NCERT Exemplar Problems Class 9 Science – Natural Resources

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NCERT Exemplar Problems Class 9 Science – Natural Resources

Question 1:
The atmosphere of the earth is heated by radiations which are mainly
(a) radiated by the sun (b) re-radiated by land
(c) re-radiated by water (d) re-radiated by land and water
(d) When solar radiations fall on earth, some are absorbed by the land but majority of these are reflected back or re-radiated by land and water bodies. These re-radiated or reflected solar radiation, heat up the atmosphere. This results in convection currents.

Question 2:
If there were no atmosphere around the earth, the temperature of the earth will
(a) increase
(b) go on decreasing
(c) increase during day and decrease during night
(d) be unaffected
(c) Air is an inexhaustible natural resource. In the absence of air, there would be no life, no winds, clouds or rain, no fires and no protection against harmful solar radiations. This is because the atmosphere covers the earth, like a blanket. As air is a bad conductor of heat the atmosphere keeps the average temperature of the earth fairly steady during the day and even during the course of whole year.
The atmosphere prevents only sudden increase in temperature during the daylight hours. And at night, atmosphere slows down the escape of heat into outer space. For instance moon, which is about the same distance from the sun that the earth is, but it lacks an earth-like atmosphere. As a consequence of absence of atmosphere the temperature of moon ranges from -190° C to 110° C.

Question 3:
What would happen, if all the oxygen present in the environment is converted to ozone?
(a) We will be protected more
(b) It will become poisonous and kill living forms
(c) Ozone is not stable, hence it will be toxic
(d) It will help harmful sun radiations to reach earth and damage many life forms
(b) Ozone is triatomic [03] aliotrope (occurence of one element in more than one form) of oxygen. It is a blue gas with purgent odour and is highly poisonous to the living system.
It is present in stratosphere region of atmosphere however, it does not exists in the lower atmosphere region called troposphere, hence does not show any fatal effect on life forms. If its value in the atmosphere increases the appreciable amount, it may become fatal, i.e., become poisonous and will kill all the living forms.

Question 4:
One of the following factors does not lead to soil formation in nature
(a) the sun (b) water (c) wind (d) polythene bags
(d) Soil is formed from the rocks undergoing the following process
(a) Weathering Breakdown of bigger rocks into smaller mineral particles
(b) Paedogenesis Decomposition by bacteria.
Polythene bags are non-biodegradable substances. These cannot be broken down by paedogenesis or weathering (physical, chemical or biological), therefore they do not contribute in soil formation. Besides these are the major non- degradable pollutants of the soil.

Question 5:
The two forms of oxygen found in the atmosphere are
(a) water and ozone (b) water and oxygen
(b) ozone and oxygen (d) water and carbon-dioxide
(c) Oxygen is present in our atmosphere in the form of a diatomic molecule. It is present in troposphere in approx imatily 20.93 (percentage of volume). Oxygen is requirecLijyalT the living beings for respiration and burning (combustion) of materials. Its percentage in air is balanced by photosynthesis.
Ozone is an aliotrope of oxygen present in significant amount in stratosphere. It is a triatomic blue gas with pungent odour. The ozone is highly important as it filter out high energy UV radiations from the sun. These radiations split ozone into molecular and atomic oxygen.

Question 6:
The process of nitrogen fixation by bacteria does not take place in the presence of
(a) molecular form of hydrogen (b) elemental form of oxygen
(c) water (d) elemental form of nitrogen
(b) Certain bacteria such as Azotobacter (occur freely in the soil) and Rhizobium (occur in root nodules of leguminous plants as pea, gram, bean, etc), convert the atmospheric nitrogen into water soluble nitrates. The process of biofixation of nitrogen is called as nitrogen fixation.
Enzymes required for biofixation of nitrogen are nitrogenases. There nitrogenases are very suseptible to destruction by oxygen. Many bacteria cease the production of enzymes in the presence of oxygen. Many nitrogen fixing bacteria grow in anaerobic conditions or respire to bring down oxygen levels or binding oxygen with the proteins like leg haemoglobin.

