SOLUTION OF SCIENCE (NCERT) CLASS VIII BOOK
NCERT Solutions for Class 8 Science Chapter 8 Cell Structure and Functions
NCERT Solutions for Class 8 Science Chapter 12 Friction
1. 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.
(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 lesser than the static friction.
2. 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 is (c) static, sliding, rolling
3. 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 order will 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 is (a) wet marble floor, dry marble floor, newspaper and towel.
4. 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.
When book slides down on the desk, a frictional force acts between the book and the surface of the desk. The direction of the friction force on the book is opposite to the direction of its motion and acts in an upward direction. It is shown in the diagram below.
5. You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?
It is possible to walk on the floor because of the friction present between our feet and the ground. For walking, we push the ground in a backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction decreases between the ground and the feet when there is soapy water spilt on the floor. Hence, it becomes difficult to walk on the soapy floor.
6. Explain why sportsmen use shoes with spikes.
Sportsmen use shoes with spikes because of the better grip given by spikes while running. This is because the force of friction between the shoes and the ground increases with the help of spikes.
7. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Due to the interlocking of the irregularities on the two surfaces in contact, the force of friction arises. On the floor when a heavy object is placed, the interlocking of irregularities on the surface of the 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 greater force than Iqbal.
8. Explain why sliding friction is less than static friction
When irregularities present in the surfaces of two objects in contact get interlocked with each other, friction come into play. The time given in sliding for interlocking is very small. Thus, interlocking is not strong. Therefore, less force is required to overcome this interlocking. Due to this reason, sliding friction is less than static friction.
9. Give examples to show that friction is both a friend and a foe.
Advantages of the friction
a) Due to friction, we are able to walk.
b) We are able to write because of the friction between the tip of the pen and paper.
Disadvantages of friction
a) Because of friction, the tires and soles of shoes wear out.
b) Friction produces heat between different parts of the machines. This can damage the machines.
10. Explain why objects moving in fluids must have special shapes.
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 the objects a special shape, the force of friction acting on it can be minimized. Hence, it becomes easier for a body to move through the fluid.
Chapter 7 Conservation of Plants and Animals
1. Fill in the blanks.
(a) A place where animals are protected in their natural habitat is called _________ .
(b) Species found only in a particular area is known as _______ .
(c) Migratory birds fly to faraway places because of __________ changes.
(a) A place where animals are protected in their natural habitat is called Sanctuary.
(b) Species found only in a particular area is known as endemic .
(c) Migratory birds fly to faraway places because of climatic changes.
2. Differentiate between the following.
(a) Wildlife sanctuary and biosphere reserve
(b) Zoo and wildlife sanctuary
(c) Endangered and extinct species
(d) Flora and fauna
An area within which wild animals are protected from external dangers like hunting
An area constructed for the conservation of biodiversity
Adequate habitat and protection are provided for the wild animals living in a sanctuary
Several life forms like plants, animals, and micro-organisms conservation are possible in a reserve
Eg: Corbett national park
Eg: Nilgiri Biosphere reserve
Animals are kept in artificially constructed facilities for public exhibition.
An area within which wild animals are protected from external dangers like hunting.
It is an artificial habitat and the animals may or may not adapt to the new conditions.
The animals are conserved within their natural habitat itself. So, they won’t have a problem with their surroundings.
The species which are on the verge of extinction are called as Endangered species.
The species of animals or plants that no longer exist are called Extinct species.
Eg: Bengal Tiger, Blue Whale, etc.
Eg: Caribbean monk seal, Great Auk, Passenger pigeon.
It is the collective name for the plant life from a particular area.
It is the collective name for the animal life from a particular area
Eg: Spurge and Hogweed from the Nilgiris region
Eg: Langur, Tiger, Tahr from the Nilgiris region.
3. Discuss the effects of deforestation on the following.
(a) Wild animals
(c) Villages (Rural areas)
(d) Cities (Urban areas)
(f) The next generation
(a) Effects of deforestation on wild animals:
Removal of trees and plant life from a particular area for the construction of industries, agriculture or for other such purposes is termed as Deforestation. Plant life forms an integral part of the ecosystem and these plants form the natural habitat of various animals of that ecosystem. Destroying the plant life will ultimately threaten the existence of animals in that particular ecosystem and may lead to their extinction.
