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. 

Limestone

Limestone is mainly calcium carbonate, CaCO₃, which when heated breaks down to form calcium oxide and carbon dioxide. Calcium oxide reacts with water to produce calcium hydroxide. Limestone and its products have many uses, including being used to make cement, mortar and concrete.

Carbonates react with acids to produce carbon dioxide, a salt and water. For example:

calcium carbonate + hydrochloric acid → carbon dioxide + calcium chloride + water

CaCO₃ + 2HCl → CO₂ + CaCl₂ + H₂O

Since limestone is mostly calcium carbonate, it is damaged by acid rain. Sodium carbonate, magnesium carbonate, zinc carbonate and copper carbonate also react with acids: they fizz when in contact with acids, and the carbon dioxide released can be detected using limewater.

When limestone is heated strongly, the calcium carbonate it contains decomposes to form calcium oxide. This reacts with water to form calcium hydroxide, which is an alkali. Calcium hydroxide is used to neutralise excess acidity, for example, in lakes and soils affected by acid rain.