Acid Rain

Carbon dioxide in the air can dissolve in rain water to form carbonic acid, H₂CO₃.

CO₂ + H₂O  H₂CO₃

Carbonic acid is a weak acid. It partially ionises to form hydrogen ions.

H₂CO₃  H⁺ + HCO₃^-

The hydrogen ions from carbonic acid give natural rain water a slightly acid pH value of 5.6. Over millions of years this very dilute acidic solution has been responsible for the formation of caves in areas of limestone rocks. Limestone is made of calcium carbonate, which reacts with acids.

calcium carbonate + acid  calcium salt + water + carbon dioxide

During the last century the rain water in some parts of the world has become far more acidic. This acid rain has been caused by the emission of pollutant gases such as sulfur dioxide. When coal is burned in electricity power stations, sulfur impurities form sulfur dioxide.

S + O₂ SO₂

The gas is also produced when fuels obtained from crude oil are burned. When sulfur dioxide is released into the air it reacts with water and oxygen to form sulfuric acid, H₂SO₄.

Sulfuric acid is a strong acid. It ionises completely to hydrogen ions.

H₂SO₄ 2H⁺ + SO₄^2-

This gives rain water a pH below 5.0. Rain water that has this higher level of acidity can cause damage to buildings and statues, particularly those made of limestone. It can also reduce the growth of, or even kill, trees and crops. Acid rain may even lower pH of water in lakes, killing fish.

When acid rain occurs, it can damage buildings and statues that have been in place for hundreds of years.