Question 7:
Rainfall patterns depend on
(a) the underground water table
(b) the number of water bodies in an area
(c) the density pattern of human population in an area
(d) the prevailing season in an area
(b) The rainfall pattern and amount of rainfall directly depends on the number of water bodies in an area when the water bodies of an area are heated during the day, a large . amount of water evaporates and goes into the air.
Some amount of water vapour also get into the atmosphere because of various biological activities. This air also gets heated due to the sunlight. The hot air rises up carrying the water vapour with it.
As the air rises, it expands and cools. This cooling causes the water vapour in the air to condense in the form of tiny water droplets. The condensation of water is facilitated if some particles like dust and other suspended particles could act as the ‘nucleus’ or center particle for these drops to form around.
Once the water droplets are formed, they grow bigger by the condensation. When the drops have grown big and heavy enough they fall down in the form of rain.

Question 8:
Among the given options, which one is not correct for the use of large amount of fertilisers and pesticides?
(a) They are eco-friendly
(b) They turn the fields barren after some time
(c) They adversally affect the useful component from the soil
(d) They destroy the soil fertility
(a) Use of pesticides and fertilisers over long period of time can destroy soil fertility or soil structure by killing the soil microorganism that recycle nutrients in the soil. It also kills earthworms which are instrumental in making rich humus.

Question 9:
The nitrogen molecules present in air can be converted into nitrates and nitrites by
(a) a biological process of nitrogen fixing bacteria present in soil
(b) a biological process of carbon fixing factor present in soil
(c) any of the industries manufacturing nitrogenous compounds
(d) the plants used as cereal crops in field
(a) Nitrogen gas makes up approx imatily. 78% of our atmosphere and nitrogen is also a part of molecules essential for life like proteins, nucleic acids (DNA and RNA). It is thus an essential nutrients for all life forms.
The nitrogen molecules present in air can be converted into nitrates and nitrites by a biological process ot nitrogen fixing bacteria (present in the soil or present in association with plants). (Also refer to Q. 6)

Question 10:
One of the following processes is not a step involved in the watercycle operating in nature
(a) evaporation (b) transpiration (c) precipitation (d) photosynthesis
Various process involved in water cycle are evaporation, transpiration and precipitation.

Question 11:
The term ‘waterpollution’ can be defined in several ways. Which of the following statements does not give the correct definition?
(a) The addition of undesirable substances to water-bodies
(b) The removal of desirable substances from water-bodies
(c) A change in pressure of the water-bodies
(d) A change in temperature of the water-bodies
(c) An undesirable change in the physical, biological or chemical qualities of water that adversely affects the aquatic life and makes the water unfit for use, is called water pollution. Pollution of water is one of the most serious environmental problems of the world.
Water pollution may be of following three types
(i) Surface water pollution (or inland water pollution).
(ii) Underground water pollution.
(iii) Marine water pollution.
The contaminants leading to pollution in water include a wide spectrum of chemicals, pathogens and physical or sensory changes such as elevated temperature and discolouration. Pathogens cause water borne diseases.
Alteration of physical and chemical, properties of water including change in pH, electrical conductivity, temperature and eutrophication (increase in concentration of chemical nutrients), also causes water pollution change in pressure of water bodies do not causes water pollution.

Question 12:
Which of the following is not a green house gas?
(a) Methane (b) Carbon dioxide
(c) Carbon monoxide (d) Ammonia
Greenhouse effect is a process by which thermal radiations from a planetary surface, (e.g., earth) are absorbed by atmospheric greenhouse gases and are re-radiated in all the directions causing an increase in the overall temperature of the planet (earth). By percentage contribution to the greenhouse effect on the earth the four major gases are
(i) Water vapour 36-70% (ii) Carbon dioxide 9-26%
(iii) Methane 4-9% (iv) Ozone 3-7%
Other greenhouse gases are nitrous oxide (N20) and CFC, i.e., chloroflouro carbons.

