CBSE Class 9 Science Natural Resources Sure Shot Questions Set 04

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This CBSE Class 9 Science assignment set (Set 4) covers Chapter 14, Natural Resources. It has 56 questions on the atmosphere and its gases, the ozone layer, the water, carbon and nitrogen cycles, soil formation, water and air pollution, and the role of the sun and living organisms in nature, with answers and Teacher's Notes.

ASSIGNMENT QUESTIONS SET - 4

CHAPTER - 14

NATURAL RESOURCES

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

Answer: (d) re-radiated by land and water

1. Sunlight passes through the atmosphere and heats the land and water surfaces.
2. Land and water then re-radiate this heat as infrared radiation, which heats the air above them.
3. Hence, the correct option is (d).

Teacher's Note:
a) The sun's rays mostly pass through the air without heating it directly.
b) Both land and water are needed for this; the option naming only one is incomplete.

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

Answer: (c) increase during day and decrease during night

1. The atmosphere retains heat and evens out temperature between day and night.
2. Without it, the surface would receive full sunlight by day and lose heat quickly at night.
3. Hence, the correct option is (c).

Teacher's Note:
a) The moon has no atmosphere, and its temperatures swing widely between day and night.

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.

Answer: (b) It will become poisonous and kill living forms

1. Ozone is a toxic gas that damages the lungs and plant tissues at ground level.
2. If all oxygen were converted to ozone, living organisms would be poisoned.
3. Hence, the correct option is (b).

Teacher's Note:
a) Ozone protects us only in the upper atmosphere, not near the ground.
b) Living organisms need oxygen for respiration, so losing it would be fatal.

4. One of the following factors does not lead to soil formation in nature
(a) the sun
(b) water
(c) wind
(d) polythene bags

Answer: (d) polythene bags

1. The sun, water and wind break rocks into soil through weathering and transport.
2. Polythene bags are man-made and do not form soil.
3. Hence, the correct option is (d).

Teacher's Note:
a) Soil forms slowly from rock fragments mixed with humus and living matter.
b) Polythene is non-biodegradable and adds pollution to the soil instead.

5. The two forms of oxygen found in the atmosphere are
(a) water and ozone
(b) water and oxygen
(c) ozone and oxygen
(d) water and carbon-dioxide

Answer: (c) ozone and oxygen

1. Oxygen in the atmosphere exists as O₂, the usual form.
2. Ozone is O₃, a second allotropic form of oxygen.
3. Hence, the correct option is (c).

Teacher's Note:
a) Water is a compound of hydrogen and oxygen, not a form of oxygen.
b) Ozone is formed high in the atmosphere by the action of ultraviolet light.

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

Answer: (b) elemental form of oxygen

1. Nitrogen-fixing bacteria use the enzyme nitrogenase, which is inactivated by oxygen.
2. Free oxygen therefore stops nitrogen fixation.
3. Hence, the correct option is (b).

Teacher's Note:
a) Many nitrogen-fixing bacteria live in low-oxygen conditions, such as root nodules.
b) Nitrogen gas itself is needed by these bacteria, so option (d) is the nitrogen they fix.

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

Answer: (b) the number of water bodies in an area

1. Water bodies such as lakes, rivers and seas give off water vapour by evaporation.
2. More water vapour in the air leads to more clouds and rainfall.
3. Hence, the correct option is (b).

Teacher's Note:
a) Forests and vegetation also add water vapour to the air through transpiration.
b) Population density has little direct effect on rainfall.

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

Answer: (a) They are eco-friendly

1. Excess fertilisers and pesticides pollute water and soil.
2. They kill useful soil organisms and reduce soil fertility, which leads to barren fields.
3. Hence, the statement that they are eco-friendly is not correct, and the answer is (a).

Teacher's Note:
a) Organic manure and crop rotation keep soil healthy without these side effects.
b) The printed word "adversally" should read "adversely" in option (c).

