace Copper, but
Non-metals
do not show
any such
displacement
reaction
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 24
Copper which is less
reactive will not
replace Zinc.
Acid are sour to
taste
Bases are bitter to
taste and soapy to
touch)
Metals are generally
hard. Exceptions are
sodium and potassium
that can be cut with a
knife.
Non-metals
are generally
brittle and
turn into
powder easily
Citric acid is
found in citric
fruits such as
oranges and
lemons
Bases in nature
occur as Calcium
hydroxide in Lime
water (choona
pani – used for
white-washing the
walls)
Mercury is the only
metal that exists as a
liquid at room
temperature.
All other metals are
solids at room
temperature. They need
to be heated to
different heat level to
reach their melting
point. Only in their
molten form the metals
can be cast in to shapes
Ascorbic acid
(Vitamin C) in
Amla and citrus
fruits
Sodium hydroxide
/ potassium
hydroxide in soap
All metals are reactive
to air and water. Some
like sodium and
potassium are so
intensely reactive to
oxygen both in air and
water, that they need to
be stored in kerosene.
Tartaric acid in
tamarind, grapes
and raw mango
Magnesium
hydroxide in milk
of magnesia
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 25
Lactic acid in
curds
Oxalic acid in
spinach or palak
Litmus is a natural
indicator made
from lichen that
grow on the floor
of the oceans. It
has a mauve
(purple colour)
when soaked in
distilled water. It
is used to test the
nature of acids and
bases. Litmus is
available as strips
of paper in red and
blue colour, and
also as a solution.
When blue litmus
is placed in an
acid it turns red.
When red litmus is
placed in a base it
turns blue
-Here it is recalled
that red, blue and
yellow are the
only three primary
colours in nature.
All other colours
are made by
mixing these three
in different
proportions.
- Also that white
and black are the
only two colours
that can be defined
scientifically as
‘white reflects
back all the light
waves, while
black absorbs all
the light waves’.
CHAPTER 5 COAL AND PETROLEUM
Q1 What are the advantageous of using LPG and CNG as fuels?
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 26
The advantages of using LPG and CNG are that both are high octane
fuels that provide more heat or caloric value per kilogram. Both are also clean
and less polluting than fossil fuels. They can also be transported more easily
through pipes.
Q2 Name the petroleum products used for surfacing the roads.
The petroleum products used for surfacing the roads are coal-tar and
bitumen.
Q3 Describe how coal is formed from dead vegetation. What is this process
called?
The process through which dead vegetation is converted in to coal is called
‘carbonization’. About 300 million years ago the surface of the Earth was
covered with thick forests. The presence of forests caused heavy rains and
floods. The floods caused landslides that caused forests in large areas to get
buried under the soil. Forests also got buried due to earthquakes. With
successive soil deposits and floods the buried forests sank deeper below the
crust of the earth. The higher temperatures below the surface of the earth caused
the wood from the vegetation to get converted into coal.
Q 6 Explain why fossil fuels are exhaustible natural resources?
All natural resources are exhaustible. Coal and petroleum are fossil
fuels that are also exhaustible natural resources. They have been created through
the slow, natural process of carbonization due to reaction of pressure and
temperature on buried dead vegetation and animal remains.
This process took millions of years to create the fossil fuels. Also special
climatic conditions existed millions of years ago that made the process possible.
These cannot be recreated in modern times. As such if these fossil fuels are
overused, they will get exhausted and not be available in future as no fresh
stocks of such fuels are being created.
Q7 Describe characteristics and uses of coke
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 27
Coke is the most superior quality of coal. It is tough and porous and almost
the pure form of carbon. It gives the highest caloric value of heat per kilogram.
As such it is the most expensive coal and used only in the manufacture of steel
and for the extraction of many metals from their ores.
Q8 Explain the process of formation of petroleum
For millions of years ever since the creation of Oceans, organisms and fish in
the oceans have been dying. Their remains sink to the ocean floor and
accumulate. These accumulated remains get covered with layers of sand and
clay that buries them deeper and deeper. High pressure and temperatures and
millions of years transformed the accumulated remains into petroleum and
natural gas. Due to continuous movement of the crust of the earth along the
continental belt volcanic and earthquake activities raise the ocean bed near the
coastal regions. Therefore, rich deposits of petroleum and gas reserves and are
found in many coastal areas, and on land that was under the sea millions of
years ago.
