Ncert Solutions Chemistry Class 11th

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Ozone in the stratosphere is formed when UV radiation falls on the dioxygen (O₂ ) molecules. The UV radiations split apart molecular oxygen into free oxygen (O) atoms, these oxygen atoms combine with molecular oxygen to form ozone. The reactions involved can be given as-

O₂ (g)  O (g) + O (g)

O (g) + O₂ (g) +M O₃ (g) + M

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NO₂ absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atom and this can be shown as- 

NO₂ (g) NO (g)+O (g)

Here the oxygen atom produced is very reactive and combines with oxygen molecule in the air to give rise to ozone. 

O (g) +O₂ (g) + M → O₃ (g) + M

Here 'M' is an inert gas like Nitrogen gas and ozone gets produced during the formation of smog.

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The uncatalyzed oxidation of SO₂ is a slow process. However, the presence of particulate matter in the polluted air catalyses the oxidation of SO₂ to SO₃. The reaction for the conversion can be given as-

2SO₂ (g)+O₂ (g)→2SO₃ (g)

The reaction can be promoted by O3 or by H₂O₂.

SO₂ (g)+O₃ (g)→2SO₃ (g)+O₂ (g)

SO₂ (g)+H₂O₂ (l)→H2SO₄ (aq)

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The symptoms which were observed in  the village indicate that oxides of sulphur and nitrogen are released from the chimneys of the factory. These gases are obtained as the product of combustion of fossil fuels such as coal,   gasoline etc. In an automobile engine, at high temperature, when fossil fuel is burnt, dinitrogen and dioxygen get combined to give significant quantities of nitric oxide and nitrogen dioxide. The chemical reactions can be given as-

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Due to the excessive growth of algae water contains a lot of phosphate due to the inflow of the fertilizers from the surroundings. The decomposition of algae leads to the bad smell and taste and even makes it unfit for  drinking, bathing, washing, swimming, boating etc. and concentration of dissolved oxygen also decreases which may be harmful for aquatic life. 

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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.