|forest damaged by acid rain|
Acid precipitation is rain, snow, or mist which has a pH lower than unpolluted precipitation. Increased levels of acid precipitation have significant effects on food chains and ecosystems.
Precipitation—rain, snow, hail, sleet, or mist—is naturally acidified by carbonic acid (H2CO3). Carbon dioxide (CO2) in the atmosphere reacts with water molecules, lowering the pH of precipitation to 5.6. A pH scale is used to measure a solution’s acidity or alkalinity; pH is defined as the negative logarithm of the concentration of hydrogen ions, H+ . A solution with a pH of 7.0 is neutral. A pH lower than 7 is acidic, and a pH greater than 7 is alkaline.
Other acidic substances are also present in the atmosphere, causing "unpolluted" precipitation to have a pH approaching 5.0. Solutions with a pH of 5.0 or less have concentrations of hydroxyl ion, or OH– , and carbonate ion, or CO3– , approaching zero.