Acid Rain

Chemists define the quanity pH = -Log [H⁺], where [H⁺] is the aqueous concentration of H⁺ ions. Pure rainwater is a neutral solution that has a pH of 7 when the droplets form. Will these droplets falling through clean, unpolluted air have a pH of 7 by the time they hit the ground?

Answer

Not so! The falling raindrops will not remain neutral at pH 7. Pure rainwater falling through unpolluted air is an acid, with a pH of about 5.6, because as the drops form and fall they dissolve carbon dioxide in the air and react to produce carbonic acid, H₂CO₃. Officially, therefore, acid rain is defined to have a pH of less than 5.0, a condition that tends to occur more often in industrialized areas of the world than in remote regions.

Human activities can increase the amount of CO₂ in the air, but so do many natural resources, such as volcanoes, lightning strikes, cows, bacteria, and fires. When industrial and automobile exhausts release sulfur compounds and nitrogen compounds, these molecules combine with oxygen to form sulfuric acid and nitric acid, which can harm ecosystems, historic monuments and buildings, and the health of people around the world. The reduction of sulfur and nitrogen compounds released into the air has become a worldwide concern.