Why are Limestone structures eroded by acid rain?
CO2 present in the air can dissolve in rain water producing carbonic acid. This acid is weak but can give rain water a slightly acidic pH of around 5.6
When this rain water falls it reacts with the claim carbonate present in limestone, in the following reaction:
Calcium carbonate + carbonic acid -> Calcium Salt + Water + CO2
This reaction has been responsible for the natural formation of limestone caves over millions of years, but more recently more acidic rain has been produce by the excess Sulfur Dioxide produced by burning Coal. SO2 reacts with water in the atmosphere to produce Sulphuric acid (H2SO4). This acid is stronger than carbonic acid and can give water a pH of below 5. When this falls as acid rain it produces a strong reaction with calcium carbonate which may erode statues and buildings, and cause damage to environmental features such as crops and trees.
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