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NCERT Exemplar · Class 9 Science Natural Resources

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Short Answer Questions 31–40 (part 4 of 6)

  1. Exercise 31

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

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    NCERT’s answer
    Water is capable of dissolving a large number of substances. As water flows over the rocks containing soluble minerals, some of them get dissolved in the water. Thus, rivers carry many nutrients from land to the sea.
    Rivers dissolve mineral salts out of the rocks and soil they flow over, carrying them in solution to the sea. \[\text{rock/soil weathering} \;\Rightarrow\; \text{minerals dissolve in river water} \;\Rightarrow\; \text{carried to sea} \] At the sea's surface, evaporation removes only the water; dissolved salts are non-volatile and stay behind, so they accumulate. \[\text{evaporation removes H}_2\text{O only} \;\Rightarrow\; \text{salts accumulate in sea water} \] Answer: Flowing river water dissolves minerals from rocks and soil and carries them to the sea, where evaporation removes only water, leaving the salts behind.

  2. Exercise 32

    How can we prevent the loss of top soil?

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    NCERT’s answer
    Loss of top soil can be prevented by (i) increasing the vegetational cover (ii) checking the falling of trees (iii) by preventing excessive grazing by animals
    Top soil is stripped by flowing water and wind, so prevention means slowing both. Afforestation and preventing deforestation bind soil with roots and litter; contour ploughing and terracing break a slope's run-off into steps; wind-breaks cut wind speed near the ground; mulching and controlled grazing keep the surface covered. Answer: Afforestation and checking deforestation, contour ploughing/terracing, wind-breaks, mulching, and controlled grazing prevent loss of top soil.

  3. Exercise 33

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

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    NCERT’s answer
    Addition of undesirable chemicals like pesticides, fertilizers, industrial waste and domestic wastes not only kill the organisms, they also cause diseases to the aquatic organisms. Besides, the requirement of oxygen by aquatic organisms is also increased. There is a reduction in the dissolved oxygen in water which adversely affects the aquatic organisms.
    Sewage, fertiliser run-off and industrial effluents load water with excess organic matter and nutrients. \[\text{organic waste/nutrients}\uparrow \;\Rightarrow\; \text{microbial decomposition}\uparrow \;\Rightarrow\; \text{dissolved O}_2\downarrow \] \[\text{dissolved O}_2\downarrow \;\Rightarrow\; \text{aquatic organisms suffocate, sicken or die} \] Excess nutrients also trigger algal blooms that block sunlight, stopping photosynthesis in submerged plants; toxic effluents poison organisms directly. Answer: Pollution lowers dissolved oxygen and blocks light, so aquatic organisms suffocate, fall ill or die, and toxins poison them directly.

  4. Exercise 34

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

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    NCERT’s answer
    Hint— Air near water bodies is cooled due to evaporation of water.
    Water has a far higher specific heat capacity than land, and its exposed surface keeps evaporating. \[Q = mc\Delta T,\qquad c_{\text{water}}\approx 4.2\ \text{J g}^{-1\,\circ}\text{C}^{-1},\quad c_{\text{land}}\approx 0.8\ \text{J g}^{-1\,\circ}\text{C}^{-1} \] For the same heat \(\displaystyle Q\) absorbed from the sun, \(\displaystyle \Delta T_{\text{water}} \ll \Delta T_{\text{land}}\), so the lake stays far cooler than the surrounding land. \[\text{evaporation} \;\Rightarrow\; \text{latent heat drawn from the air above the lake} \;\Rightarrow\; T_{\text{air}}\downarrow \] Answer: Evaporation from the lake draws latent heat from the air just above it, cooling that air; felt near the lake as relief from the heat.