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

Answer: (a) a biological process of nitrogen fixing bacteria present in soil

1. Nitrogen fixing bacteria in soil convert atmospheric nitrogen into nitrates and nitrites.
2. Carbon fixing factors and cereal crops do not fix nitrogen from air.
3. Hence, the correct option is (a).

Teacher's Note:
a) Lightning and industrial processes also fix nitrogen, but the biological process is the one asked here.
b) Rhizobium in root nodules is a common example.

10. One of the following processes is not a step involved in the water-cycle operating in nature
(a) evaporation
(b) transpiration
(c) precipitation
(d) photosynthesis

Answer: (d) photosynthesis

1. The water cycle includes evaporation, transpiration, condensation, and precipitation.
2. Photosynthesis is part of the carbon cycle, not the water cycle.
3. Hence, the correct option is (d).

Teacher's Note:
a) Photosynthesis uses water, but it does not move water through the cycle.
b) Transpiration is the loss of water vapour from leaves.

11. The term "water-pollution" 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

Answer: (c) A change in pressure of the water bodies

1. Water pollution is any change in water quality that harms living things.
2. Adding harmful substances (a), removing useful substances such as oxygen (b), and raising the temperature (d) all pollute water.
3. A change in pressure is not a pollution definition, so the answer is (c).

Teacher's Note:
a) Thermal pollution from hot industrial water is a real form of water pollution.
b) Pressure changes in water bodies do not usually harm organisms.

12. Which of the following is not a green house gas?
(a) Methane
(b) Carbon dioxide
(c) Carbon monoxide
(d) Ammonia

Answer: (c) Carbon monoxide

1. Methane and carbon dioxide are major greenhouse gases.
2. Carbon monoxide does not trap heat in the same way and is not counted among the main greenhouse gases.
3. Hence, the correct option is (c).

Teacher's Note:
a) Carbon monoxide has an indirect effect, because it reacts to form carbon dioxide.
b) Ammonia is also not one of the main greenhouse gases.

13. Which step is not involved in the carbon-cycle?
(a) Photosynthesis
(b) Transpiration
(c) Respiration
(d) Burning of fossil fuels

Answer: (b) Transpiration

1. Photosynthesis takes carbon dioxide from the air into plants.
2. Respiration and burning of fossil fuels return carbon dioxide to the air.
3. Transpiration is the loss of water vapour and does not involve carbon, so the answer is (b).

Teacher's Note:
a) Decomposition also returns carbon to the atmosphere.
b) Transpiration belongs to the water cycle.

14. 'Ozone-hole' 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

Answer: (a) a large sized hole in the ozone layer

1. The ozone hole is a region of very severe depletion of the ozone layer.
2. It appears as a large area over Antarctica, not as small scattered holes.
3. Hence, the correct option is (a).

Teacher's Note:
a) Thinning is the general term for ozone depletion, while the ozone hole is a severe, localised case.
b) The ozone hole mainly forms over the poles in spring.

15. Ozone-layer is getting depleted because of
(a) excessive use of automobiles
(b) excessive formation of industrial units
(c) excessive use of man-made compounds containing both fluorine and chlorine
(d) excessive deforestation.

Answer: (c) excessive use of man-made compounds containing both fluorine and chlorine

1. Chlorofluorocarbons (CFCs) are man-made compounds containing chlorine and fluorine.
2. In the upper atmosphere, UV light releases chlorine from CFCs, which breaks down ozone.
3. Hence, the correct option is (c).

Teacher's Note:
a) Refrigerators, aerosol sprays and air conditioners used CFCs in the past.
b) The Montreal Protocol was signed to phase out these compounds.

16. Which of the following is a recently originated problem of environment?
(a) Ozone layer depletion
(b) Green house effect
(c) Global warming
(d) All of the above

Answer: (d) All of the above

1. Ozone layer depletion, the greenhouse effect and global warming have all become serious problems since the industrial age.
2. Each of these problems is recent compared with the history of the earth.
3. Hence, the correct option is (d).