(b) Effects of deforestation on the environment:
In plants, Photosynthesis takes place by the absorption of CO2 from the earth’s atmosphere. The percentage of CO2 in the atmosphere increases rapidly if the plant life of an area is destroyed. Increased concentrations of CO2 result in the trapping of excessive heat radiations within the Earth’s atmosphere contributing to global warming. This results in the rise of the temperature of the Earth and disturbs the water cycle which occurs in nature. Hence, rainfall pattern changes leading to droughts and floods.
(c) Effects of deforestation on villages:
The soil particles are held together in a place by the roots of the plants, trees and other vegetation. In the event of water flow or a high-speed wind over the top layer of the soil, the soil particles get removed easily as there are no plants to hold it in place. Thus, Soil erosion is increased by deforestation activities. Soil loses fertility and loses humus. Hence, a fertile land, fit for agriculture turns into a desert.
(d)Effects of deforestation on cities:
The risk of natural calamities like floods and drought in urban areas is increased by deforestation. It will also lead to global warming because of increased CO2 levels in the atmosphere. This is a result of continuous pollution from automobiles and industries. The water cycle in the vicinity is also affected.
(e) Effects of deforestation on Earth:
Deforestation leads to increased chances and occurrences of desertification, droughts and floods. Carbon Dioxide level in the earth’s atmosphere also increases which leads to global warming. The water cycle is disrupted and also there is an increased risk of natural calamities.
(f) Effects of deforestation on the next generation:
Our Environment is slowly changed by the activities of deforestation, both directly and indirectly. Soil erosion, global warming, desertification, drought, greenhouse effect, floods and many other problems are caused only because of deforestation. Ultimately, the next generation will be facing the consequences of deforestation.
4. What will happen if
(a) we go on cutting trees
(b) the habitat of an animal is disturbed
(c) the top layer of soil is exposed
(a) If we go on cutting the trees
If we go on cutting the trees, the animals will lose their natural habitat as essential part of their ecosystem are destroyed. This reduces the biodiversity of the affected areas. The temperature of the Earth also increases and gives way to global warming. Global warming, in turn, affects the water cycle. Hence, rainfall pattern changes leading to droughts and floods. Risks of soil erosion, global warming, desertification, greenhouse effect increases.
(b) If the habitat of an animal is disturbed
All the basic necessities for an animal, like food, water, shelter and protection is provided by the habitat in which the animal lives-in disturbing the habitat of the animal forces it to move to another place in search of food, water, shelter and protection. The animal, in due course, may get killed by other animals.
(C) If the top layer of the soil is removed
Removing the top layer of the soil gradually exposes the lower layers of the soil. The lower layers of the soil are hard and rocky in nature and are less fertile. This is because of the reduced quantity of humus. Soil erosion on a continued basis will turn it barren and infertile.
5. Answer in brief.
(a) Why should we conserve biodiversity?
(b) Protected forests are also not completely safe for wild animals. Why?
(c) Some tribals depend on the jungle. How?
(d) What are the causes and consequences of deforestation?
(e) What is Red Data Book?
(f) What do you understand by the term migration?
(a) The number and variety of various life forms such as plants, animals and micro-organisms in the area are called Biodiversity. Both plants and animals have a mutual dependence on each other for their survival. As they are related to one another, destruction of one will automatically destroy the other. Hence, biodiversity has to be conserved in order to maintain nature’s balance.
(b) People who live near the forests depend on the resources of the forests in order to fulfil their day-to-day needs. Therefore, it is not safe for animals living in protected forests. The animals would be threatened by the presence of the human population. This results in the killing of wild animals and selling their products for huge sums of money.
(c) The forests provide food, fodder and other resources to the tribal people. It is inevitable for them to depend on forests and their resources for everyday needs.
(d) Causes of deforestation:
There are a lot of causes for deforestation. A few of them are listed below.
I. In order to accommodate the ever-expanding urban population, forest areas are often cleared and converted into lands for various uses.
II. For agricultural activities like crop cultivation and cattle grazing, forest lands are cleared.
III. Firewood is a major product from the forests and it is one of the main reasons for cutting the trees on a large scale.
Consequences of deforestation:
There are a lot of fatal consequences caused by deforestation. A few of them are listed below.
1. Soil erosion
2. Loss of Biodiversity
5. Global warming as a result of climate change
6. Disruption of the water cycle
(e) Red data book is basically a sourcebook which has an international list of all plant and animal species which are endangered, that is, on the verge of extinction. The International Union for Conservation of Nature and Natural resources (IUCN) maintains the books and adds/ removes the names of the species by conducting a comprehensive survey.
