Environmental chemistry

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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. 

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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. 

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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. 

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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.

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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.

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

This chlorine radical then reacts with stratospheric ozone to form chlorine monoxide radicals and molecular oxygen. The reaction involved is-

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

This shows that chlorine radicals are continuously regenerated and cause the breakdown of ozone. Hence, CFCs are transporting

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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.

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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 reaction 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)