Is Rainwater an Acid?
Rainwater is slightly acidic.
Pure rainwater, uncontaminated by atmospheric pollutants, has a pH of around 5.0 to 5.5. This slight acidity is due to the absorption of carbon dioxide from the atmosphere, forming carbonic acid. However, this is naturally occurring and generally not harmful.
Several sources confirm this: The U.S. Geological Survey (https://www.usgs.gov/special-topics/water-science-school/science/acid-rain-and-water) and the EPA (https://www3.epa.gov/acidrain/education/site_students/phscale.html) both state that normal, clean rain is slightly acidic with a pH between 5.0 and 5.5. A Reddit discussion (https://www.reddit.com/r/water/comments/14cxrx5/ph_test_rain_water_left_well_water_right_i/) also highlights the slightly acidic nature of rainwater due to atmospheric carbon interaction.
Acid Rain: A Different Story
It's important to distinguish between normal rainwater and acid rain. Acid rain, a significantly more acidic form of precipitation, results from atmospheric pollutants like sulfur dioxide and nitrogen oxides reacting with water and other chemicals. These pollutants, primarily emitted from power plants and automobiles, create sulfuric and nitric acids, dramatically lowering the pH of rainwater. This is detailed in resources like the New Hampshire Department of Environmental Services fact sheet (https://www.des.nh.gov/sites/g/files/ehbemt341/files/documents/2020-01/bb-8.pdf) and the EPA's explanation of acid rain (https://www.epa.gov/acidrain/what-acid-rain). A study on rainwater as a drinking water source (https://pmc.ncbi.nlm.nih.gov/articles/PMC6657612/) also mentions the formation of sulfuric and nitric acids in rainwater due to reactions with atmospheric gases.
In Summary
While rainwater naturally exhibits a slight acidity due to dissolved carbon dioxide, it's crucial to differentiate this from the significantly more harmful acid rain caused by pollution. The level of acidity can vary considerably depending on the level of air pollution in a given location.
