Acids formed in the atmosphere can fall as acid rain. This can be a major problem, killing trees and fish in lakes. The acid rain also corrodes limestone buildings and marble statues since these are both made of calcium carbonate (CaCO₃). Some metals such as iron are also attacked by acid rain.


Sulfur dioxide released into the atmosphere from the burning of fossil fuels can react with water and oxygen to make sulfuric acid (H₂SO₄):

2SO₂ (g)         +         2H₂O (l)         +         O₂ (g)         →         2H₂SO₄ (aq)


Also, if sulfur dioxide in the atmosphere reacts with just water, a weaker acid called sulfurous acid (H₂SO₃) is formed:

SO₂ (g)         +         H₂O (l)         →         H₂SO₃ (aq)


In car engines the temperature is high enough for the nitrogen in the air to react with oxygen to produce oxides of nitrogen, e.g:

N₂ (g)         +         O₂ (g)         →         NO₂ (g)

In the atmosphere these nitrogen oxides can produce nitric acid (HNO₃).