(f) The movement of an organism or a group of organisms from its natural habitat to another place on a standard basis at a particular time of each and every year is known as migration. The organisms do so in order to avoid uninhabitable conditions of climate or for the process of breeding.
6. In order to meet the ever-increasing demand in factories and for shelter, trees are being continually cut. Is it justified to cut trees for such projects? Discuss and prepare a brief report.
No. Cutting trees in order to meet the ever growing demands of the human population is not at all justified. There are a vast number of organisms like wild animals, insects, and birds living in the forests. The forests provide good quality air for both animals and humans. This is because of the process of plants respiration by which, they consume the harmful carbon dioxide and give out good quality Oxygen. Thus, keeping greenhouse gases and global warming under check. The roots of the flora of the forests help in preventing soil erosion. Natural Calamities such as floods and droughts could also be prevented by their presence. They help in increasing the soil’s fertility and conserving the biodiversity of the ecosystem.
Overutilization of forests and its resources by cutting a large number of trees in order to satisfy the demands of the ever-increasing human population leading to many long-term problems like
- Soil erosion
- Greenhouse effect
- Global warming
Besides the above-listed problems, the effects of large-scale deforestation will be leaving an everlasting mark on the face of human civilization. The balance of nature is disturbed by the destruction of trees. Hence, it is of utmost priority and importance to conserve forests.
7. How can you contribute to the maintenance of green wealth of your locality? Make a list of actions to be taken by you.
I can take care of the green cover of my locality by growing plants in my locality. I shall motivate the people of my neighbourhood to plant more trees and take care of the existing trees by creating awareness about the importance of plant life in an ecosystem. In order to make this possible, I would initiate the proceedings and organize weekly events on the same. Creating awareness among the children and youth would be of top priority as they will easily understand the issues that we are facing. Planting new trees is as important as looking after the existing ones. So, my volunteers and I would water the plants and trees on a regular basis.
Chapter 6 COMBUSTION AND FLAME
1. List conditions under which combustion can take place.
The burning of a substance in the presence of oxygen is defined as combustion.
The conditions under which combustion can take place are
- The presence of air or oxygen.
- The presence of fuel plays a significant role.
- Ignition temperature is maintained (It is defined as the substance that catches fire at its lowest temperature.)
2. Fill in the blanks.
(a) Burning of wood and coal causes __________of air.
(b) A liquid fuel, used in homes is__________ .
(c) Fuel must be heated to its ____________ before it starts burning.
(d) The fire produced by oil cannot be controlled by___________ .
(a) Burning of wood and coal causes Pollution of air.
(b) A liquid fuel, used in homes is Kerosene.
(c) Fuel must be heated to its Ignition Temperature before it starts burning.
(d) The fire produced by oil cannot be controlled by Water.
3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
CNG played an important role in reducing pollution among automobiles for the following reasons
- CNG is comparatively a cleaner fuel.
- The CNG can be an alternative for diesel, petrol and propane/LPG.
- It usually contains a few undesirable gases than the other fuels mentioned above.
- The combustion of fuels like petroleum causes many unburnt carbon particles along with carbon monoxide which leads to respiratory diseases.
4. Compare LPG and wood as fuels
- It is considered as a traditional fuel used for both domestic and industrial purposes.
- Wood produces a lot of smoke which pollutes the atmosphere and cause respiratory diseases.
- The usage of wood to a large extent causes deforestation.
- The calorific value of wood ranges between 17000 to 22000 kJ/kg
- However, wood may be used as a furnace, stove or fireplace in indoors while it is used for a campfire, furnace at outdoors.
- The usage LPG (Liquefied petroleum gas) has replaced wood.
- It doesn’t release smoke and other pollutants
- It is a cleaner fuel
- The fuel efficiency of LPG is more than that of wood.
- The calorific value of LPG is 55000 kJ/kg
- Hence LPG is mostly preferred choice
5. Give reasons.
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
- Water is a good conductor of electricity.
- If added to an electrical fire, the water would just spread the electricity further.
- The person dousing the fire might get an electric shock
- LPG being a cleaner fuel than wood doesn’t release smoke and other pollutants.
- Wood, on the other hand, releases a lot of smoke and fumes polluting the atmosphere causing pollution and leading to respiratory diseases.
- Hence LPG is a better domestic fuel than wood.