CHAPTER 6 COMBUSTION AND FLAME
Q1 List conditions under which combustion can take place
Combustion is a chemical process in which a substance reacts with
oxygen to give off heat and light. The conditions needed for combustion to take
place are the presence of oxygen, a combustible substance or an inflammable
substance and an ignition temperature.
Q3 Explain how the use of CNG in automobiles has reduced pollution in our
cities.
Air pollution is cities is caused by emission of carbon monoxide by
vehicles that use fossil fuels such as petrol and diesel. Diesel in particular is
used by buses and trucks and other heavy vehicles and is a major cause of
pollution. CNG or Compressed Natural Gas is a clean fuel because it does not
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 28
emit any carbon-monoxide to pollute the air. Therefore its use in automobiles
can reduce air pollution is cities.
Q4 Compare LPG and wood as fuels
LPG Wood
LPG is an abbreviation that stands
for Liquefied Petroleum Gas. It is
created by subjecting gas to such
high pressure that it converts in to
liquid form.
Wood is the solid fuel grown on
mature trees in the forest.
LPG is an expensive fuel compared
to wood
Wood is a much cheaper fuel
compared to LPG
LPG needs a special gas stove for
its use
Wood can be burnt as a fuel even
on an informal way by arranging a
few piece in pile. It does not need
any special equipment or stove
LPG needs to be stored in a special
cylinder and sealed when not in use
Wood can be stored openly but
needs to be protected from rain,
because when wet it is difficult to
burn wood.
LPG does not emit any smoke
while burning and is therefore a
non-polluting fuel
Wood emits smoke while burning
and is therefore, polluting
LPG is very easy to alight and use Wood is more difficult to alight and
use
Per kg of LPG gives 55000 caloric
value
Per kg of wood gives 17000 to
22000 caloric value
Q5 Give reasons
(a) Water is not used to control fires involving electrical equipment
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 29
Water is a good conductor of electricity. Therefore, though it is
used for controlling all other forms of fire, it is not used for
controlling fires involving electrical equipment because then in
place of cutting off the current supply it is likely to spread the
current. This can even cause death by electrocution.
(b) LPG is a better domestic fuel than wood
Liquefied Petroleum Gas is a better domestic fuel than wood
because it has a higher caloric value, is clean and non-smoky, and is
easier to use.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped
around an aluminum pipe does not.
When a piece of paper is wrapped around an aluminum pipe it
reduces its temperature is lower than its ignition temperature. The
supply of oxygen on the lower surface of paper is also cut out due to
wrapping it. This further delays its combustion.
Q 7 Name the unit in which the caloric value of a fuel is expressed.
The caloric value of fuel is expressed in a unit called ‘kilojoule per kg’
which is written in abbreviation as ‘kJ / kg’
Q8 Explain how CO2 is able to control fires.
CO2 is useful for controlling fires of electrical equipment, oil and
petrol. It cuts out the supply of oxygen to the fire and thereby extinguishes it.
CO2 is stored in an extinguisher in liquid form. When released it expands in
volume and cools down. This expansion creates a blanket over the fire that cuts
out the supply of oxygen from the air.
Q9 It is difficult to burn a heap of green leaves but dry leaves catch fire
easily. Explain.
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 30
Green leaves contain a lot of water that inhibits burning. Dry leaves become
more combustible due to absence of any water.
Q10 Which zone of a flame does a goldsmith use for melting gold and silver
and why?
A goldsmith uses the outermost zone of a flame for melting gold and
silver. The outermost non-luminous zone of the flame has the highest
temperature. Therefore, it is most effective for melting of metals such as gold
and silver.
Q11 In an experiment 4.5 kg of a fuel was completely burnt. The heat
produced was measured to be 180.000 kJ. Calculate the caloric value of the fuel.
Ans. The caloric value of the fuel is 40.00 kJ / kg.
Q12 Can the process of rusting be called combustion? Discuss.
No, the process of rusting cannot be called combustion because
combustion occurs only when burning in air is involved and both heat and light
are generated in the process. In the process of rusting iron reacts with oxygen to
create iron oxide. This does not cause any burning, or heat or light. Therefore,
rusting cannot be called combustion.
