Module-8 - Module8: AcidRain Howdoesthishappen CO2(g H2O...

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Module 8: Some Environmental Concerns Acid Rain How does this happen? When atmospheric carbon dioxide dissolves in water it forms carbonic acid. CO 2  (g) + H 2 O (  H 2 CO 3  (aq) The acid formed dissociates partially generating small amounts of hydrogen ions. H 2 CO 3  (aq)   H +  (aq) + HCO 3 -  (aq) The hydrogen ions formed are responsible for the weakly acidic nature of rain. The pH Scale The acidity (a measure of hydrogen ion concentration) of a solution is conveniently described in  terms of pH scale. According to this scale, if: pH < 7: solution is acidic pH > 7: solution is basic pH = 7: solution is neutral pH Laboratory This virtual laboratory provides a visual example of how different substances can effect the pH  balance. This activity should also remind you of the health and safety aspects of working in a  laboratory discussed in Module 4.   Acidity =       pH Based on the concentration of carbonic acid and small amounts of other natural acids, the  estimated pH of rain (as well as fog, dew and snow) under normal atmospheric conditions should be around 5.3.
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Field measurements in many areas, on the other hand, show much lower pH values (4.1-  4.5). Higher acidity (lower pH) is due to the reactions of NO x  and SO x  present in the atmosphere. Various reactions of these oxides lead to the formation of acids. The dissociation of formed acids generates hydrogen ions and hence higher acidity (or lower pH). 2 SO 2  (g) + O 2  (g) + 2 H 2 O (  2 H 2 SO 4  (aq) H 2 SO 4  (aq)   H +  (aq) + HSO 4 -  (aq) HSO 4 -  (aq)   H +  (aq) + SO 4 2-  (aq) 4 NO 2  (g) + O 2  (g) + 2 H 2 O (  4 HNO 3  (aq) HNO 3  (aq)   H +  (aq) + NO 3 -  (aq) Environmental Effects Material Damage       Marble and limestone (CaCo3) used in many historic and irreplaceable statues and  buildings (e.g. the Parthenon in Greece, the Taj Mahal in India) react with acid in the  rain. The reaction leads the formation of calcium salts the formation of calcium salts. The  solubility of these salts in water is much greater than that of marble in water. The  dissolutionof formed calcium salts leads to roughened surfaces, removal of material  and loss of carved details from the buildings and statues. Forest Damage       : Considerable damage to trees has been reported in various parts of North America and  Europe. Acid rain has been implicated with slower growth, injury and destruction of  forests and has been attributed to be a contributing factor to the declining health of trees  by damaging their leaves, limiting the nutrients available to them or exposing them to  toxic substances released from soil due to the change in pH.
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