Teacher's Note:
a) The greenhouse effect itself is natural and keeps the earth warm.
b) Global warming is the increase in average temperature caused by extra greenhouse gases.

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 CO2 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.

Answer: (b) It comes out with the CO2 during exhalation

1. Nitrogen is not used by the body in respiration.
2. It passes into the lungs and is breathed out again during exhalation.
3. Hence, the correct option is (b).

Teacher's Note:
a) Nitrogen is an inert gas for human respiration.
b) Oxygen is the only gas taken up in significant amounts by the blood.

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.

Answer: (d) Humus, living organisms and soil particles.

1. Top-soil is rich in humus, the dark organic matter formed from dead plants and animals.
2. It also contains many living organisms and mineral soil particles.
3. Hence, the correct option is (d).

Teacher's Note:
a) Top-soil is the most fertile layer of the soil.
b) Plants grow in top-soil, but their roots are not counted as soil content.

19. Choose the correct sequences
(a) CO2 in atmosphere → decomposers → organic carbon in animals → organic carbon in plants
(b) CO2 in atmosphere → organic carbon in plants → organic carbon in animals → inorganic carbon in soil
(c) Inorganic carbonates in water → organic carbon in plants → organic carbon in animals → scavengers
(d) Organic carbon in animals → decomposers → CO2 in atmosphere → organic carbon in plants

Answer: (b) CO2 in atmosphere → organic carbon in plants → organic carbon in animals → inorganic carbon in soil

1. Plants take in CO₂ from the atmosphere and make organic carbon by photosynthesis.
2. Animals eat plants and receive organic carbon.
3. On death, decomposers convert organic carbon into inorganic carbon in the soil, which the cycle reuses.
4. Hence, the correct option is (b).

Teacher's Note:
a) Decomposers act on dead matter, so they come after animals in the cycle.
b) The carbon cycle has no step where animals produce organic carbon first.

20. Major source of mineral in soil is the
(a) parent rock from which soil is formed
(b) plants
(c) animals
(d) bacteria

Answer: (a) parent rock from which soil is formed

1. Weathering of the parent rock releases minerals such as potassium, calcium and iron into the soil.
2. Plants, animals and bacteria only return minerals that they have taken from the soil.
3. Hence, the correct option is (a).

Teacher's Note:
a) Living things recycle minerals but do not create them.
b) Soil fertility depends on the minerals from the rock below.

21. Total earth's surface covered by water is
(a) 75%
(b) 60%
(c) 85%
(d) 50%

Answer: (a) 75%

1. Oceans and seas cover about 71 per cent of the earth's surface.
2. Including lakes, rivers and ice, the water cover is about 75 per cent.
3. Hence, the correct option is (a).

Teacher's Note:
a) Most of this water is salty and unfit for drinking.
b) Only a small fraction of the earth's water is fresh and usable.

22. Biotic component of biosphere is not constituted by
(a) producers
(b) consumers
(c) decomposer
(d) air

Answer: (d) air

1. Biotic components are living things, such as producers, consumers and decomposers.
2. Air is a non-living (abiotic) component of the biosphere.
3. Hence, the correct option is (d).

Teacher's Note:
a) Sunlight, water, soil and air are abiotic components.
b) Producers, consumers and decomposers together form the biotic components.

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

Answer: (d) abundance of desert plants

1. More carbon dioxide traps more heat, so the environment retains more heat and global warming increases.
2. Carbon dioxide is a raw material of photosynthesis, so it can increase photosynthesis in plants.
3. It does not directly cause desert plants to multiply, so the answer is (d).

Teacher's Note:
a) Extra carbon dioxide can boost plant growth, but this does not solve the warming problem.
b) Desert plants depend on water and soil, not directly on carbon dioxide levels.