Question 13:
Which Step is not involved in the carbon-cycle? .
(a) Photosynthesis (b) Transpiration
(c) Respiration (d) Burning of fossil fuels
(b) The steps involved in carbon cycle are

Question 14:
‘Ozonehole’ means
(a) a large sized hole in the ozone layer
(b) thinning of the ozone layer
(c) small holes scattered in the ozone layer
(d) thickening of ozone in the ozone layer
(b) The part of atmosphere which is rich in ozone may be called ozone layer,
ozonosphere, ozone umbrella or ozone shield. In 1985, Farman found that ozone layer had thinned out over Antarctica. It was called on ozone hole. A similar but smaller hole was also found over arctic region by Augestein (1987).
It has grown in size from 13 million km2 in 2000. Atmospheric ozone is measured by spectrometer and is expressed in Dobson Units (DU).

Question 15:
Ozone layer is getting depleted because of
(a) excessive use of automobiles
(b) excessive formation of industrial units
(c) excessive use of manmade compounds containing both fluorine and chlorine
(d) excessive deforestation
(c) Reduction in the concentration of ozone layer is called ozone depletion. It is caused by certain chemicals which are generally rich in some of the chlorine, fluorine and carbon and are called Ozone Depleting Substances (ODS). Some of the important ODSs are chlorofluorocarbons (CFCs), halones, methyl bromide, nitrogen oxides and Carbon Tetra Chloride (CCI3) and dichlorodifuoromethane (CCI2F2).
CFCs are commonly used as aerosol propellants, refrigerants, shaving foams, spray agent in scents, etc. Some ODS are released in the stratosphere by jets flying at height.

Question 16:
Which of the following is a recently originated problem of environment?
(a) Ozone layer depletion (b) Greenhouse effect
(b) Global warming (d) All of these
(d) Due to extensive use of vehicles and thermoelectric plants, gaseous emission from industries, use of aerosol propellants, refrigerants, shaving foams, spray agents etc. leads to the emission of various harmful gases like S02, C02, CO, CFCs, H2S, HCN, etc. in the atmosphere. .
These gases are major constituents of air pollution and are inter-linked in causing ozone layer depletion, greenhouse effect and global warming.
(Also refer to Q. 12,0. 14 and Q. 15)

Question 17:
When we breathe in air, nitrogen also goes inside along with oxygen. What is the fate of this nitrogen?
(a) It moves along with oxygen into the cells
(b) It comes out with the C02 during exhalation
(c) It is absorbed only by the nasal cells
(d) Nitrogen concentration is already more in the cells so it is not at all absorbed.
Elemental nitrogen in the atmosphere cannot be used directly by either plants or animals, and must be converted to a reduced state to be useful for higher plants and animals. Precipitation often contains little quantities of ammonium and nitrate resulting from nitrogen fixation by lightning.
Certain specific bacteria possess nitrogenase enzymes that can fix atmospheric nitrogen into a form (ammonium ion) that is chemically useful to higher organisms.

Question 18:
Top soil contains the following
(a) Humus and living organisms only
(b) Humus and soil particles only
(c) Humus, living organisms and plants
(d) Humus, living organisms and soil particles
(d) The term soil profile is the vertical section of earth’s crust, which is made up of a succession of horizontal layers also called as horizons, each of which varies in thickness, colour, texture, structure, consistency, porosity, acidity and composition. The upper or A-horizon is the top soil which contains most of the litter living organisms and humus. It also has a zone of leaching, through which dissolved materials weep downward. The roots of small plants are embedded in top soil. The second or B-horizon is composed of the mineral soil (sub soil).
The third or C-horizon contains the unconsolidated parent material. The last or D-horizon comprises of rock or unmodified parent material.

