Normal rainwater is slightly acidic, with a pH between:

1. Rain picks up gases from the air as it falls. 2. Carbon dioxide is naturally present in the atmosphere because most organisms produce it as a waste product of metabolism. 3. Normal rainwater has a pH between 5 and 6 due to the presence of dissolved carbon dioxide, which forms carbonic acid. 4. So even unpolluted rain is mildly acidic. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 14 'Acid-Base Equilibria', sections 14.1-14.7_