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Asia-Pacific Forum
on Science Learning and Teaching, Volume 5, Issue 1, Article 4 (Apr.,
2004) Man Wai FONG A demonstration of acid rain
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| |
Asia-Pacific Forum
on Science Learning and Teaching, Volume 5, Issue 1, Article 4 (Apr.,
2004) Man Wai FONG A demonstration of acid rain
|
What acid rain is
As atmospheric CO2 dissolves in water to form carbonic acid, rain is naturally acidic. Normal rain has a pH of about 5.6. It has been generally considered that rain whose pH is lower than 5.6 to be acid rain because this is the pH value of carbon dioxide in equilibrium with distilled water.
The main cause of acid rain is the presence of strong mineral acids, mainly sulfuric (H2SO4) and nitric (HNO3) acids, derived from the atmospheric oxidation of sulfur dioxide (SO2) and nitrogen oxides (NOX). These gases are emitted from both natural and anthropogenic sources.
Volcanic eruptions release gaseous SO2 and H2S that are eventually converted to sulfuric acid. Thus rain that falls far from inhabited or industrialized areas, through supposedly unpolluted air, has a pH lower than 5.6 - generally about 5.0.
The burning of fossil fuels is the most important anthropogenic source. Sulfur as pyrite is present as impurities in coal, gasoline and petroleum products and is oxidized to SO2 when these fuels are burned. Nitrogen oxides released from the burning of fossil fuels in automobiles and power plants come from the air itself rather than from impurities in fuels. Nitrogen and oxygen of the air can be converted to NO at high temperatures. The temperatures of combustion chambers of the internal combustion engine are effective in this conversion. NO reacts with oxygen to form NO2 which dissolves in water to from nitric acid.
Copyright (C) 2004 HKIEd APFSLT. Volume 5, Issue 1, Article 4 (Apr., 2004). All Rights Reserved.