CHAPTER 11 FORCE AND PRESSURE
Answer 1 Two examples in which an object is pushed or pulled to change
the state of motion of objects are: (a) Opening and shutting of a door. (b)
Opening and shutting of a table drawer
Answer 2 Two examples of situations in which applied force causes a change
in the shape of an object are (a) pressing a balloon ( b) pressing a soft rubber
ball.
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 31
Answer 3 Fill in the blanks : answers (a) pull (b) attracts (c) push (d) repels
Answer 4 (a) shape (b) mascular (c) non-contact (d) gravity and friction or air
Answer 5 (a) Agent are the fingers and the object is the lemon that is squeezed
and the force is muscular force
( b) Agent is the hand that presses the tube, object is the toothpaste
tube, and force applied is the muscular force.
(a) Agent is the spring, load is the object, and force is gravitational
force and non-contact force
(b) Athlete is the agent, the bar is the object and force is mascular
force.
Answer 6. It changes the shape of the piece of iron
Answer 7. Electromagnetic force is responsible for attraction between the
balloon and the wall.
Answer 8. Air pressure and gravitational force are the forces acting on the
bucket. These forces do not change the state of its motion because the object is
in a state of rest.
Answer 9. Friction and gravity are the two forces acting on the rocket
Answer 10. Is due to atmospheric pressure.
CHAPTER 12. FRICTION
Answer 8 The sliding friction is less than static friction because a sliding
object does not get sufficient time to interlock with the contact points of the
irregularities of the floor surface. When the object static its surface touching the
floor interlocks with the irregularities of the floor surface. Therefore static
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 32
friction is more than sliding object and sliding friction is slightly less than static
friction.
Answer 9 Give examples to show that friction is both a friend and a foe
Friction is a friend because without it no control over movement
would exist. Without friction a moving person or object would have difficulty in
stopping or being in a state of rest. Also without friction it would not have been
possible to hold anything in one’s hand, or to write on a paper or a black-board.
In winter rubbing of one’s palms can help to warm them through friction.
Friction is a foe because it causes wear and tear of various objects that are
involved in any movement, such as shoes and tyres of vehicles. Friction also
damages machine parts in factories and vehicles. Due to friction more muscular
energy gets wasted.
Answer 10 Objects moving in fluids must have special shapes in order to
minimize wear and tear due to friction. Generally these shapes are spherical or
rounded as in the case of ball bearing.
CHAPTER 13
SOUND
The background context for this Chapter is given first. Also see ‘Fundamentals of
Physics’ in the Fundamental unit accessible from Home page.
All stars emit radio frequency waves along with light frequency waves.
Radio frequency waves fall in the frequency band 20 cycles per second to 107
kilocycles per second meaning 1010
cycles per second.
These waves are produced by the oscillation of a current of electrons in
specially designed circuits. Radio waves are subdivided into several groups
according to wave length or frequency.
The broad divisions are:
Wave length in
metres
Frequency, kilocycles
per second
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 33
Audio frequencies Over 10,000 Below 30
Long and medium wave 10,000 to 100 30 to 3,000
Short wave 100 to 10 3,000 to 30,000
Very high frequency (VHF) 10 to 1 30,000 to 3 x 105
Ultra high frequency (UHF) 1 to 0.1 3x105 to 3 x 10
6
Radar frequency 0.1 to 0.01 3x 106 to 3x10
7
In the above division the lowest frequency is also called the audio frequency
because they are the frequencies of sound waves that can be detected by the
human ear.
Chapter begins here:
All Sound waves or audio frequencies have three characteristics, namely
(a) pitch
(b) loudness
(c) quality or timbre.
The (a) Pitch of a sound depends entirely on its frequency. A pure musical
note consists of one basic frequency. For example the middle note of the
‘saptasur’ has a basic frequency of 256 cycles per second. The highest note has
double this frequency.
( see Indian classical Music slide 2 under Fundamentals )
The (b) loudness of a sound is the amount of energy contained by the wave by
which it is transmitted. This is related directly to the amplitude.
The (c) quality or timbre of sound depends on the number and strength of the
overtones which are produced together with the basic frequency.