Question 19:
Choose the correct sequences
(a) C02 in atmosphere → Decomposers → Organic carbon in animals → Organic carbon in plants
(b) C02 in atmosphere → Organic carbon in plants → Organic carbon in animals → Inorganic carbon in soil
(c) Inorganic carbonates in → Organic carbon in plantsOrganic carbon in animals → scavengers
(d) Organic carbon in animals → Decomposers → CO;, in atmosphere → Organic carbon in plants
(b) The steps of carbon cycle are
Question 20:
Major source of mineral in soil is the
(a) parent rock from which soil is formed
(b) plants
(c) animals
(d) bacteria
(a) The physical and chemical weathering of rocks is the process involved in formation of soil. Rocks are the naturally occurring soil aggregates of one or more minerals or mineraloids. For example common granite is combination of quartz, feldspar and biotite minerals.

Question 21:
Total earth’s surface covered by water is
(a) 75% (b) 60% (c) 85% (d) 50%
Water covers approximately 75% of the earth’s surface and is vital for all known forms of life. On earth 96.5% of planets water is found in seas and oceans, 17% in . groundwater and 1.7% in glaciers and ice caps and 0.001% in air as water vapour, clouds and precipitation.
Only 2.5% of earth’s water is freshwater and 98.8% of that water is in ice and groundwater. The remaining occurs as surface water (rivers, dams, cakes and ponds) and soil water.

Question 22:
Biotic component of biosphere is not constituted by
(a) producers (b) consumers (c) decomposer (d) air
Biosphere is the global sum of all ecosystems, integrating all living beings and their relationship including their interaction with the elements of lithosphere (land), hydrosphere (water) and atmosphere (air).
Biosphere is composed of
(i) Biotic Components are the living things that shape an ecosystem. Biotic components usually include producers, consumers and decomposers.
(ii) Abiotic Components are the non-living chemical and physical part of the ecosystem like air, water, soil, temperature and sunlight.

Question 23:
An increase in carbondioxide content in the atmosphere would not cause
(a) more heat to be retained by the environment
(b) increase in photosynthesis in plants
(c) global warming
(d) abundance of desert plants
An increase in carbon dioxide content in the atmosphere would not cause abundance of desert plants as when the temperature rises (due to excessive carbon dioxide emission) their ability to take up carbon dioxide reduces.

Question 24:
Oxygen is returned to the atmosphere mainly by
(a) burning of fossil fuel (b) respiration
(c) photosynthesis (d) fungi
(c) Burning of fossil fuels results in the emission of C02 in the atmosphere. Respiration is involved in water cycle whereas photosynthesis is the process by which plants make their food in the presence of C02 and sunlight, thus oxygen in the atmosphere.

Question 25:
Low visibility during cold weather is due to
(a) formation of fossil fuel
(b) unburnt carbon particles or hydrocarbons suspended in air
(c) lack of adequate power supply
(d) None of the above
(b) The combustion of fossil fuels increases the amount of suspended particles in air.
These suspended particles could be unburnt carbon particles (i.e., hydrocarbons). Presence of high levels of these pollutant cause visibility to be lowered, especially in cold weather when water also condenses out of air. This is known as smog and it is a visible indication of air pollution.

Question 26:
Growth of lichens on barren rocks is followed by the growth of
(a) moss (b) ferns
(c) gymnosperms (d) algae
(a) Licheus are the pioneer of succession which produce acid to errode the barren so etc.
Most lichens grow on stable rock surfaces or the bark of old trees. They are an important part of soil stabilisation and help to retain water in the soil. This enables masses to graw an the soil after lichen which is then followed by gymnosperms.

Question 27:
Marked temperature changes in aquatic environment can affect
(a) breeding of animals (b) more growth of aquatic plants|
(c) process of digestion in animals (d) availability of nutrients
(a) Most of the aquatic animals are cold blooded, i.e., their body temperature may vary according to the environment moreover the eggs and larvae of aquatic animals are highly suseptible to temperature changes.

