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NCERT Solutions · Class 9 Science Earth as a System: Energy, Matter, and Life

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Revise, Reflect, Refine 13.1–13.10 (part 2 of 3)

  1. Exercise 13.1

    Choose the most appropriate option to describe the role of biogeochemical cycles in an ecosystem. (i) To provide food directly to all organisms. (ii) To recycle essential nutrients between biotic and abiotic components. (iii) To create new elements for use by living things. (iv) To remove pollutants and toxins from the organism.

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    NCERT’s answer
    (ii)
    (ii) To recycle essential nutrients between biotic and abiotic components.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-1
    The chapter defines a biogeochemical cycle as the cyclic movement of matter and energy between the abiotic (non-living) and biotic (living) components of the Earth, which keeps carbon, nitrogen and oxygen available to support life.
    (i) is wrong — the cycles supply raw materials, not food; only producers actually make food, by photosynthesis.
    (iii) is wrong — elements are never created; they are only moved between reservoirs such as air, water, soil, rock and organisms.
    (iv) is wrong — removing toxins from inside an organism is excretion, which is not what a biogeochemical cycle does.

  2. Exercise 13.2

    Which of the following is primarily responsible for warming of the Earth? (i) Solar radiation is immediately absorbed by carbon dioxide, which then releases it as heat. (ii) The atmosphere’s tiny particles absorb incoming solar radiation, which directly heats the Earth. (iii) The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases. (iv) The Earth’s environment is heated only by the solar radiation reflected by the clouds.

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    NCERT’s answer
    (iii)
    (iii) The Earth's surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-2
    The chapter states that infrared radiation warms the Earth's surface, which then re-radiates this heat back into the atmosphere, and a portion of the outgoing heat is trapped by greenhouse gases such as \(\displaystyle CO_{2} \), \(\displaystyle CH_{4} \) and water vapour.
    (i) is wrong — \(\displaystyle CO_{2} \) does not absorb the incoming sunlight; it absorbs the outgoing infrared radiated by the warmed surface.
    (ii) is wrong — particles, clouds and gases do absorb and scatter some incoming radiation, but that reduces what reaches the ground; it is not what heats the Earth.
    (iv) is wrong — radiation reflected by clouds is sent back towards space, so it is energy lost, not energy that warms the Earth.

  3. Exercise 13.3

    Explain how climate change affects the water cycle. Illustrate with examples.

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    Climate change speeds up the water cycle and makes it uneven — more moisture moves through the air, but it is delivered as extremes rather than steady rain.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-3
    Heavier rain in some places, drought in others: a warmer atmosphere holds more moisture, so monsoons intensify over some regions while others are left dry.
    Melting glaciers: extra meltwater swells rivers now, and in the long run raises sea level, threatening coastal cities such as Mumbai and Chennai.
    More run-off, less infiltration: sudden bursts of intense rainfall run off the surface instead of soaking in — this erodes soil and reduces the recharge of groundwater.
    Agriculture suffers: with less groundwater stored, sustaining crops through the dry months becomes difficult.
    All spheres are linked by it: the disturbed water cycle connects the cryosphere (glaciers), hydrosphere (rivers and oceans), atmosphere (moisture), geosphere (soil erosion and poor infiltration) and biosphere (crops and fisheries).

  4. Exercise 13.4

    Describe how albedo affects the Earth’s surface temperature and its climate.

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    Albedo is the fraction of solar radiation that a surface reflects — the higher the albedo, the less radiation is absorbed and the cooler the surface stays.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-4
    High albedo keeps a region cold: snow reflects \(\displaystyle 0.80-0.90 \) and ice \(\displaystyle 0.50-0.70 \) of the incoming radiation, which is a major reason the polar regions are so cold.
    Low albedo makes a region warm: black soil, ocean water and crushed rock (\(\displaystyle 0.25-0.30 \)) reflect little and absorb much, so they heat up quickly — the same reason a dark road becomes hotter than a light-coloured surface, and dark clothes feel hotter than white ones.
    Climate effect: these differences in absorption create the uneven heating of the Earth's surface, and that uneven heating is what drives winds and ocean currents.
    Feedback effect: warming melts bright snow and ice and exposes darker land and sea beneath; the lowered albedo means still more absorption, which warms the region further.
    Human land use changes albedo too: cities of dark concrete and asphalt absorb and re-radiate heat, giving the urban heat island effect, and clearing forests also alters the surface albedo.

  5. Exercise 13.5

    How are mountain and valley breezes formed? Suppose there are two mountains, one covered with grass and another covered with barren rocks; would the temperature of the two mountain breezes be different? If so, how?

