In its purest form, water is H2O — two parts hydrogen and one part oxygen. Water is also known as the universal solvent in that it will dissolve bits of everything it comes into contact with. As water forms clouds in the atmosphere, it absorbs impurities in the air (dust, gases, smog, and pollution). And as the water condenses and falls to earth as rain, hail, sleet, or snow, the process of absorbing contaminants continues.