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NCERT Exemplar · Class 9 Science Natural Resources

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Long Answer Questions 51–56 (part 6 of 6)

  1. Exercise 51

    How do fossil fuels cause air pollution?

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    NCERT’s answer
    The fossil fuels like coal and petroleum contain small amounts of nitrogen and sulphur. When fossil fuels are burnt, it produces oxides of nitrogen and sulphur. These gases cause inhalation problems and in presence of rain forms acid rain. Burning of fossil fuels also increase the amount of suspended particles in air that reduce the visibility.
    Fossil fuels (coal, petroleum, natural gas) burn in air and release pollutants straight into the atmosphere. \[C + O_2 \rightarrow CO_2 \] \[2C + O_2 \rightarrow 2CO \] \[S + O_2 \rightarrow SO_2 \] \[N_2 + O_2 \xrightarrow{\text{combustion}} 2NO \] \(\displaystyle CO_2\) traps outgoing infrared radiation and warms the atmosphere; \(\displaystyle CO\) binds haemoglobin and lowers blood's oxygen-carrying capacity; \(\displaystyle SO_2\) and \(\displaystyle NO\) dissolve in atmospheric moisture to form acid rain that damages plants, buildings and water bodies. Unburnt carbon escapes as soot and suspended particulate matter, causing smog and irritating the respiratory tract. Answer: Burning fossil fuels adds \(\displaystyle CO_2\), \(\displaystyle CO\), \(\displaystyle SO_2\), \(\displaystyle NO_x\) and soot to air, causing global warming, acid rain and respiratory illness.

  2. Exercise 52

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

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    Water pollution can be caused by addition of (i) undesirable substances like fertilizers and pesticides or any poisonous substances. (ii) sewage directly entring a water body. (iii) hot water from the power plant that increases the temperature and reduces the dissolved oxygen in water thus killing the aquatic organisms. (iv) industrial effluents or radioactive substances in water body. We can take following measures to check water pollution (i) The sewer lines should not be directly connected to the water body. (ii) We should not throw our garbages or domestic waste in the water body. (iii) Prevent dumping of toxic compounds in the water bodies. (iv) Washing of clothes should be avoided near water bodies as it adds lot of detergents to it. (v) Plant trees near the banks of the river to check soil erosion otherwise erosion leads to siltation of water body.
    Untreated domestic sewage and detergents, industrial effluents carrying chemicals and heavy metals, agricultural runoff of fertilisers and pesticides, oil spills, and hot water discharged by power plants and factories all enter rivers, lakes and groundwater. Fertiliser runoff triggers eutrophication and algal blooms that deplete dissolved oxygen, killing aquatic life. I can help by not dumping waste, plastic or chemicals into water bodies, using fewer detergents and pesticides at home, getting sewage treated before release, and not washing clothes or vehicles at a river or pond. Answer: Sewage, industrial and agricultural effluents, oil spills and thermal discharge pollute water; reducing detergent/pesticide use, treating sewage and never dumping waste in water bodies helps control it.

  3. Exercise 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?

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    Infra-red radiations in sunlight pass through the glass and heat the interior of the car. The radiation emitted by upholstery and other inner parts of the car cannot pass out of the glass, so the heat trapped inside raises the temperature of the interior. This is because glass is transparent to infra- red radiation from the sun having smaller wavelength than that emitted by the interior of the car which are of longer wavelength to which the glass is opaque.
    Sunlight is short-wavelength visible radiation, to which glass is transparent; the heat radiated by warmed objects is long-wavelength infrared, to which glass is largely opaque. \[\text{sunlight (short }\lambda\text{)} \rightarrow \text{enters through glass} \rightarrow \text{absorbed by dashboard/seats} \rightarrow \text{re-emitted as infrared (long }\lambda\text{)} \rightarrow \text{glass blocks IR} \Rightarrow \text{heat trapped} \Rightarrow T_{\text{inside}}\uparrow \] Answer: Visible sunlight passes in through the glass and warms the interior, which re-radiates the heat as infrared that the glass will not let escape — trapped heat raises the car's inside temperature, the same mechanism as the atmospheric greenhouse effect.

  4. Exercise 54

    Justify “Dust is a pollutant” ?

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    NCERT’s answer
    Dust remains present in air as suspended particles can cause allergy and other respiratory diseases. It also affects plant growth, by covering stomata on leaf surface. It acts as the carrier of toxic compounds like heavy metals.
    Dust particles stay suspended in air, cutting visibility as haze and smog. \[\text{dust inhaled} \rightarrow \text{lodges in air passages/alveoli} \Rightarrow \text{irritation, coughing, asthma} \] Settling on leaves, dust coats stomata and reduces the light reaching chlorophyll, lowering photosynthesis; settling on water and soil it can carry adsorbed toxic chemicals or heavy metals that enter food chains. Answer: Dust reduces visibility, damages the respiratory tract, blocks stomata to cut photosynthesis, and carries toxins and heavy metals into food chains.

  5. Exercise 55

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

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    NCERT’s answer
    Hint— The rocks are heated by the sun; they contract during night but not at same rate —resulting in cracks in rocks and ultimately to smaller particles.
    The Sun supplies the energy that breaks bare rock into soil. \[\text{Sun's heat} \rightarrow \text{rock heats \& expands by day, cools \& contracts by night} \Rightarrow \text{outer layer expands/contracts more than the core} \Rightarrow \text{cracks} \Rightarrow \text{rock fragments} \] Solar heating of the atmosphere also drives wind, and through evaporation the water cycle; wind and flowing rainwater erode these fragments further. Sunlight lets lichens colonise the rock, whose acids etch it further, while plant roots growing into the cracks widen them and, on decay, add humus. Answer: The Sun's uneven day-night heating cracks rock into fragments, and solar-driven wind, rain and living organisms break these down further and mix in organic matter to form soil.

  6. Exercise 56

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

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    NCERT’s answer
    Hint— Increasing concentration (more than normal) of \(\displaystyle CO_{2}\) is harmful and considered as a pollutant. Higher concentrations of \(\displaystyle CO_{2}\) is one of the causes of green house effect/global warming.
    \(\displaystyle \text{CO}_2\) at its normal \(\displaystyle \approx 0.03\%\) of air is essential: plants use it in photosynthesis and every organism releases it in respiration.\[\text{Fossil-fuel burning + deforestation} \Rightarrow [\text{CO}_2]\uparrow \text{ in atmosphere}\]The extra \(\displaystyle \text{CO}_2\) traps outgoing infrared radiation that would otherwise escape to space:\[[\text{CO}_2]\uparrow \Rightarrow \text{IR trapped}\uparrow \Rightarrow T_{\text{atmosphere}}\uparrow \quad (\text{enhanced greenhouse effect})\]\[T_{\text{atmosphere}}\uparrow \Rightarrow \text{polar ice melts},\ \text{sea level}\uparrow,\ \text{climate disturbed}\]Answer: Only the excess \(\displaystyle \text{CO}_2\) is a pollutant; it traps outgoing infrared radiation and drives global warming, while \(\displaystyle \text{CO}_2\) at its normal level remains essential for plants.