Question 28:
Soil erosion can be prevented by
(a) raising forests (b) deforestation
(c) excessive use of fertiliser (d) overgrasing by animals
(a) Various methods used to prevent soil erosion are

  1. Intensive cropping Crops keeps the soil binded.
  2. Sowing grasses and plantihg xerophytes Sowing grasses on barren soil or planting of xerophytes will bind the loose soil.
  3. Terrace farming (terracing) In terracing the slopes are divided into a number of flat fields to slow down the flow of water. Moreover, eroded soil from upper regions of hills gets deposited in lower terraces.
  4. Contour bunding Small bunds (embankments or dikes) are raised on the edges of fields to prevent loss of top soil through either wind or water.
  5. Conservation tillage Instead of conventional tillage, reduced or no tillage can be praticed.
  6. Wind breaks Rows of trees and shrubs are planted at right angles to the prevailing wind flow.
  7. Proper drainage canals around the fields This method involves the removal of excess rain water through small drainage canals formed around the fields.

Question 29:
What happens when rain falls on soil without vegetational cover?
(a) Rain water percolates in soil efficiently
(b) Rain water causes loss of surface soil
(c) Rain water leads to fertility of the soil
(d) Rain water does not cause any change in soil
(b) Vegetational cover on the ground helps to the hold the soil in place. It also helps in percolating water into the deeper layers of the soil. Absence of vegetational cover may lead to soil erosion.

Question 30:
Oxygen is harmful for
(a) ferns (b) nitrogen fixing bacteria
(c) chara (d) mango tree
Nitrogen fixing bacteria are found in anaerobic conditions, i.e., absence of oxygen. {Also, refer to Q. 6)

Short Answer Type Questions

Question 31:
Rivers from land, add minerals to sea water. Discuss how?
The soil is made up of rocks (which are aggregation of minerals). Due to heavy rains and hoods the unprotected top soil gets washed into the streams and rivers. Feat moving water removes the top soil of the fields and near the river banks and carries it away with it. Almost all the rivers along with the eroded soil and mineral particles fall into oceans and seas thus adding mineral content to them.

Question 32:
How can we prevent the loss of top soil?
Top soil is composed of humus, earthworm and other living is mainly lost due to excessive winds, floods, rain water, etc. The major causes of soil erosion (loss of top soil) are deforestation, desertification etc. It can be prevented by various methods like cantour farming, afforestation, etc.
(Also, refer to Q. 28)

Question 33:
How is the life of organisms living in water affected when water gets polluted ?
Almost all types of water pollutants affect the life forms living in the water. The pollutants encourage the growth of some life forms and can harm the growth of other life forms. This affects the balance between various organisms that persists in that system (ecosystem). With the increase in the amount of organic wastes in water, bacteria multiply very rapidly thus using up the available oxygen. The lack of oxygen hence caused kills the fish and other aquatic animals.
Aquatic organisms are used to a certain range of temperature in the water body where they live, a sudden marked change in this temperature can be very dangerous for them (they are cold blooded animals), e.g., it affects breeding of aquatic animals. The eggs and larvae of various animals are particularly susceptible to temperature changes.

Question 34:
During summer, if you go near the lake, you feel relief from the heat, why?
Due to the flow of high pressure air present over the lake blows towards the low pressure air present over land, we feel relief from heat. The air present over lake is comparatively cooler as compared to the air present over the land

Question 35:
In coastal area, wind current moves from the sea towards the land during day; but during night it moves from land to the sea. Discuss the reason.
In coastal areas, during daytime, there is a regular flow of cool air from the sea towards the land. At night, there is a reverse flow of air from land to sea. This happens because during the daytime, land gets heated faster than water and radiated heat from land, heats the air above it. The hot air rises and creates an area of low pressure
Sea water does not get heated so rapidly, so air above the sea is comparatively cool A high pressure area forms above sea water as compared to air over land.
Therefore, cooler air over the sea, flows towards the land (region of low pressure) area. The movement of air from one region to the other creates winds.
During the night, reverse flow of air occurs. Land cools down rapidly and the air above the land becomes cooler. Sea water cools down slowly so, the air above the sea is hotter and has a lower air pressure as compared to air pressure above the land. Therefore, cooler air present over land flows towards sea.