  5. Exercise 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.

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    NCERT’s answer
    Hint— Air above the land gets heated quickly during day and starts rising. This creates a region of low pressure as a result air over sea rushes into this area of low pressure. This movment of air from one region to the other creates winds. During night, as water cools down slowly, the air above water is warmer than the air on land. So air moves from land to sea creating winds.
    Land and sea gain and lose heat at very different rates: land's specific heat capacity is far lower than water's. Day: \[T_{\text{land}}\uparrow\ (\text{faster than } T_{\text{sea}}) \;\Rightarrow\; \text{air over land rises} \;\Rightarrow\; P_{\text{land}}\downarrow \;\Rightarrow\; \text{sea breeze: sea}\to\text{land} \]Night: \[T_{\text{land}}\downarrow\ (\text{faster than } T_{\text{sea}}) \;\Rightarrow\; \text{air over sea rises} \;\Rightarrow\; P_{\text{sea}}\downarrow \;\Rightarrow\; \text{land breeze: land}\to\text{sea} \] Answer: Land heats and cools faster than the sea; low pressure forms over land by day (sea breeze) and over the sea by night (land breeze).

  6. Exercise 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.

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    NCERT’s answer
    Hint— Lichens and Mosses (a) and (b). Lichens and mosses release substances which break down the stones resulting in the formation of soil.
    Lichen (a) and moss (b) colonise bare rock; mango (c) and cactus (d) need soil already in place. \[\text{lichen: weak acids} \;\Rightarrow\; \text{rock corroded, pitted} \;\Rightarrow\; \text{thin mineral + humus layer} \] Moss roots in this thin layer and also releases substances that break the rock down further; its remains add more humus, thickening the soil for larger plants. Answer: (a) lichen and (b) moss -- both release substances that corrode bare rock, and moss's remains add humus that thickens the soil.

  7. Exercise 37

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

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    NCERT’s answer
    Abiotic factors making soil— sun, water, wind Biotic factors— lichens, mosses and trees
    Abiotic: sun (heating/cooling), water (rain, flow, freezing), wind. Biotic: lichens and mosses, plant roots, decomposers (bacteria, fungi), burrowing animals (earthworms). Answer: Abiotic -- sun, water, wind. Biotic -- lichens/mosses, plant roots, decomposers, earthworms.

  8. Exercise 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.

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    NCERT’s answer
    Hint— Through photosynthesis and absorption from soil
    Carbon, hydrogen, oxygen enter via photosynthesis, which fixes atmospheric \(\displaystyle \text{CO}_2\) and water into carbohydrate; animals get them by eating plants or other animals. \[6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow[\text{chlorophyll}]{\text{sunlight}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \] Nitrogen enters only as fixed compounds: \[\text{N}_2 \xrightarrow{\text{N}_2\text{-fixing bacteria}} \text{NH}_3 \xrightarrow{\text{nitrifying bacteria}} \text{NO}_3^- \] roots absorb the nitrate and build proteins. Sulphur and phosphorus weather out of rocks as \(\displaystyle \text{SO}_4^{2-}\), \(\displaystyle \text{PO}_4^{3-}\), and roots absorb these too. Answer: C, H and O enter via photosynthesis; N, S and P are absorbed from soil as nitrate, sulphate and phosphate.

  9. Exercise 39

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

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    NCERT’s answer
    Hint— Cycling of these gases maintains consistency.
    Natural cycles return nearly what they remove, keeping the proportions of these gases almost steady. \[\text{CO}_2 \xrightarrow{\text{photosynthesis}} \text{organic carbon} \xrightarrow{\text{respiration, combustion}} \text{CO}_2 \] \[\text{N}_2 \xrightarrow{\text{fixation}} \text{nitrates} \xrightarrow{\text{denitrification}} \text{N}_2 \] Photosynthesis also releases \(\displaystyle \text{O}_2\) at close to the rate respiration and combustion use it up. Answer: The oxygen, carbon and nitrogen cycles return each gas to the air at nearly the rate it is removed, so the proportions stay almost constant.

  10. Exercise 40

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

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    NCERT’s answer
    Hint— Absence of atmosphere on the moon.
    The moon has no atmosphere. \[\text{no air} \;\Rightarrow\; \text{no greenhouse trapping, no convection, no insulation} \] \[\text{lit side: direct sunlight} \;\Rightarrow\; T \to 110\,^\circ\text{C} \] \[\text{dark side: heat radiates freely to space} \;\Rightarrow\; T \to -190\,^\circ\text{C} \] Earth's atmosphere absorbs and redistributes this heat, so its day-night swing stays mild by comparison.Answer: With no atmosphere to trap or spread heat, the moon's lit face overheats and its dark face loses heat unchecked, unlike earth.