If a string or a wire is stretched, as in the case of all string musical instrument
such as a sitar or a sarod, the length, the thickness, the tension of its pull, the
division points at which it is fixed, will determine the number of overtones it
will have along with each of its fixed frequency. These overtones give the
distinguishing quality to the sound produced by different musical instruments.
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 34
The transmission of energy through sound waves is very similar to
transmission of energy through electromagnetic waves.
The differences are :
(a) sound waves cannot be radiated through empty space. They always need
a medium of matter whether in solid form, or fluid form, or gaseous form.
(b) sound waves travel much more slowly than electro-magnetic waves or
light waves. The speed of sound waves depends on the material through which
they are traveling. They are faster in solids than in liquids and gases, and are
faster in liquids than in gases.
The velocity of sound in air at sea level is 740 miles per second.
(c) The sound waves are longitudinal waves while the electro-
magnetic or light waves are transversal waves.
(d) In the sound waves the movement of a particle is back and forth in
the same direction as the wave. In the transversal wave the
movement is at an angle.
Sound waves can be converted into electrical waves through instruments such
as a microphone.
Their volume can be increased through simple devices such as a loudspeaker.
Answer 8 Music is the pleasant frequency of sound. Musical sound
is the sound that is pleasant to hear. The human ear has an audible
range from 20 hertz to 20,000 hertz. Loudness of sound is proportional
to the amplitude of the vibration producing the sound. Loudness is
expressed in unit called decibel (dB) A whisper is sound at 30 dB while
a normal conversation is 60 dB. Above 80 dB sound becomes
physically painful. Noise is the unpleasant frequency of sound.
Excessive or unwanted sound is noise. Music also can become noise if
its loudness increases to above 80 dB
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 35
Answer 13. Lightning is seen earlier and thunder is heard later
because the speed of light is faster than the speed of sound.
CHAPTER 14 CHEMICAL EFFECTS OF ELECTRIC
CURRENT
In the last class it was seen that in every chemical reaction
chemical energy is converted in to heat. This chapter is part of the
process of how chemical energy can be converted directly in to electric
energy.
For this, chemical reaction has to take place in an electric cell. A
simple electric cell is made by dipping a rod of zinc (Zn) and a rod of
copper (Cu) in a solution of sulphuric acid (H2SO4) in water (H2O)
Zinc dissolves in acid forming zinc ions, sulphate ions and hydrogen
ions. If the two rods are connected with a piece of metal wire the
electrons (ions) will start flow from the zinc rod to the copper rod. The
copper rod will get negatively charged. This will attract the positively
charged hydrogen ions towards the copper rod.
The two metal rods in this electric cell are called the ‘electrodes’,
and the solution is called ‘electrolyte’. Different metals chosen as
electrodes will produce different quantities of electric energy from such
a chemical reaction. Nearly all reactions that occur in the electric cell
are reversible. This means if electric energy is introduced in the
chemical reaction the reaction will go backwards. This is called the
process of ‘electrolysis’ through which gold, silver, or tin are ‘plated’
on to articles made of cheaper metals. This chapter is the first step in
understanding this process.
In general, electrical field across which energy is transmitted exists
between any two charged particles of matter. The attractive force of
two charges has the ability to bridge small gaps or distance between the
two. This results in attraction which is similar to the attraction in a
magnetic field. As a single electron behaves like a magnet, the electric
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 36
current which is a flow of electrons also exerts a magnetic field. This is
called the electro-magnetic field.
Chapters 14 and 15 make a set and need to be understood together.
From the Chapter
Answer 6 Pure water does not conduct electricity. If it is to made a
good conductor of electricity a pinch of salt needs to be added to it to
make is a solution. All solutions whether acids, bases or salts are good
conductors of electricity. As tap water contains some mineral salts
naturally dissolved in it, ordinary water is a good conductor of
electricity.
Answer 7 Normal water is a good conductor of electricity. To
prevent damage through electric shocks, before using water hoses to
douse a fire, firemen switch off the main electric supply in the area.
Answer 9 It is not safe for the electrician to carry out electrical
repairs outdoors during heavy downpour because rainwater has several
dissolved minerals and salts which make it a good conductor of
electricity.
