acid deposition  Acid rain  Acid rain is rain or any other form
of precipitation that is unusually acidic. It has harmful
effects on plants, aquatic animals, and infastructure. Acid rain
is mostly caused by human emissions of sulfur and nitrogen
compounds which react in the atmosphere to produce acids. In
recent years, many governments have introduced laws to reduce
these emissions.   "Acid rain" is a popular term referring to
the deposition of wet (rain, snow, sleet, fog and cloudwater,
dew) and dry (acidifying particles and gases) acidic components.
A more accurate term is “acid deposition”. Distilled water,
which contains no carbon dioxide, has a neutral pH of 7. Liquids
with a pH less than 7 are acidic, and those with a pH greater
than 7 are basic. “Clean” or unpolluted rain has a slightly
acidic pH of about 5.2, because carbon dioxide and water in the
air react together to form carbonic acid, a weak acid (pH 5.6 in
distilled water), but unpolluted rain also contains other
chemicals.  H2O (l) + CO2 (g) →