Question 36:
Following are a few orgainsms
(a) lichen (b) mosses (c) mango tree (d) cactus
Which among the above can grow on stones; and also help in formation of soil? Write the mode of their action from making soil.
(a) Out of the above given organisms lichens can grow on rocks. During growth, they release certain
substances that cause the rock surface to powder down and form a thin layer of soil. Lichens grow on the rock surface, extract minerals from them. This creates small crevices at places where a thin layer of soil builds up.
Mosses grow over these crevices and cause deepening of the crevices and result in build up of more soil inside them. Deeper crevices form cracks and cracks become wider and deeper when the roots of short lived herbs pass into them.
With the passage of time, the roots of bigger plants, (e.g., peepal, banyan tree) pass into the cracks. Cracks gradually widen and cause slow fragmentation and eventually pulverisation of rocks.

Question 37:
Soil formation is done by both abiotic and biotic factors. List the names of these factors by classifying them as abiotic and biotic ?
Abiotic factors involved in soil formation are
(a) Rocks (b) Water
(c) Sun (d) Wind
Biotic factors involved in soil formation are (a) Lichens (b) Bryophytes|
(c) Bacteria (d) Fungi
(E) Nematodes (f) Earthworm
(g) Arthropods
e.g., ants.

Question 38:
All the living organisms are basically made up of C, N, S, P, H and 0. How do they enter the living forms?
The elements like carbon, nitrogen, sulphur, phosphorus, hydrogen and oxygen enter into living forms through plants. Most of these elements first enter plants and become components of their organic material during the process of photosynthesis. From plants, these elements pass on to other organisms through the food chain. e.g., Plant (grasses) is eaten by goats which are further consumed by man. Thus, the organic materials are trasferred from grass to goat and then to man

Question 39:
Why does the percentage of gases like oxygen, nitrogen and carbon dioxide remain almost the same in the atmosphere?
In the environment the percentage of the gases like oxygen, nitrogen and carbon dioxide is maintained by their cyclic flow.
The oxygen cycle, nitrogen cycle and carbon cycle respectively operate continuously in the nature by the constant interaction between the biotic and abiotic components of the biosphere. These interactions consist of a transfer of matter and energy between the different components of the biosphere.
Thus, their levels in the atmosphere is maintained constantly up to required limit.

Question 40:
Why does moon have very cold and very hot temperature variations e.g., from – 190°C to 110°C even though it is at the same distance from the sun as the earth is?
The moon is at the same distance from the sun as the earth is but there is very cold and hot temperature variations, e.g. from -190°C to 110°C. The main reason behind this huge variation in temperature of moon is that it has no atmosphere.
The atmosphere plays very important role in the temperature control. Air is a bad conductor of temphaturs thus, it acts as temperature buffer and checks excessive rise of temperature during the day and excessive cooling during the night.

Question 41:
Why do people love to fly kites near the seashore ?
In sea shore or coastal regions, during the day, the air above the land gets heated faster and starts rising. As this air rises, a region of low pressure is created and air over the sea moves into this area of low pressure.
This movement of air from one region to the other creates winds. During the day, the direction of wind would be from the sea to the land. So, flying kites near sea shore is easy as compared to anywhere else.|(Also, refer to 0. 35)

Question 42:
Why does Mathura refinery pose problems to the Taj Mahal ?
Mathura refinery releases many air pollutants such as sulphur dioxide. S02 reacts with water in the atmosphere to forms sulphuric acid. This sulphuric acid is washed away into the soil by rain (acid rainfall). This acid errodes the stone, surface of buildings, affects salmon reproduction, brickworks, etc.
Thus posing a great threat to Taj Mahal as they can destroy the marble quality and its colour, etc.

