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

Step-by-step working for every question of End-of-chapter exercises in NCERT's Class 9 Science book, chapter “Earth as a System: Energy, Matter, and Life”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. Exercises Q1

   The role of biogeochemical cycles in an ecosystem is best described as: (i) giving food directly to all organisms (ii) recycling essential nutrients between living and non-living parts (iii) making new elements for living things (iv) removing pollutants and toxins from organisms

   1. Biogeochemical cycles move matter between living and non-living parts of the Earth.
   2. This process recycles essential nutrients like carbon, nitrogen, and oxygen to keep them available for life.
   3. The cycles do not create new elements or directly provide food to all organisms.

   Answer: (ii) To recycle essential nutrients between biotic and abiotic components.

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2. Exercises Q2

   What mainly warms the Earth? (i) carbon dioxide absorbs sunlight at once and releases it as heat (ii) tiny particles in the air absorb sunlight and heat the Earth directly (iii) the surface absorbs sunlight, which is re-radiated and trapped by greenhouse gases (iv) only sunlight reflected by clouds heats it

   1. (i) The Earth's surface absorbs incoming solar radiation from the Sun.
   2. (ii) The surface then re-radiates this energy as heat in the infrared region.
   3. (iii) Greenhouse gases like carbon dioxide and water vapour trap this outgoing heat.
   4. (iv) This trapping process keeps the Earth warm enough to support life.

   Answer: (iii) The Earth's surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.

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3. Exercises Q3

   How does climate change affect the water cycle? Give examples.

   1. A warmer atmosphere holds more moisture, which leads to heavier rainfall in certain regions.
   2. An example of this is the intensification of monsoons in some areas.
   3. Conversely, other areas may experience droughts due to changes in the water cycle.
   4. Melting glaciers contribute additional water to the system, affecting the cryosphere.
   5. Intense rainfall causes more runoff that erodes soil, while less infiltration reduces groundwater recharge.
   6. This reduction in groundwater recharge is particularly noticeable during dry months.

   Answer: Climate change affects the water cycle by causing heavier rains like intensified monsoons, droughts elsewhere, and melting glaciers, while intense rainfall leads to soil erosion and reduced groundwater recharge.

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4. Exercises Q4

   How does albedo affect the Earth's surface temperature and climate?

   1. Albedo is the fraction of solar radiation that a surface reflects, with the term derived from the Latin word for whiteness.
   2. Surfaces with high albedo, such as snow and ice, reflect a large portion of incoming sunlight and therefore remain comparatively cool.
   3. Surfaces with low albedo absorb more solar radiation because they reflect less, causing them to heat up more quickly.
   4. The uneven heating of the Earth's surface, influenced by factors like albedo and latitude, drives global winds and ocean currents which shape climate.

   Answer: High albedo surfaces reflect more sunlight and stay cool, while low albedo surfaces absorb more heat and warm up, contributing to uneven heating that drives global climate patterns.

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5. Exercises Q5

   How do mountain and valley breezes form? Would the breezes from a grass-covered mountain and a bare rocky mountain differ in temperature? How?

   1. During the day, the sun heats the mountain slopes and the surrounding air.
   2. The air near the mountain surface becomes warm, expands, and rises up the slopes.
   3. This rising air creates a low-pressure zone that draws cooler air from the valley up the mountain, forming an upslope breeze.
   4. At night, the mountain slopes cool down faster than the valley floor.
   5. The air near the mountain surface becomes cold, dense, and sinks down the slopes.
   6. This sinking air pushes the warmer air in the valley up the slopes, forming a downslope breeze.
   7. Yes, the breezes would differ. A bare rocky slope heats up strongly in the day, so the air rising from it is warmer and drier.
   8. A grass-covered slope is shaded by the plants and loses water from them by transpiration, which keeps the surface cooler.
   9. So by day the up-slope breeze from the grassy mountain is cooler and more moist than the one from the bare rock.
   10. At night the rock gives off the heat it stored, so the air sliding down a bare slope can be a little less cold than the air from a grassy slope.