- The paper by itself catches fire easily because of its low ignition temperature
- The piece of paper wrapped around an aluminium pipe doesn’t catch fire because aluminium is a good conductor of electricity.
- While the paper wrapped around an aluminium pipe results in an increase in ignition temperature. So there is a transfer of heat from paper to the aluminium pipe. Hence it doesn’t catch fire.
6. Make a labelled diagram of a candle flame.
Draw Diagram from book.
7. Name the unit in which the calorific value of a fuel is expressed.
Calorific value is defined as the energy contained in the fuel. It is expressed in the form kJ/kg
kJ=kilo joules and kg=kilogram
8. Explain how CO2 is able to control fires.
CO2 is a non-combustible gas and extinguishes fire in two ways:
(i) It is heavier than oxygen and it covers the fire like a blanket and cuts off the contact between oxygen and fuel.
(ii) In cylinders, CO2 is kept in the liquid form. When released, it expands enormously. This brings down the temperature of the fuel, which helps in controlling the fire.
9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
A heap of green leaves contains a lot of moisture in it, hence its ignition temperature is high. Therefore it does not catch fire easily.
But dry leaves have no moisture content in it, hence its ignition temperature is low. Therefore it catches fire easily.
10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
The goldsmith mainly uses non-luminous flame which is termed to be the outermost part of the flame. This part of the flame is used because the outermost flame undergoes complete combustion and is considered as the hottest part of the flame.
11. In an experiment, 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.
12. Can the process of rusting be called combustion? Discuss.
No, because rusting is an exothermic process as heat is liberated during rusting. On the other hand, combustion is a chemical process in which a substance reacts with oxygen to release energy in the form of heat or light.
13. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?
The water placed in the outermost part of the flame will be heated in a short time since it is non-luminous flame and is regarded as the hottest part of the flame. So Ramesh’s beaker will be heated first. However, Abida who placed the beaker in the luminous flame (yellow flame) is comparatively less hot.
Access Answers of Science NCERT class 8 Chapter 5 Coal and Petroleum
1. What are the advantages of using CNG and LPG as fuels?
The advantages of using CNG and LPG as fuels are as follows
- CNG and LPG burn easily.
- CNG and LPG give a lot of heat energy when burnt.
- CNG and LPG can be transported easily through pipelines.
- CNG and LPG are clean fuels and they do not release smoke when burnt.
2. Name the petroleum product used for surfacing of roads.
Bitumen is used for surfacing of roads.
3. Describe how coal is formed from dead vegetation. What is this process called?
Dense forests got buried under the soil due to natural processes, millions of years ago. More and more soil got deposited over them and they got compressed more. This led them to get exposed to very high temperature and pressure. They slowly got converted into coal under these extreme conditions. The whole process of formation of coal from dead vegetation is known as carbonization.
4. Fill in the blanks.
(a) Fossil fuels are ____________, ____________, and ____________ .
(b) Process of separation of different constituents from petroleum is called _____________.
(c) Least polluting fuel for a vehicle is ______________.
(a) Fossil fuels Coal, Petroleum, and natural gas.
(b) Process of separation of different constituents from petroleum is called refining.
(c) Least polluting fuel for a vehicle is Compressed Natural Gas (CNG).
5. Tick True/False against the following statements.
(a) Fossil fuels can be made in the laboratory. (T/F)
(b) CNG is more polluting fuel than petrol. (T/F)
(c) Coke is the almost pure form of carbon. (T/F)
(d) Coal tar is a mixture of various substances. (T/F)
(e) Kerosene is not a fossil fuel. (T/F)
6. Explain why fossil fuels are exhaustible natural resources.
The process of formation of fossil fuels requires millions of years. Dead vegetation and animals that get buried deep inside the earth require high temperature and pressure for the formation of fossil fuels, which cannot be done in the laboratory. Thus, fossils are limited. Therefore, the use of fossil fuels at a rapid rate will lead to their exhaustion.
7. Describe the characteristics and uses of coke.
i) Coke is tough.
ii) Coke is porous.
iii) Coke is black in colour.
i) In the manufacture of steel.
ii) In the extraction of metals (as a reducing agent).
8. Explain the process of the formation of petroleum.
Dead organisms that got buried in the sea millions of years ago got covered with layers of sand and clay. Due to lack of air, high temperature, and high pressure, these dead organisms got transformed into petroleum and natural gas.