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    Valley breeze (daytime, air flows up the slope): the Sun-facing mountain slopes heat up faster than the valley floor, so the air above them becomes warm and rises, creating a low pressure region; cooler air from the valley then moves up the slopes to replace it.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-5
    Mountain breeze (after sunset, air flows down into the valley): the slopes lose heat faster and become cooler while the valley floor stays relatively warmer; the air over the slopes becomes cool and dense and flows down into the valley.
    Such daily reversals of wind direction are common in hilly regions like Shimla, Dehradun and other Himalayan valleys.
    Yes — the two mountain breezes would differ in temperature. The breeze off the barren rocky mountain would be comparatively warmer (and drier).
    Bare rock has a low albedo (crushed rock reflects only \(\displaystyle 0.25-0.30 \)), so during the day it absorbs a large amount of solar radiation and stores heat.
    At night the rock re-radiates that stored heat into the air above it — exactly the reason a concrete house feels hot at night while a thick mud-and-wood house stays cool — so the air draining off the rocky slope is warmer.
    The grass-covered mountain stays cooler through the day by shade and transpiration and stores less heat, so the air sinking off it at night is cooler; the transpired water vapour also makes that breeze more moist.

  6. Exercise 13.6

    You have witnessed weather phenomena, such as winds, storms, rainfall, etc. Which atmospheric layer is mainly responsible for such phenomena and what is the primary reason for its occurrence?

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    The troposphere — the lowest layer of the atmosphere, of average height about \(\displaystyle 12 \) km — is responsible for winds, storms and rainfall.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-6
    Primary reason: it is heated from the Earth's surface below, not directly by the Sun. The ground absorbs solar radiation and warms the air touching it, so temperature in this layer decreases with height at about \(\displaystyle 6.5\ ^{\circ}\mathrm{C} \) per km.
    Warm air near the ground is lighter and rises; cooler air moves in to replace it. This convection is what generates winds and storms.
    Nearly all the atmosphere's water vapour is in this layer, so evaporation, condensation and precipitation take place here.
    Above it, the stratosphere (\(\displaystyle 12-50 \) km) behaves oppositely: the ozone layer absorbs UV, so temperature rises with height. This calms the layer, prevents vertical mixing of air, and keeps weather confined to the troposphere.
    The troposphere's height is greatest above the equator and least above the poles, which is why equatorial regions have the most vigorous weather.

  7. Exercise 13.7

    Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?

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    The nitrogen cycle is the overall movement of nitrogen between air, soil, water and organisms; atmospheric \(\displaystyle N_{2} \) is the largest reservoir but is non-reactive, so it must first be converted into soluble compounds.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-7
    Nitrogen fixation — nitrogen-fixing bacteria such as Rhizobium in the root nodules of legumes and Azotobacter in the soil convert atmospheric \(\displaystyle N_{2} \) into ammonia (\(\displaystyle NH_{3} \)). Lightning fixes a tiny amount as nitrogen oxides, and industrially the Haber–Bosch process does most of it today.
    Nitrification — Nitrosomonas converts ammonia into nitrite (\(\displaystyle NO_{2}^{-} \)), and Nitrobacter converts nitrite into nitrate (\(\displaystyle NO_{3}^{-} \)).
    Assimilation — plants take up these nitrogen compounds from the soil; animals obtain their nitrogen by eating plants or other animals.
    Ammonification — when plants and animals die or produce waste, decomposers such as bacteria and fungi break down the organic matter and return ammonia to the soil.
    Denitrification — denitrifying bacteria such as Pseudomonas convert some nitrates back into nitrogen gas, returning it to the atmosphere and completing the cycle.
    If nitrogen were not cycled: the huge atmospheric store would stay useless, because \(\displaystyle N_{2} \) cannot be absorbed directly by plants or animals.
    No nitrate or ammonia in the soil means plants cannot synthesise proteins and nucleic acids, so growth, seed formation and crop yields would fail.
    Animals, which get all their nitrogen from plants, would be starved of protein, and food chains would collapse.
    Nitrogen locked up in dead bodies and waste would never be released back to the soil, so ecosystems could not recover or maintain their balance.

  8. Exercise 13.8

    What are the impacts of deforestation on the Earth’s oxygen and carbon cycles? What are the other consequences of deforestation?

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    Oxygen cycle: fewer trees means less photosynthesis, so less \(\displaystyle O_{2} \) is returned to the atmosphere, while respiration and the combustion of fuels keep on consuming it — the production side of the balance is weakened.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-8
    Carbon cycle (uptake): forests are natural carbon sinks; clearing them means less atmospheric \(\displaystyle CO_{2} \) is drawn down by photosynthesis, and the remaining sinks become saturated.
    Carbon cycle (release): the carbon stored in the wood is returned to the air as \(\displaystyle CO_{2} \) when the trees are burnt or decay, so deforestation is a source of \(\displaystyle CO_{2} \) as well as a loss of sink.
    Net effect: atmospheric \(\displaystyle CO_{2} \) rises, intensifying the greenhouse effect and global warming — deforestation, along with fossil-fuel burning, is one of the two causes the chapter names for the \(\displaystyle 35\% \) rise since 1960.
    Less rainfall: reduced transpiration puts less moisture into the air, which can cause a decline in local rainfall.
    Altered albedo: removing forest cover changes the fraction of sunlight the surface reflects, changing local heating.
    Soil erosion: without tree roots to hold the soil together, erosion increases; run-off rises and infiltration falls.
    Loss of biodiversity: habitats are destroyed over time, and many species decline as they lose their natural homes.