CHAPTER 15 SOME NATURAL PHENOMENA
Read this chapter with Chapter 14
Answer 4 Sometimes a crackling sound is heard while taking off a sweater
in winter because when the sweater is being taken off it is rubbed against the
shirt or the inner clothes. This rubbing generates a charge of static energy that
produces the crackling sound
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 37
Answer 5 A charged body loses it charge if touched by hand because the
human body is a good conductor of electricity and the charge flows through the
hand in to the ground.
Answer 7 Three measures that can protect against lightning are (a) having a
lightning conductor installed on the roof of the house (b) Take shelter in a house
or a building and do not stay in the open. (c) In a forest take shelter under short
trees and avoid tall trees.
CHAPTER 16 LIGHT
Light is one of the six forms of energy that is useful to humans. These six
forms are (1) Chemical energy (2) Heat energy (3) Mechanical energy (4)
Electrical energy (5) Radiant energy (light) ( 6) Atomic energy. These six
forms energy can be transformed from one form to another through a variety of
complex processes through the medium of matter. If matter is absent as in outer
space, then energy can exist only in the form of radiant energy in the form of
ultraviolet rays, x-rays, y-rays, and cosmic rays. In this chapter only the first
simple rules of light and reflection are introduced, but you need to keep in mind
the larger picture. Try reading chapter 16 and chapter 17 together as these two
give the small and the larger picture about light
Light is a very small portion of radiant energy that is seen by the
human eye as the visible spectrum. The retina of the human eye is sensitive to a
very narrow waveband of 4.1 x 10-5
cm. to 6.5 x 10-5
cm. If a ray of sun light is
passed through a prism it splits up in to the range of different colour waves.
This is the structure of the ‘visible spectrum’. The wave length of red light is the
longest and therefore it is on the top of the spectrum at 6.5 x 10-5
The wave
length of violet light is the shortest and therefore it is at the lowest level at 4.1 x
10-5
Wavelengths shorter than the violet rays are not visible to the human eye and
therefore they are called ‘Ultra-violet’ rays. Though the visible spectrum shows
seven different colours they all blend and merge in to the next wave band.
Therefore, between each narrow band of the pure colour exists a band of
blended colours also
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 38
The following table will help in understanding the relative position of the
frequencies.
TABLE OF RADIANT ENERGY WAVE FREQUENCIES
Wave
length in
metres
Frequency
in kilocycle
per second
10-15
Cosmic ray frequencies
10-14
1020
10-13
y-ray frequencies
1019
10-12
1018
10-11
x-ray frequencies
1017
10-10
1016
10-9
1015
10-8
Ultra-violet frequencies
1014
10-7
1013
10-6
Visible light 1012
10-5
Infra-red frequencies
1011
10-4
1010
10-3
Radio frequencies
109
10-2
108
10-1
107
1 106
10 105
102 10
4
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 39
103 10
3
104
Audio frequencies
102
105 10
106 1
107 0.1
0.01
From the Chapter
Answer 2 Regular reflection takes place on smooth, polished and
regular surface. Diffused reflection takes place on irregular and rough
surface. The two laws of reflection state that a reflection takes place
when the angle of incidence is equal to the angle of reflection, and when
the reflected ray and the normal line from point of incidence fall in the
same plane. In diffused reflection these two laws do not fail, but
diffused reflection is caused by the irregularities due to the rough
surface.
Answer 3
(a) Polished wooden table: gives diffused reflection because
even after polish the irregular structure of the wood will
not enable the angle of reflection and angle of incidence
to fall on the same plane
(b) Chalk powder: gives diffused reflection because the
powder is not a plane surface
(c) Cardboard surface: will give diffused reflection due to
irregularities on the surface of the cardboard.
(d) Marble floor with water spread over it: will give regular
reflection as the water will enhance the capacity of the
polished surface to reflect
NCERT Science Textbook for Class 8th
Chapters 1 to 16
2016
© 10x10learning.com, Page 40
(e) Mirror : will give a regular reflection as the angle of
reflection and the angle of incidence will be equal due to
the smooth surface of the mirror
(f) Piece of paper will give a diffused reflection as the surface
of the paper is not a polished plane surface for the purpose
of reflection.
Answer 4 The first law of reflection states that the angle of incidence is
always equal to the angle of reflection.
The second law states that the ray of incidence, the normal ray and the
reflected ray all fall in the same plane.
See Chapters on Light in ‘Fundamentals of Physics’