24. Oxygen is returned to the atmosphere mainly by
(a) burning of fossil fuel
(b) respiration
(c) photosynthesis
(d) fungi

Answer: (c) photosynthesis

1. Green plants use carbon dioxide and water to make food, releasing oxygen as a by-product.
2. Respiration and burning use up oxygen instead of releasing it.
3. Hence, the correct option is (c).

Teacher's Note:
a) Most of the oxygen in the air is produced by marine algae and plants.
b) Burning fossil fuels uses oxygen and releases carbon dioxide.

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 these

Answer: (b) unburnt carbon particles or hydrocarbons suspended in air

1. In cold weather, smoke and fog hang close to the ground and do not disperse.
2. The smoke carries unburnt carbon particles and hydrocarbons, which reduce visibility.
3. Hence, the correct option is (b).

Teacher's Note:
a) This mix of fog and smoke is called smog.
b) Burning of wood and coal in winter adds to the problem.

26. Growth of Lichens on barren rocks is followed by the growth of
(a) moss
(b) ferns
(c) gymnosperms
(d) algae

Answer: (a) moss

1. Lichens form the first colonisers on bare rock and break it down into soil.
2. Once a thin soil layer forms, mosses grow, followed by ferns and then larger plants.
3. Hence, the correct option is (a).

Teacher's Note:
a) This sequence is called ecological succession.
b) Mosses hold moisture and add organic matter to the thin soil.

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.

Answer: (a) breeding of animals

1. Many aquatic animals breed only within a narrow range of temperature.
2. Sudden temperature changes disturb breeding cycles and can cause death.
3. Hence, the correct option is (a).

Teacher's Note:
a) Temperature also affects the amount of oxygen dissolved in water.
b) Options (b), (c) and (d) are secondary effects, not the direct one asked.

28. Soil erosion can be prevented by
(a) raising forests
(b) deforestation
(c) excessive use of fertilizer
(d) overgrazing by animals

Answer: (a) raising forests

1. Tree roots bind the soil and prevent it from being washed or blown away.
2. Leaves and branches also slow down rain and wind.
3. Hence, the correct option is (a).

Teacher's Note:
a) Deforestation, overgrazing and fertilizer misuse all increase erosion or soil damage.
b) Planting grass and trees on slopes is a simple remedy.

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

Answer: (b) Rain water causes loss of surface soil

1. Without plant cover, the rain falls directly on the soil and loosens its surface.
2. The loose soil is then washed away as run-off, which is soil erosion.
3. Hence, the correct option is (b).

Teacher's Note:
a) Roots and leaves protect the soil by holding particles together and slowing down water.
b) Surface soil is the most fertile layer, so losing it reduces crop yield.

30. Oxygen is harmful for
(a) ferns
(b) nitrogen fixing bacteria
(c) chara
(d) mango tree

Answer: (b) nitrogen fixing bacteria

1. Many nitrogen fixing bacteria are anaerobic and work in low-oxygen conditions.
2. The enzyme nitrogenase is damaged by oxygen, so free oxygen harms these bacteria.
3. Hence, the correct option is (b).

Teacher's Note:
a) Ferns, chara and mango trees all need oxygen for respiration.
b) Root nodules protect nitrogen fixing bacteria from too much oxygen.

31. Rivers from land, add minerals to sea water. Discuss how?

Answer:
1. Rivers flow over rocks and soil and dissolve minerals such as sodium, potassium, calcium and magnesium salts.
2. Rivers carry these dissolved minerals and fine particles down to the sea.
3. When the river water mixes with sea water, the minerals are added to the sea, which is why sea water is salty.

Teacher's Note:
a) Weathering of rocks releases minerals, and rain water carries them away.
b) Evaporation of sea water leaves the salts behind, but the minerals enter the sea through rivers.

32. How can we prevent the loss of top soil?

Answer:
1. Plant trees and grasses, because their roots bind the soil.
2. Prevent overgrazing and deforestation.
3. Use contour ploughing and terrace farming on slopes to slow down water run-off.
4. Add organic matter and practise crop rotation to keep the soil structure healthy.

