Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions (low pH). It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen oxides which react with the water molecules in the atmosphere to produce acids.
The term acid rain is used to describe all precipitations – rain, snow, fog, dew – which are more acidic than normal. The normal rain is slightly acidic having a pH of about 5.6 as carbon dioxide gas reacts with it to form a weak carbonic acid.
As carbon dioxide is present in unpolluted air in traces the acidity is extremely mild and may be regarded as neutral. Acid rain, thus, refers to any precipitation which has pH less than 5.6. The pH of the cid rain can range between 5.6 and 3.5 and in some cases even lower up to 2. Acid rain results from the presence of two strong acids in polluted air: H2SO4, and to a lesser extent, HNO3.
Nitric acid comes largely from a three step process. The first step involves the formation of NO from the elements by high temperature combustion, as in automobile engines or lightning in the atmosphere during the rainy season. This is hen oxidized to NO2 at normal atmospheric conditions. NO2 combines with water to form a mixture of nitrous and nitric acids.
Coal, used in power plants, contains up to 4% sulphur, mostly in the form of minerals such as pyrite, FeS2 The combustion of coal forms sulphur dioxide. Petroleum products also on combustion release sulphur dioxide.
Large quantities of SO2 are also produced in the metallurgical processes used to treat sulphide ores.
Natural processes such as volcanic eruptions, forest fire and bacterial decomposition of organic matter can produce the oxides of sulphur and nitrogen. When it comes in contact with water, forms sulphurous acid (H2SO3).
SO2 is partly and slowly converted into sulphur trioxide by molecular oxygen in presence of dust particles
SO2 is also oxidized to SO3 by ozone.
SO3 thus formed readily combines with wate3r and produces sulphuric acid as tiny droplets high in the atmosphere. These may be carries by the prevailing winds as far as 1500 km.
(Sulphuric Acid)The sulphuric acid and nitric acid, thus formed remain as vapour under high temperature and begin to condense slowly as the temperature falls. These acids mix with rain or snow, i.e., rain water gets acidified on its way down to the earth. The acidified rain water eventually mixes with the already existing water on the earth. The resulting pH determines the pH of the aquatic environment.
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