The water in a raindrop is one of the cleanest sources of water available. Rainwater can absorb gases such as carbon dioxide, oxygen, nitrogen dioxide, and sulfur dioxide from the atmosphere. It can also capture soot and other microscopic particulates as it falls through the sky. Nevertheless, rainwater is almost 100% pure water before it reaches the ground.
Rainwater is soft water and leaves no limescale; washing clothes and hair in soft water requires less detergent and so reduces water pollution from these compounds. Plants LOVE rainwater. It doesn’t contain chlorine, which is carcinogenic.
Water is made “hard” by dissolving calcium or magnesium ions, neither of which is present in rain water (at least, not until it runs over calcium or magnesium containing compounds on the Earth’s surface). Pure water is considered the universal solvent; it can absorb or dissolve contaminants from almost anything it comes into contact with. That is why it is especially important to design and operate your system so that the rainwater picks up as few contaminants as possible before you consume it.
Although rainwater can be contaminated by absorbing airborne chemicals, most of the chemicals present in harvested rainwater is introduced during collection, treatment, and distribution. By properly designing and operating your rainwater harvesting system, you can minimize your exposure to a variety of chemical contaminants that include organic chemicals, such as volatile and synthetic organics, and inorganic chemicals, such as minerals and metals.
Chemical composition of rain water : Rainwater gets its compositions largely by dissolving particulate materials in the atmosphere (upper troposphere) when droplets of water nucleate on atmospheric particulates, and secondarily by dissolving gasses from the atmosphere.
Rainwater compositions vary geographically. In open ocean and coastal areas they have a salt content essentially like that of sea water (same ionic proportions but much more dilute) plus CO2 as bicarbonate anion (acidic pH).
Terrestrial rain compositions vary significantly from place to place because the regional geology can greatly affect the types of particulates that get added to the atmosphere. Likewise, sources of gaseous acids (SO3, NO2) and bases (NH3) vary as a function of biome factors and anthopogenic land use practices. Each of these gasses can be added in varying proportions from natural and non natural input sources (non-natural sources of SO3 and NO2 far outweigh natural ones). Particulate load to the atmosphere can also be greatly affected by human activities.
Finally, local climate (especially the amount of precipitation in one area compared to another) will affect the solute concentrations in terrestrial rainwaters. The result is highly variable compositions, so there isn’t one simple formula.
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Courtesy and Sources : Wholly H2O, Ask-An-Earth-Scientist and Google.
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