Teacher's Note:
a) Top-soil takes hundreds of years to form, so its loss is very serious.
b) Mulching cover on bare soil also reduces the damage from rain.

33. How is the life of organisms living in water affected when water gets polluted?

Answer:
1. Toxic chemicals kill fish and other aquatic organisms directly.
2. Organic wastes use up the dissolved oxygen in water, so fish and other organisms suffocate.
3. Pollution disturbs the food chain, which affects the whole aquatic ecosystem.

Teacher's Note:
a) Dissolved oxygen is the key requirement for fish.
b) Some pollutants build up in the bodies of organisms and pass up the food chain.

34. During summer, if you go near the lake, you feel relief from the heat, why?

Answer:
1. Water has a high specific heat, so it absorbs a large amount of heat without getting hot quickly.
2. Water evaporates from the lake surface, and this evaporation takes heat from the air.
3. The air near the lake is therefore cooler, which gives relief from the heat.

Teacher's Note:
a) Evaporation is a cooling process because it takes in heat.
b) The same effect is why sprinkling water on a floor makes a room feel cooler.

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.

Answer:
1. During the day, land heats up faster than the sea, so the air over the land becomes warm and rises.
2. Cooler, denser air from over the sea moves in to take its place, which gives a sea breeze.
3. At night, land cools faster than the sea, so the air over the sea is warmer and rises, and the cooler air from the land moves towards the sea, which gives a land breeze.

Teacher's Note:
a) Water has a higher specific heat, so it heats and cools more slowly than land.
b) The wind always blows from the cooler area towards the warmer area.

36. Following are a few organisms (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 for making soil.

Answer:
1. Lichen can grow on stones and also helps in forming soil.
2. Lichen secretes acids that slowly dissolve the surface of the rock and break it into small particles.
3. When lichen dies, its remains add organic matter to these particles, which gradually forms a thin layer of soil.

Teacher's Note:
a) Lichens are the first organisms to colonise bare rock.
b) Mosses can also grow on rocks once a thin soil layer is formed, but lichen is the one that starts the process.

37. Soil formation is done by both abiotic and biotic factors. List the names of these factors by classifying them as abiotic and biotic?

Answer:
Abiotic factors: sunlight (temperature changes), water (rain, rivers and glaciers), wind, and rocks (weathering).
Biotic factors: lichens, mosses, plants, bacteria, fungi, earthworms and other animals.

Teacher's Note:
a) Abiotic factors are non-living and break rocks physically or chemically.
b) Biotic factors add humus and mix the soil.

38. All the living organisms are basically made up of C, N, S, P, H and O. How do they enter the living forms? Discuss.

Answer:
1. Carbon and oxygen enter living things through carbon dioxide and oxygen from the air, and through water.
2. Hydrogen and oxygen enter through water, which plants absorb through their roots.
3. Nitrogen enters through nitrogen fixing bacteria, which convert atmospheric nitrogen into nitrates that plants absorb.
4. Sulphur and phosphorus enter from soil and water as sulphates and phosphates, which plants absorb through their roots.
5. Animals get these elements by eating plants or other animals.

Teacher's Note:
a) Plants are the main entry point for most elements into the food chain.
b) Elements are not created by living things; they are recycled through the environment.

39. Why does the percentage of gases like oxygen, nitrogen and carbon dioxide remain almost the same in the atmosphere?

Answer:
1. Photosynthesis releases oxygen and uses carbon dioxide, while respiration and burning do the reverse.
2. The nitrogen cycle fixes nitrogen and returns it to the atmosphere through denitrification, so nitrogen levels stay steady.
3. These cycles balance the gases, so their percentages in the air remain almost constant.

Teacher's Note:
a) Nitrogen makes up about 78 per cent of the air, and oxygen about 21 per cent.
b) Human activities can disturb this balance, as seen with rising carbon dioxide.

40. Why does moon have very cold and very hot temperature variations eg, from -190°C to 110°C even though it is at the same distance from the sun as the earth is?

