Rain Water as it passes through the air and through and over the land, it dissolves many chemical species. Passing through the atmosphere, for example, it dissolves the gases which are the constituent of air including nitrogen, oxygen and carbon dioxide. The fact that it dissolves carbon dioxide from air is very important because when carbon dioxide is present in the water it forms carbonic acid and this acid enhances capability of the water to dissolve chemical species contained in rocks and soil.
H2O + CO2 ↔ H2CO3 (carbonic acid)
Which then enacts with Ca2+, Mg2+, Na+, Fe3+, SO42-, SiO22-, Cl-
Water become hard with: Temporary Hardness and Permanent Hardness