Why Is Normal Unpolluted Rain Water Acidic?

Updated March 23, 2017

Normal unpolluted rain water is acidic because it contains carbonic acid, according to the Virtual Chembook of Elmhurst College. The pH or acidity of rainfall is typically less than 6, with 7.0 being neutral.

Carbonic Acid

Carbonic acid forms when carbon dioxide in the atmosphere dissolves in water droplets. The clouds containing these droplets produce rainfall or other precipitation with carbonic acid. Human activities can further acidify precipitation.

Acid Rain

The term "acid rain" refers to rain that contains acids that form from man-made pollutants, including nitrogen and sulphur oxides. The pH of acid rain can range from about 4 to 5.5. Lower acidity is associated with more industry, power plants and cars, according to the U.S. Geological Survey.

Measuring Acidity

Rain samples are collected around the country and their pH is measured to assess the distribution of acidity. Natural sources such as volcanoes, organic decay and phytoplankton also produce compounds like sulphur dioxide that can acidify rain, according to The source of acid in rainfall is sometimes impossible to determine.

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About the Author

Lexa W. Lee is a New Orleans-based writer with more than 20 years of experience. She has contributed to "Central Nervous System News" and the "Journal of Naturopathic Medicine," as well as several online publications. Lee holds a Bachelor of Science in biology from Reed College, a naturopathic medical degree from the National College of Naturopathic Medicine and served as a postdoctoral researcher in immunology.