9. The following table shows the total power shortage in India from 1991–1997. Show the data in the form of a graph. Plot shortage percentage for the years on the Y-axis and the year on the X-axis.
Answers of Science NCERT class 8 Chapter 4 Materials: Metals and
1. Which of the
following can be beaten into thin sheets?
Soln: Answer is Zinc
Here, Zinc is a metal with malleability and
ductility whereas Phosphorus, Sulphur and Oxygen are nonmetals which lack
malleability and ductility.
2. Which of the
following statements is correct?
(a) All metals are
(b) All non-metals
(c) Generally, metals
(d) Some non-metals
is (c) Generally, metals are ductile.
Ductility is a property where a substance can
be drawn into thin wires; generally metals are ductile with mercury as the
3. Fill in the
(a) Phosphorus is a
(b) Metals are
conductors of heat and ____________ .
(c) Iron is
____________reactive than copper.
(d) Metals react
with acids to produce ____________ gas.
(a) Phosphorus is a very reactive non-metal.
(b) Metals are good conductors of heat and electricity.
(c) Iron is more reactive than copper.
(d) Metals react with acids to produce hydrogen gas.
4. Mark ‘T’ if the
statement is true and ‘F’ if it is false.
non-metals react with acids. ( )
(b) Sodium is a
very reactive metal. ( )
displaces zinc from zinc sulphate solution. ( )
(d) Coal can be
drawn into wires. ( )
5. Some properties
are listed in the following table. Distinguish between metals and non-metals on
the basis of these properties.
Have property of Malleability
Do not have a property of Malleability
Have property of Ductility
Do not have the property of Ductility
Good conductor of Heat
Bad Conductor of Heat
6. Conduction of
Good conductor of Electricity
The bad conductor of Electricity
6. Give reasons for
(a) Aluminium foils
are used to wrap food items.
(b) Immersion rods
for heating liquids are made up of metallic substances.
(c) Copper cannot
displace zinc from its salt solution.
(d) Sodium and
potassium are stored in kerosene
a) Aluminium is malleable and can be drawn
into thin sheets hence Aluminium foils are used to wrap food items
b) Immersion rods for heating liquids are
made up of metallic substances because metals are good conductors of heat and
c) Copper cannot displace zinc from its salt
solution because Zinc is more reactive than copper.
d) Sodium and Potassium are highly reactive a
metal which readily reacts with atmospheric Oxygen to catch fire hence Sodium
and Potassium are stored in kerosene.
7. Can you store
the lemon pickle in an aluminium utensil? Explain.
Soln: Pickle consists of acids
which react with Aluminium metal to produce salt and Hydrogen. Hence pickle is
not stored in aluminium utensil.
8. Match the
substances given in Column A with their uses given in Column B.
(c) Wrapping food
(b) Electric wire
9. What happens when?
sulphuric acid is poured on a copper plate?
(b) Iron nails are
placed in a copper sulphate solution?
equations of the reactions involved.
No reaction occurs when dilute sulphuric acid is poured on a copper plate.
However, when concentrated sulphuric acid is poured on a copper plate, hydrogen
gas evolves along with the formation of blue coloured copper sulphate crystals.
The chemical reaction for the reaction between concentrated sulfuric acid and copper
Cu + H2SO4 (conc.) -> CuSO4 + H2
(ii) Iron being more reactive displaces
copper from copper sulphate. In this reaction, the blue colour of copper
sulphate fades and there is a deposition of copper on the iron nail.
Fe + CuSO4 →
FeSO4 + Cu
10. Saloni took a
piece of burning charcoal and collected the gas evolved in a test tube.
(a) How will she
find the nature of the gas?
(b) Write down word
equations of all the reactions taking place in this process.
a) In a test tube containing gas, add a few
drops of water. Now cover the test tube and shake well. After shaking, test the
solution with blue litmus. It will change from blue to red. Thus, gas is acidic
b) Charcoal reacts with oxygen to form carbon
11. One day Reeta
went to a jeweller’s shop with her mother. Her mother gave old gold jewellery
to the goldsmith to polish. Next day when they brought the jewellery back, they
found that there was a slight loss in its weight. Can you suggest a reason for
the loss in weight?
In order to polish the gold ornament, it is
to be dipped into a liquid called aqua regia (a mixture of hydrochloric acid
and nitric acid). On getting dissolved in the environment of aqua regia, the
outer layer of gold dissolves and an inner shiny layer appear. The dissolving
of the layer causes a reduction in the weight of the jewellery.