No, rainwater is not perfectly pure. While often considered the purest form of natural water, it's not lab-grade pure. Rainwater undergoes a natural distillation process as it forms, but it still picks up various impurities along the way.
Impurities in Rainwater
Several factors contribute to rainwater's impurities:
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Atmospheric Gases: Rainwater absorbs gases like carbon dioxide from the atmosphere, resulting in a slightly acidic pH. This is mentioned in the Reddit AskScience thread (https://www.reddit.com/r/askscience/comments/3dugya/is_rain_water_directly_from_the_sky_clean/).
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Airborne Particles: Raindrops form around tiny particles like dust, pollen, bacteria, and other airborne pollutants. These particles become incorporated into the raindrop, contaminating the water (https://www.sciencefocus.com/planet-earth/does-rain-ever-fall-as-pure-water). A Quora answer further explains that the rainwater picks up "a bunch of other stuff from the atmosphere" (https://www.quora.com/How-clean-is-rain-water).
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Pollutants: Depending on air quality, rainwater can collect pollutants such as industrial emissions, vehicle exhaust, and agricultural chemicals. This significantly impacts the purity of rainwater, as noted by https://smartwateronline.com/news/is-rainwater-clean.
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Initial Condensation: Even as it forms, a raindrop isn't pure because it condenses around a nucleation point, which is often a speck of dust or airborne bacterium (https://www.reddit.com/r/askscience/comments/2atxta/is_rain_totally_pure_clean_water/).
Despite these impurities, rainwater is still relatively pure compared to other water sources. However, the level of purity varies considerably based on location and atmospheric conditions. Many commercial bottled water brands utilize rainwater as their source (https://responsiblyrain.com/products/24-pack-rain-pure-mountain-spring-water), although often with added filtration for enhanced purity.