  9. Exercise 13.9

    Explain with suitable diagram the path that carbon takes to go back to the atmosphere. You may start from plants using CO2\displaystyle CO_{2} from the atmosphere.

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    Carbon starts in the air as \(\displaystyle CO_{2} \), is fixed by plants, and returns to the air by four routes: respiration, decomposition, combustion of fossil fuels, and exchange with ocean water.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-9
    Step $\displaystyle 1$ — Photosynthesis: plants take \(\displaystyle CO_{2} \) from the atmosphere and, using sunlight, convert it into glucose.
    Step $\displaystyle 2$ — Respiration (fast route, days to years): plants themselves respire and release some \(\displaystyle CO_{2} \) straight back; animals that eat plants or other animals respire and release \(\displaystyle CO_{2} \) too.
    Step $\displaystyle 3$ — Decomposition: when plants and animals die, decomposers break down the organic matter and \(\displaystyle CO_{2} \) returns to the air.
    Step $\displaystyle 4$ — Combustion of fossil fuels (slow route, millions of years): buried dead plants and animals are converted over millions of years into coal, oil and gas; burning these for heating, cooking, transport and industry releases that carbon as \(\displaystyle CO_{2} \) on a very short time scale.
    Step $\displaystyle 5$ — Ocean exchange: the atmosphere and ocean water continuously exchange \(\displaystyle CO_{2} \); dissolved \(\displaystyle CO_{2} \) becomes carbonate and bicarbonate ions used by phytoplankton and by shell-forming organisms, and when they die and sink, carbon is stored on the ocean floor for long periods.
    What your diagram must show
    Draw a closed loop. Put a box at the top labelled "\(\displaystyle CO_{2} \) in the atmosphere".
    A downward arrow from that box to a box "Plants", labelled Photosynthesis.
    An arrow from "Plants" to a box "Animals", labelled "feeding".
    Two upward arrows back to the \(\displaystyle CO_{2} \) box — one from "Plants" and one from "Animals" — each labelled Respiration.
    Arrows from "Plants" and "Animals" down to a box "Dead plants and animals", then an upward arrow from it to the \(\displaystyle CO_{2} \) box labelled Decomposition.
    A branch from "Dead plants and animals" down to a box "Fossil fuels — coal, oil, gas", labelled "burial over millions of years", and an upward arrow from that box to the \(\displaystyle CO_{2} \) box labelled Combustion of fossil fuels.
    To one side, a box "\(\displaystyle CO_{2} \) dissolved in ocean water — plankton and shells" joined to the \(\displaystyle CO_{2} \) box by a double-headed arrow labelled Absorption / release.
    Mark the respiration and decomposition arrows as the fast cycle (days to years) and the fossil-fuel branch as the slow cycle (millions of years).

  10. Exercise 13.10

    Why is an excess of CO2\displaystyle CO_{2} in the atmosphere considered undesirable even though it is required by plants?

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    \(\displaystyle CO_{2} \) is genuinely needed — plants use it for photosynthesis and it keeps the Earth warm enough to sustain life — but the balance is critical, and an excess breaks it.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-10
    Enhanced greenhouse effect: extra \(\displaystyle CO_{2} \) traps more of the outgoing infrared radiated by the Earth's surface, causing global warming.
    Melting ice and rising seas: glaciers and Arctic sea ice melt, sea level rises, and low-lying coastal cities are threatened.
    Extreme weather: in India this may mean more intense monsoons (warmer air holds more moisture) and changed rainfall patterns that threaten agriculture.
    Ocean acidification: the ocean absorbs the excess \(\displaystyle CO_{2} \) and sea water becomes more acidic, threatening plankton and coral reefs and disrupting marine ecosystems.
    Plants cannot simply mop it up: photosynthesis is limited by water, light, nutrients and available forest area, and deforestation is shrinking that sink even as emissions grow.
    Scale of the problem: \(\displaystyle CO_{2} \) has risen by about \(\displaystyle 35\% \) since $\displaystyle 1960$, from \(\displaystyle 315 \) ppm to \(\displaystyle 420 \) ppm — a rise unprecedented in the history of human civilisation.