Robert Angus Smith, an English chemist, first used the phrase "acid rain" in 1852 when he noted the connection between London's polluted skies and the acidity of its rainfall.
Most Scientists agree that "normal" rainfall has a pH of 5.6. Rain in the atmosphere reacts with carbon dioxide (CO2) to form a weak carbonic acid, altering the rain pH to 5.6.
Acid rain is defined as any form of wet precipitation which has a pH less than 5.6 (on a scale of 0 to 14, with 7 being neutral). The "rain" becomes acidic when water molecules (H2O) react with gases in the air.
These gases are primarily sulfur dioxide (SO2) and various nitrogen oxides (NOx). This combination of gases and water molecules takes place when the water captures (attracts) hydrogen ions (H+) from the gases (ions are electrically charged particles in molecules).