Chapter 14: Environmental Chemistry

Ans: 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.

Ans: 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.

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

\[{{\text{H}}_{\text{2}}}\text{O(l)+C}{{\text{O}}_{\text{2}}}\text{(g)}\to {{\text{H}}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}\text{(aq)}\]

\[{{\text{H}}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}\text{(aq)}\to {{\text{H}}^{\text{+}}}\text{(aq)+HC}{{\text{O}}_{\text{3}}}\text{(aq)}\]

If the pH of the rainwater falls below 5.6 then it is known as acid rain. \[\text{S}{{\text{O}}_{\text{2}}}\]and \[\text{N}{{\text{O}}_{\text{2}}}\] after oxidation and reaction with water are major contributors to the acid rain.

\[\text{2S}{{\text{O}}_{\text{2}}}\text{(g)+}{{\text{O}}_{\text{2}}}\text{(g)}+\text{2}{{\text{H}}_{\text{2}}}\text{O(l)}\to \text{2}{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\text{(aq)}\]

\[\text{4N}{{\text{O}}_{\text{2}}}\text{(g)+}{{\text{O}}_{\text{2}}}\text{(g)+2}{{\text{H}}_{\text{2}}}\text{O(l)}\to \text{4HN}{{\text{O}}_{\text{3}}}^{-}\text{(aq)}\]

Ans: Options (i) and (ii) are correct since 75% of the solar energy reaching the earth is being absorbed by the earth’s surface and this increases its temperature. The rest of the heat radiates back to the atmosphere. The average global temperature will increase to a level, and this leads to melting of polar ice caps and flooding of low-lying areas all over the earth. Increase in the global temperature increases the incidence of inectious diseases like malaria, dengue,  yellow fever etc.