Answer:
1. The moon has no atmosphere to retain heat or to spread heat around the surface.
2. During the day the surface receives full sunlight and heats up to about 110°C.
3. At night the surface loses heat quickly into space, so it cools to about -190°C.

Teacher's Note:
a) The earth's atmosphere keeps temperatures steady between day and night.
b) The moon's day and night each last about 14 earth days, which adds to the extremes.

41. Why do people love to fly kites near the seashore ?

Answer:
1. Near the seashore, the sea breeze blows steadily and strongly during the day.
2. A steady, strong wind lifts and keeps the kite in the air easily.
3. Hence, people enjoy flying kites by the sea.

Teacher's Note:
a) The sea breeze is caused by uneven heating of land and water.
b) Open, flat areas with few obstacles also help the wind flow smoothly.

42. Why does Mathura refinery pose problems to the Taj Mahal?

Answer:
1. The Mathura refinery releases sulphur dioxide and other gases into the air.
2. These gases form acid rain, which reacts with the marble of the Taj Mahal.
3. The marble, which is made of calcium carbonate, gets eroded and discoloured over time.

Teacher's Note:
a) Acid rain attacks calcium carbonate, which is the main mineral in marble.
b) Pollution control in the region protects monuments like the Taj Mahal.

43. Why do not lichens occur in Delhi whereas they commonly grow in Manali or Darjeeling?

Answer:
1. Lichens are very sensitive to air pollution, especially sulphur dioxide.
2. Delhi has high levels of air pollution from vehicles and industries, which prevents lichens from growing.
3. Manali and Darjeeling have cleaner air, so lichens grow there.

Teacher's Note:
a) Lichens are therefore used as indicators of air quality.
b) Clean air is the main requirement for lichen growth.

44. Why does water need conservation even though large oceans surround the land masses?

Answer:
1. Most of the water in oceans is salty and cannot be used for drinking, farming or industry.
2. Fresh water is only a very small part of the total water on earth.
3. Fresh water is limited, so it must be conserved for daily use.

Teacher's Note:
a) Desalination is possible but costly, so it is not used everywhere.
b) Rainwater harvesting and reducing waste help conserve fresh water.

45. There is mass mortality of fishes in a pond. What may be the reasons ?

Answer:
1. Pollution from factory waste or sewage may have made the water toxic.
2. Decaying algae and organic waste may have used up the dissolved oxygen in the water.
3. A sudden rise in temperature may have lowered the oxygen content of the water and stressed the fish.

Teacher's Note:
a) Low dissolved oxygen is one of the most common reasons for fish deaths.
b) Testing the water for oxygen and pollutants can confirm the cause.

46. Lichens are called pioneer colonisers of bare rock. How can they help in formation of soil?

Answer:
1. Lichens grow on bare rock and secrete acids that slowly dissolve the rock surface.
2. This breaks the rock into small particles.
3. When lichens die, their remains mix with these particles and add humus, which forms a thin layer of soil.

Teacher's Note:
a) This is the first stage of soil formation on bare rock.
b) Once the soil layer thickens, mosses and other plants can grow.

47. "Soil is formed by water." If you agree to this statement then give reasons

Answer:
1. Partly agree. Water breaks rock into smaller pieces through the action of rain, running water and freezing and thawing.
2. Flowing water carries the rock particles and deposits them, which forms soil in rivers, valleys and plains.
3. Water also dissolves minerals and supports living organisms that add humus, so soil formation depends on water along with other factors.

Teacher's Note:
a) Water is one of several agents of soil formation, along with wind, temperature and living organisms.
b) Soil formed by water is often deposited as alluvial soil in river plains.

48. Fertile soil has lots of humus. Why?

Answer:
1. Humus holds water and minerals, so the soil stays moist and nutrient-rich.
2. Humus improves the soil structure, which allows air and water to move through the soil.
3. As humus decomposes, it releases nutrients such as nitrogen and phosphorus that plants need.