Question 43:
Why do not lichens occur in Delhi whereas they commonly grow in Manali or Darjeeling?
Lichens are bioindicators of air pollution. They are highly sensitive to sulphur dioxide (S02) pollution in air from automobiles and other factories.
Delhi has maximum air pollution due to a large number of vehicles compared to Manali and Darjeeling. Therefore, the growth of lichens does not occur in Delhi or any place, where air pollution is high.

Question 44:
Why does water need conservation even though large oceans surround the land masses?
Water is one of the most unusual natural compound found on earth and it is also very important constituent of life. It exists as snow, water and water vapour. Terrestrial life forms require fresh water for their existence and the fresh water is not available easily.
(Also, refer to Q. 21)

Question 45:
There is mass mortality of fishes in a pond. What may be the reasons?
The reason of mass mortality of fishes in a pool may be water pollution.
(Also, refer to Q.33)

Question 46:
Lichens are called pioneer colonisers of bare rock. How can they help in formation of soil ?
Licheus grow on the rock surface extract minerals from them this creates small creviees at places where a thin layer of soil build sup.
(Also refer to Q. 36)

Question 47:
‘Soil is formed by water.’ If you agree to this statement then give reasons.
Water helps in breakdown of bigger rocks into smaller mineral particles. It could get into cracks in the rocks formed due to uneven heating by the sun. If this water freezes, it would cause cracks to widen.
Flowing water wears away even hard rock over long periods of time. Fast flowing water often carries big and small particles of rocks downstream. These rocks rub against other rocks and resultant abrasion causes the rock to broken down into smaller and smaller particles. The water then takes these particles alongwith it and deposit them further.

Question 48:
Fertile soil has lots of humus, why?
Answer :
Fertile soil has lots of humus because it contains microorganisms that decompose dead organic matter present in soil and help in the formation of humus. Flumus is a sources of mineral, absorbs water and makes the soil porous for easy passage of plant roots.

Question 49:
Why step farming is common in hills?
Step farming is a common preventive measures for soil erosion caused due to rain water and strong winds.
(Also, refer to 0. 28)

Long Answer Type Questions

Question 50:
Why are root nodules useful for the plants?
Root nodules are special parts on the roots of plants where nitrogen fixing bacteria are present. These bacteria convert inert aerial nitrogen into nitrates for the use of plants. The process of biofixation of nitrogen by some nitrogen fixing bacteria like Rhizobium, etc., is called nitrogen fixation. .
(Also, refer to Q. 6)

Question 51:
How do fossil fuels cause air pollution?
Air pollution is the addition of particulate matter, gases and vapours into the atmosphere these have an adverse effect on humans, animals, vegetation and human assets. Air pollution is of two main types: 1. Natural (e.g., forest fire, dust storm, pollen); 2. Human made (e.g., burning of fossil or fuels in industries, vehicles and thermoelectric plants, gaseous emission from industries, mining processing, stone crushing).
The particulate air pollutants are also called suspended particulate matter or SPM because they remain suspended in air for a good period of time. SPM consists of smokes, soot (a black powder substance that is produced when coal, wood, etc., is burnt), fly ash and dust, (i.e., particles of silica, grit, lead, asbestos, cement, sulphur, etc).
Gases that causes air pollution are sulphur dioxide, nitrogen oxides, carbon monoxide, hydrogen sulphide, hydrogen cyanides, hydrogen fluorides, chlorine, methane and ammonia. They are formed from nitrogen, sulphur and carbon compounds of fossil fuels and decaying organic matter. Pollutant vapours include unburnt hydrocarbons (benzene).

