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NCERT Exemplar · Class 9 Science Natural Resources

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Short Answer Questions 41–50 (part 5 of 6)

  1. Exercise 41

    Why do people love to fly kites near the seashore ?

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    Hint— Due to wind created during the day time
    Land heats and cools faster than the sea. \[\text{land } T\uparrow \text{(day)} \;\Rightarrow\; \text{air over land rises} \;\Rightarrow\; \text{low pressure inland} \] \[\text{sea stays cooler} \;\Rightarrow\; \text{high pressure over water} \] Air flows from sea to land to fill the gap -- a steady sea breeze.Answer: Differential heating of land and sea sets up a steady onshore breeze, and that steady wind is what keeps a kite up.

  2. Exercise 42

    Why does Mathura refinery pose problems to the Taj Mahal?

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    Mathura refinery releases toxic gases (like oxides of sulphur) which causes acid rain and hence corrosion of the marbles of Taj Mahal.
    The refinery's stack gases carry \(\displaystyle \text{SO}_2\) and \(\displaystyle \text{NO}_x\), oxidised in moist air to acids. \[\text{SO}_2 + \text{H}_2\text{O} + \tfrac{1}{2}\text{O}_2 \rightarrow \text{H}_2\text{SO}_4 \] Acid rain then attacks the Taj's marble: \[\text{CaCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2 \] The soluble product washes off, pitting and yellowing the surface -- called marble cancer. Answer: Refinery-derived \(\displaystyle \text{SO}_2\)/\(\displaystyle \text{NO}_x\) form acid rain that reacts with the marble's calcium carbonate, corroding and yellowing the Taj Mahal.

  3. Exercise 43

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

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    Hint—It is a bio-indicator and sensitive to \(\displaystyle SO_{2}\) pollution from automobiles. Delhi has maximum number of automobiles, hence has a highly polluted environment.
    Lichens are bio-indicators, highly sensitive to \(\displaystyle \text{SO}_2\) in the air. \[\text{air SO}_2 \uparrow \;\Rightarrow\; \text{lichen survival} \downarrow \] Delhi's vehicular and industrial exhaust keeps \(\displaystyle \text{SO}_2\) levels high; Manali and Darjeeling, away from such industry, keep their air clean. Answer: Lichens cannot tolerate \(\displaystyle \text{SO}_2\) pollution; Delhi's polluted air kills them, while Manali's and Darjeeling's clean hill air lets them thrive.

  4. Exercise 44

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

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    Hint—Marine water is not useful for human and plant life directly. Uneven distribution of limited fresh water resources need conservation to cater to the demands.
    Ocean water is saline -- unusable for drinking, irrigation or most industry without costly desalination. \[\text{saline (oceans)} \approx 97\%, \qquad \text{fresh water} \approx 3\% \text{ (mostly locked in ice)} \] The small usable share left -- rivers, lakes, groundwater -- is unevenly distributed, over-drawn and increasingly polluted. Answer: Oceans are saline and cannot be used directly; the tiny, unevenly spread share of fresh water we can actually use must still be conserved.

  5. Exercise 45

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

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    Hint—(i) Thermal pollution (ii) Addition of poisonous (mercury) compounds in water (iii) Due to blockage of gills with any pollutant.
    Several kinds of pollution can each cause mass fish mortality. \[\text{hot effluent enters pond} \Rightarrow T_{\text{water}}\uparrow \Rightarrow \text{dissolved O}_2\downarrow \Rightarrow \text{fish suffocate} \quad (\text{thermal pollution}) \] Poisonous compounds such as mercury entering the water can poison the fish directly, and suspended pollutant particles can clog the gills, blocking respiration. Excess nutrients cause eutrophication: \[\text{excess nutrients} \Rightarrow \text{algal bloom} \Rightarrow \text{decomposition} \Rightarrow \text{dissolved O}_2\downarrow \Rightarrow \text{fish suffocate} \] Answer: Thermal pollution, poisonous compounds (e.g. mercury), gills clogged by suspended pollutants, or eutrophication -- each can lower dissolved oxygen or poison the fish directly.

  6. Exercise 46

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

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    Lichens release chemical substances to break the rocks into smaller particles and hence make soil.
    Lichens secrete organic acids that slowly corrode the bare rock surface. \[\text{rock} \xrightarrow{\text{acid secretion, weathering}} \text{fine mineral particles} \] Their own dead remains mix into these particles as humus, building the first thin soil layer. Answer: Lichens chemically weather rock into fine particles and add humus from their own decay, forming the first soil that mosses and later plants can root in.

  7. Exercise 47

    “Soil is formed by water.” If you agree to this statement then give reasons

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    Water helps in formation of soil in following ways (i) Water causes ‘wear off’ of rocks over a long period of time. (ii) It also causes the rocks to rub against other rocks creating small particles which are taken away downstream and deposited as soil. (iii) Water expands on freezing in crevices of rocks and cracks rocks into smaller pieces.
    Agreed. Flowing water abrades rock into ever finer grains: \[\text{rock} \xrightarrow{\text{running water, abrasion}} \text{fine rock particles} \] Water trapped in cracks freezes and expands, widening and splitting the rock: \[\text{H}_2\text{O}(l) \xrightarrow{\text{freezes}} \text{H}_2\text{O}(s) \Rightarrow \text{crack widens} \Rightarrow \text{rock splits} \] It also dissolves minerals and carries the loosened particles downhill, where they settle and mix with humus. Answer: Yes — water weathers rock (abrasion, freeze-thaw cracking, dissolving minerals) and transports the particles that settle into soil.

  8. Exercise 48

    Fertile soil has lots of humus. Why?

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    Fertile soils are rich in organisms that decomposes dead organic matter forming humus. Humus gives minerals, absorbs water and makes soil porous.
    Humus is organic matter — dead plants, animal remains and excreta — broken down by decomposer microbes: \[\text{dead organic matter} \xrightarrow{\text{decomposers}} \text{humus} \] It supplies nitrogen and other minerals plants need, binds particles to hold water and air pockets, and feeds the earthworms and microbes that keep turning and aerating the soil. Answer: Humus supplies plant nutrients and improves the soil's water-holding, aeration and texture, so humus-rich soil is fertile.

  9. Exercise 49

    Why step farming is common in hills?

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    Hint— This is practiced to check soil erosion through water currents on the slopes.
    On a steep slope, rainwater runs down fast and strips away the fertile top layer: \[\text{steep slope} \Rightarrow v_{\text{water}}\uparrow \Rightarrow \text{soil erosion}\uparrow \] Cutting the slope into flat steps breaks the fall into short, near-level stretches, so water slows and soaks in instead of racing off: \[\text{stepped slope} \Rightarrow v_{\text{water}}\downarrow \Rightarrow \text{erosion}\downarrow,\ \text{infiltration}\uparrow \]Answer: Terracing breaks a hill's slope into flat steps that slow runoff, so rainwater soaks in instead of eroding the topsoil.

  10. Exercise 50

    Why are root nodules useful for the plants?

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    In root nodules nitrogen fixing Rhizobium bacteria are present which increases the soil fertility.
    Root nodules of leguminous plants house nitrogen-fixing bacteria (Rhizobium): \[\text{N}_2 \xrightarrow{\text{Rhizobium in nodules}} \text{NH}_3 \] Plants cannot use atmospheric nitrogen gas directly, so this fixed nitrogen is absorbed and built into amino acids and proteins; in return the bacteria get shelter and food from the plant's roots.Answer: Nodule bacteria fix atmospheric nitrogen into a form the plant can use for growth, in exchange for shelter and food.