Teacher's Note:
a) Humus is the dark organic matter formed from dead plants and animals.
b) Soils without humus are sandy and dry out quickly.

49. Why step farming is common in hills?

Answer:
1. Hill slopes are steep, so rain water runs off quickly and erodes the soil.
2. Step farming turns the slope into flat steps, which slow down the water and reduce erosion.
3. The flat steps also hold water for crops and make cultivation possible on steep land.

Teacher's Note:
a) Step farming is also called terrace farming.
b) Terraces protect the top soil from being washed into the valleys below.

50. Why are root nodules useful for the plants?

Answer:
1. Root nodules contain nitrogen fixing bacteria, such as Rhizobium.
2. These bacteria convert atmospheric nitrogen into nitrates that the plant can use to make proteins.
3. This enriches the soil with nitrogen, so the plant grows better and the soil stays fertile.

Teacher's Note:
a) Legumes such as peas, beans and gram form root nodules.
b) Plants cannot take nitrogen directly from the air, so this partnership is essential.

51. How do fossil fuels cause air pollution?

Answer:
1. Fossil fuels contain sulphur and nitrogen, which burn to form sulphur dioxide and oxides of nitrogen.
2. Incomplete burning releases carbon monoxide and unburnt carbon particles, which form smoke.
3. Burning also releases carbon dioxide, which adds to global warming, and these gases together pollute the air.

Teacher's Note:
a) Sulphur dioxide and nitrogen oxides cause acid rain.
b) Using cleaner fuels such as CNG and better engines reduces this pollution.

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

Answer:
1. Causes: industrial effluents, untreated sewage, fertilisers and pesticides washed from fields, oil spills, plastic and solid waste dumped in water, and hot water released by industries.
2. My contribution: I can avoid littering near water, use less detergent and soap, and not throw waste or medicines into drains.
3. I can also save water, plant trees near water bodies and report illegal dumping to local authorities.

Teacher's Note:
a) Sewage and industrial waste are the largest sources of water pollution.
b) Small individual actions add up when many people follow them.

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?

Answer:
1. Glass lets short wave sunlight enter the car and warm the seats and dashboard.
2. These warm objects re-radiate heat as long wave infrared radiation, which glass does not let out.
3. The trapped heat keeps building up, so the temperature inside the car rises very high. This is the greenhouse effect.

Teacher's Note:
a) The same greenhouse principle keeps the earth warm.
b) Leaving windows open a little reduces the build-up of heat.

54. Justify "Dust is a pollutant" ?

Answer:
1. Dust particles in the air cause breathing problems and lung diseases such as asthma and bronchitis.
2. Dust reduces visibility and settles on leaves, which blocks sunlight and reduces photosynthesis.
3. Dust particles also settle on buildings and monuments and damage them.

Teacher's Note:
a) Fine dust can enter deep into the lungs and cause long term harm.
b) Dust comes from vehicles, construction and burning of waste.

55. Explain the role of the Sun in the formation of soil.

Answer:
1. The sun heats rocks during the day and they cool at night, which causes them to expand and contract and break into pieces.
2. The sun's energy drives evaporation, which produces rain and running water that break and carry rock particles.
3. The sun's energy also supports plants and other organisms, which add humus to the soil.

Teacher's Note:
a) Temperature changes are a physical cause of weathering.
b) Without sunlight there would be no plants and so no humus.

56. Carbon dioxide is necessary for plants. Why do we consider it as a pollutant?

Answer:
1. Carbon dioxide is needed by plants for photosynthesis, so it is a useful gas in the right amount.
2. Burning fossil fuels and deforestation have raised its level in the atmosphere far above the natural amount.
3. Excess carbon dioxide traps heat in the atmosphere and causes global warming, so it is considered a pollutant.

Teacher's Note:
a) A substance can be useful at low levels and harmful in excess.
b) Planting trees and reducing fossil fuel use help control the level of carbon dioxide.