   Answer: Valley breezes blow up the slopes by day as heated air rises, and mountain breezes blow down at night as cooled air sinks; a bare rocky mountain gives warmer, drier breezes by day than a grass-covered one, whose plants keep it cooler and moist.

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6. Exercises Q6

   Which layer of the atmosphere is mainly responsible for winds, storms and rain, and why does weather happen there?

   1. The troposphere is the atmospheric layer where winds, storms, and rain mainly occur.
   2. In this layer, temperature decreases as height increases, which causes warm air to rise.
   3. The rising warm air drives the formation of winds and storms within the troposphere.
   4. The stratosphere above is stable due to temperature increasing with height, which keeps weather confined to the lower layer.

   Answer: The troposphere is responsible for weather phenomena because temperature decreases with height there, causing warm air to rise and drive winds and storms.

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7. Exercises Q7

   Explain the steps of the nitrogen cycle. How would life be affected if nitrogen were not cycled?

   1. Nitrogen fixation converts atmospheric nitrogen gas into soluble compounds by bacteria or lightning.
   2. Assimilation occurs when plants absorb these compounds and animals obtain nitrogen by eating plants or other animals.
   3. Ammonification happens when decomposers break down dead organisms and waste, returning nitrogen as ammonia to the soil.
   4. Nitrification transforms ammonia into nitrites and then nitrates, which plants can absorb.
   5. Denitrification converts nitrates back into nitrogen gas, completing the cycle and maintaining balance.
   6. Without the nitrogen cycle, living organisms could not synthesize essential proteins and nucleic acids, making life impossible.

   Answer: The nitrogen cycle involves fixation, assimilation, ammonification, nitrification, and denitrification; without it, life would cease because proteins and nucleic acids could not be formed.

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8. Exercises Q8

   How does deforestation affect the oxygen and carbon cycles? What are its other effects?

   1. Deforestation removes trees that perform photosynthesis, which reduces the production of oxygen in the atmosphere.
      why?

      Oxidation is gain of oxygen (or loss of hydrogen); reduction is loss of oxygen (or gain of hydrogen). In a redox reaction one substance is oxidised while another is reduced.

   2. Fewer trees mean less carbon dioxide is absorbed from the air, causing atmospheric CO2 levels to rise.
   3. The loss of forests saturates natural carbon sinks, preventing them from effectively storing excess carbon.
   4. Other consequences include increased greenhouse warming, melting glaciers, and rising sea levels.
   5. Deforestation also leads to more extreme weather conditions and threats to agriculture due to changing rainfall patterns.

   Answer: Deforestation disrupts the oxygen cycle by reducing photosynthesis and the carbon cycle by increasing atmospheric CO2; other effects include global warming, extreme weather, and agricultural threats.

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9. Exercises Q9

   Explain, with a diagram, how carbon taken in by plants as CO₂ goes back to the atmosphere.

   1. (i) Plants absorb carbon dioxide from the atmosphere and use sunlight to convert it into glucose through photosynthesis.
   2. (ii) Animals eat the plants, incorporating the carbon into their bodies, while plants also release some CO2 back into the air through respiration.
   3. (iii) When plants and animals die, decomposers break down their remains, releasing carbon dioxide back into the atmosphere.
   4. (iv) The burning of fossil fuels, which are formed from ancient dead organisms, also releases stored carbon as CO2 into the air.

   Answer: Carbon returns to the atmosphere via plant and animal respiration, decomposition of dead organisms, and the combustion of fossil fuels.

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10. Exercises Q10

    Why is too much CO₂ in the atmosphere harmful even though plants need it?

    1. Plants use carbon dioxide for photosynthesis to make food, which is why it is necessary for life.
    2. However, carbon dioxide is also a greenhouse gas that traps heat re-radiated by the Earth's surface.
    3. Excess carbon dioxide from human activities enhances this trapping effect, leading to global warming.
    4. This extra heat can make the Earth too hot for life to survive, disrupting natural ecosystems.
    5. Additionally, too much carbon dioxide dissolves in oceans, making water acidic and harming marine life.

