Chemistry NCERT Exemplar Solutions Class 11th Chapter Fourteen

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Options (ii), (iii) and (iv)  are correct since rainwater has a pH of 5.6 due to the presence of H+ ions formed by the reaction of rainwater with carbon dioxide present in the atmosphere. The reactions can be shown as-

H₂O (l)+CO₂ (g)→H₂CO₃ (aq)

H₂CO₃ (aq)→H+ (aq)+HCO₃ (aq)

If the pH of the rainwater falls below 5.6 then it is known as acid rain. SO₂ and NO₂ after oxidation and reaction with water are major contributors to the acid rain.

2SO₂ (g)+O₂ (g)+2H₂O (l)→2H₂SO₄ (aq)

4NO₂ (g)+O₂ (g)+2H₂O (l)→4HNO₃⁻ (aq)

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Options (i) and (ii) are correct since fertilizers contain phosphates as additives. The addition of phosphates in water enhances the growth of the algae. Such profuse growth of algae covers the water surface and reduces the oxygen concentration in water.

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Options (iii) and (iv) are correct since clean water has BOD less than 5 ppm and polluted water has BOD value higher than 5 ppm. So, 10 BOD means highly polluted water. This process in which nutrient enriched water bodies support a dense plant population, which kills animal life by depriving it of oxygen and results in subsequent loss of biodiversity is termed as Eutrophication.

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Option (iv)

Using plastic cans for neatly storing substances is correct since use of plastic cans for neatly storing substances will not come under green chemistry because plastic is non-biodegradable and causes pollution.

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Option (iii)

Ozone hole is thinning of the ozone layer of the stratosphere at some places is correct since it was found that a unique set of conditions were responsible for the ozone hole. In summer, nitrogen dioxide and methane react with chlorine monoxide and chlorine atoms forming chlorine sinks, preventing much ozone depletion, whereas in winter, special types of clouds called polar stratospheric clouds are formed over Antarctica. 

Following reactions occur in the stratosphere.

CF₂Cl₂→CF₂Cl + Cl?

Cl? +O₃→ClO? + O₂

ClO? +  [ O ] →Cl? + O₂

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Option (i) the reaction is endothermic and requires very high temperature is correct since dinitrogen and dioxygen are the main constituents of air. These gases do not react with each other at a normal temperature. The dissociation energy of N₂is very high due to the presence of triple bond and it is very stable. 

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Option (ii) Troposphere is correct since the lowest region of atmosphere in which the human beings along with other organisms live is called troposphere. It extends up to the height of  approximately 10 km from sea level. 

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Option (ii) i.e., It has low concentration of oxidizing agents is the answer since the common components of photochemical smog are ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate (PAN). Photochemical smog causes serious health problems and both ozone and PAN act as powerful eye irritants. Photochemical smog contains high concentrations of oxidants. 

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Option (iii)

i.e., Decrease in the amount of dissolved oxygen in water is the answer since organic waste is oxidized by microorganisms in the presence of dissolved oxygen. Hence, oxygen decreases in water as a result it is harmful for aquatic life.