Question 52:
What are the causes of water pollution? Discuss how you can contribute in reducing water pollution.
Different causes of air pollution are

(i) Industrial wastes Effluents of mills and industries such as paper mills, petroleum refineries, etc., contain large quantities of harmful chemicals including acids, alkalies and heavy metals, (e.g., mercury salts from paper industries). These chemicals are discharge into water bodies (rivers and lakes).
(ii) Synthetic soaps and detergents Water containing soaps and detergents is discharged into rivers, from houses and certain factories.
(iii) Fertilisers and pesticides Fertilisers and pesticides are being used excessively in
the fields to increase crop production. These are washed by rainwater into water bodies and pollute them. .
(iv) Petroleum oil Drilling and shipping operations are common in the oceans. Leakage of petroleum oil during such operations or due to accidents (i.e., oil spill) cause water pollution.
(v) Solid particles Rain erodes soil and carries silt to water. Tiny suspended particles of clay and dust also settle in water. These soil particles cause turbidity in water.
(vi) Thermal pollution Discharge of hot water from industries and thermal plants into water body changes the normal temperature of the water. The content of oxygen decrease. Reduced oxygen content kills aquatic animals and reduces the rate of decomposition of organic matter which, therefore, accumulates.
(vii) Sewage Organic wastes are contributed as domestic and commercial sewage by food processing plants, dairy, farms, piggeries, poultry farms, slaughter houses, breweries, tanneries, etc. Animal excreta discharged into fields or dumped in to pits reaches water bodies through run off and leaching, particularly during the rainy season and cause water pollution.
Ways to reduce water pollution are
(i) If sewage and garbage generated by homes and industries is treated properly before discharging into water sources, it will reduce water pollution and cause less harm to the aquatic life.
(ii) If hot water generated by the industries is collected at common place, allowed to cool and then discharged in water-bodies, it will not affect the breeding capacity of aquatic organisms.
(iii) If commercial areas, factories and industries are shifted to the isolated area far away from residential areas, it can reduce the effect of air pollution on people.

Question 53:
A motor car, with its glass totally closed, is parked directly under the sun. The inside temperature of the car rises very high. Explain why?
Sunlight emits ultraviolet radiations which pass through the glass and heat the car. The radiations emitted by inner parts of the cannot pass out of the glass, so the heat trapped inside raises the temperature of the interior.
This is because glass is transparent to infrared raditations released from the sun, having smaller wavelength than that emitted by the interior of the car which have longer wavelength to which the glass of the car is opaque.

Question 54:
Justify ‘Dust is a Pollutant.
Dust is a pollutant because
(i) Dust consists of suspended particles. Inhalation of dust causes lots of discomfort including allergic asthma, bronchitis, cold or cough.
(ii) It reduces intensity of light.
(iii) Dust particles settle over leaf and can block stomata and reduce gaseous exchange in plants.
(iv) It causes irritation of eyes.
(iv) Dust particles reduce the photosynthetic activity of plants.
(v) It contains smoke, fly ash, particle of silica, lead, asbestos, etc., which pollute environment.

Question 55:
Explain the role of the sun in the formation of soil.
The sun heats up rocks during the day, so they expands. At night, these rocks cool down and contract. Since, all the part of rock do not expand and contract at the same rate, (ultimately huge rocks break into smaller pieces). This results in the formation of soil.
{Also, refer to Q. 37)

Question 56:
Carbon dioxide is necessary for plants. Why do we consider it as a pollutant?
Carbon dioxide gas is necessary for plants to performs photosynthesis. It is also a greenhouse gas. Upto certain concentration1 (approx imatuly 350 ppm) in atmosphere, it is both good raw material as well as essential for keeping the earthworm. But, when concentration of C02 from certain level rise^’ it becomes a pollutant because it. Causes greenhouse effect. It may cause various respiratory disorders and also helps in the formation of smog.

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