    Answer: Excess CO2 is harmful because it intensifies the greenhouse effect causing global warming and ocean acidification, even though plants need it for photosynthesis.

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11. Exercises Q11

    How does the Earth's surface lose heat, and why does this matter?

    1. The Earth's surface, warmed by sunlight, gives off heat as infrared radiation.
    2. It also passes heat to the air touching it; the warm air rises (convection), and water evaporating from the surface carries heat away too.
    3. Greenhouse gases such as carbon dioxide and water vapour absorb part of the outgoing infrared heat and send some back, so not all of it escapes to space.
    4. This balance between heat gained and heat lost keeps the Earth's average temperature steady and suitable for life; without the greenhouse effect it would be too cold, and with too much it gets too hot.

    Answer: The surface loses heat by infrared radiation, by warming the air (convection) and by evaporation; greenhouse gases hold back part of the radiated heat, and this balance keeps the Earth's temperature suitable for life.

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12. Exercises Q12

    If the Earth were a flat disc instead of a sphere, how would sunlight and temperature patterns differ?

    1. i) On a flat disc, the Sun's rays would strike the surface at similar angles everywhere, unlike the spherical Earth where rays hit different latitudes at varying angles.
    2. ii) Solar radiation would not be concentrated over smaller areas at the equator or spread over larger areas at the poles, leading to more uniform heating across the surface.
    3. iii) The significant temperature differences between equatorial and polar regions would disappear, eliminating the uneven heating that currently drives global winds and ocean currents.

    Answer: Sunlight would strike the flat Earth at similar angles everywhere, causing uniform heating and eliminating the temperature differences between equator and poles that drive winds and currents.

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13. Exercises Q13

    If the air temperature rose, how would the cryosphere, hydrosphere and biosphere be affected?

    1. Cryosphere: Glaciers, snow, and ice caps would melt faster, reducing the volume of frozen water on Earth.
    2. Hydrosphere: Meltwater from the cryosphere would flow into rivers and oceans, increasing water levels and altering the water cycle.
    3. Biosphere: Changes in temperature and water availability would disrupt habitats, affecting the survival and distribution of living organisms.

    Answer: A rise in air temperature would cause the cryosphere to melt, increase water levels in the hydrosphere, and disrupt habitats in the biosphere.

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14. Exercises Q14

    How does the atmosphere keep the Earth's temperature suitable for life?

    1. The ozone layer in the upper atmosphere absorbs most of the harmful short-wavelength ultraviolet radiation, protecting life and adding some heat to the air.
    2. Visible sunlight reaches the surface to power photosynthesis and also warms the land and water bodies.
    3. Infrared radiation from the Sun heats the ground and oceans, causing them to release heat energy back into the air.
    4. Greenhouse gases like carbon dioxide, methane, and water vapour trap a portion of this outgoing heat, keeping the planet warm enough for life.
    5. The atmosphere also filters out high-energy gamma rays and X-rays, preventing them from reaching the surface and causing damage.

    Answer: The atmosphere maintains a suitable temperature by absorbing harmful UV rays, allowing visible light to warm the surface, and trapping outgoing infrared heat via greenhouse gases.

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15. Exercises Q15

    Describe how the Earth's spheres are linked, with an example of how they stay in balance.

    1. Natural processes like solar heating, air and water movement, and nutrient cycles connect the Earth's spheres in a delicate balance.
    2. Uneven solar heating drives global winds and ocean currents, linking the atmosphere, hydrosphere, and geosphere.
    3. An example of this balance is how snow from the cryosphere melts and becomes part of a lake in the hydrosphere.

    Answer: The Earth's spheres are linked by energy and matter flows; for example, cryospheric snow melts into the hydrosphere, maintaining balance.

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