Uncontaminated rain water has a pH of 5.6 (slightly acidic)because

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Quizplus · Accepted Answer

Upon condensation, water chemically reacts with carbon dioxide forming a weak carbonic acid, which lowers the pH of the rainwater. This is the most significant factor in the acidity of rainwater, as natural sources of sulfur oxides (such as volcanoes) are usually not enough to cause a significant pH decrease. Similarly, while water droplets can condense on atmospheric particulate, this alone is not enough to cause a significant pH decrease. While sea water is slightly acidic, the majority of atmospheric water vapor comes from freshwater sources such as lakes and rivers, which are not significantly acidic.

Uncontaminated Rain Water Has a PH of 5