Chapter 14: Environmental Chemistry

Ans: Apart from carbon dioxide gas other greenhouse gases are methane, water vapour, oxides of sulphur, nitrous oxide, CFCs, and ozone. Methane is found to be produced naturally when vegetation is burnt, digested, or rotted in the absence of oxygen. Large amounts of methane are released in paddy fields, coal mines, from rotting garbage dumps and by fossil fuels. Chlorofluorocarbons (CFCs) are man-made industrial chemicals used in air conditioning etc. CFCs are also damaging the ozone layer. Nitrous oxide occurs naturally in the environment but is increasing day by day by human activities.

Ans: 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 reaction 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: The atmosphere is divided into certain levels like troposphere, stratosphere, mesosphere, and thermosphere. The stratosphere consists of a considerable amount of ozone, and this protects us from the harmful ultraviolet  radiations coming from the sun. These radiations cause skin cancer in humans. So, it is important to maintain the ozone shield. The main reason for ozone layer depletion is believed to be the release of chlorofluorocarbon compounds (CFCs), also known as Freon. UV radiation can cause genetic mutation and destroy crops, aquatic plants and animals are also affected by ultraviolet radiations.

Ans: The bacteria is held responsible for degrading biodegradable detergent feed on it and grows rapidly. While growing, they may use up all the oxygen dissolved in the water. The lack of oxygen kills all other forms of aquatic life such as fish and plants. Fertilizers contain phosphates as additives. The addition of phosphates in water further enhances algae growth. Such profuse growth of algae covers the water surface and reduces the oxygen concentration in water.

Ans: In the stratosphere, chlorofluorocarbons get broken down by powerful UV radiations, releasing chlorine free radical. This can be shown as-

\[\text{C}{{\text{F}}_{\text{2}}}\text{C}{{\text{l}}_{\text{2}}}\to \text{C}{{\text{F}}_{\text{2}}}\text{Cl+}\overset{\bullet }{\mathop{\text{Cl}}}\,\]

This chlorine radical then reacts with stratospheric ozone to form chlorine monoxide radicals and molecular oxygen. The reaction involved is-\[\overset{\bullet }{\mathop{\text{Cl}}}\,\text{+}{{\text{O}}_{\text{3}}}\to \text{Cl}\overset{\bullet }{\mathop{\text{O}}}\,\text{+}{{\text{O}}_{\text{2}}}\]

Reaction of chlorine monoxide radical with atomic oxygen produces more chlorine radicals and can be shown as-

\[\text{Cl}\overset{\bullet }{\mathop{\text{O}}}\,+\text{ }\!\![\!\!\text{ O }\!\!]\!\!\text{ }\to \overset{\bullet }{\mathop{\text{Cl}}}\,+{{\text{O}}_{\text{2}}}\]

This shows that chlorine radicals are continuously regenerated and cause the breakdown of ozone. Hence, CFCs are transporting agents for continuously generating chlorine radicals into the stratosphere and damaging the ozone layer.

Ans: This is observed that the improper disposal of wastes is one of the major causes of environmental pollution. Whenever domestic waste is not properly managed, it goes to the sewers and is eaten by cattle. Non-biodegradable wastes like metals, glass etc, choke the sewers or gut of the ruminants also many a times polythene bags are swallowed by cattle resulting in their deaths. Similarly, when industrial waste is not properly managed it leads to the air, water, and soil pollution.

Ans: The domestic wastes and organic compounds such as detergents provide nutrients to enhance the growth of algae and aquatic plants. These are decomposed by bacteria and give unpleasant odour. This process is being termed as eutrophication.

Ans: The pollutants which get decomposed by bacteria like waste of vegetable and fruits, sewage, cow dung etc. are known as biodegradable pollutants. Non-biodegradable pollutants are those which do not get decomposed by bacteria e.g., mercury, polythene, aluminium, DDT etc. 

Ans: The oxygen reaches water through the atmosphere or by the process of photosynthesis carried out by green plants during daytime. But, during night as photosynthesis stops , the plants continue to respire, resulting in the reduction of dissolved oxygen. This dissolved oxygen is even used by microorganisms to oxidize organic matter. The sources of dissolved oxygen in water can be 

(i) Photosynthesis by aquatic plants. 

(ii) Due to the direct contact of the water surface with air. 

(iii) Mechanical aeration. 

Ans: The amount of BOD of water is the amount of organic material present in water in terms of how much oxygen will be required to break it down biologically. Clean water has a BOD value less than 5 ppm. While highly polluted water has a BOD value of 17 ppm or more.