All pretty much right on in all the contributions. Ion exchange is probably the overall process or set of processes which describe what is going on in the water cycle. At the surface of the ocean is the first exchange. The water is in constant contact with one of the most reactive elements in our world, oxygen. In general the water becomes oxygenated and this is great for all the little fishies in the deep blue sea. On occasion an actual chemical reaction takes place and H20 becomes H2O2, hydrogen peroxide. The prefix "per" indicates an over abundance of an element, in this case oxygen, and that makes the compound unstable. Pure H2O2 is extremely dangerous and has been used as a component in rocket fuel, sometimes with disastrous results. The excess oxygen makes H2O2 a base, as opposed to acid. Any water anywhere on earth exposed to normal atmosphere will naturally have a percentage of H2O2 and over time the water will tend to neutralize any acids present.

As evaporation takes place, pure water in the form of vapor and is whisked off into the wild blue yonder. When it runs into colder air it precipitates out as droplets or ice crystals. This usually occurs around microscopic dust motes or other heavier aerosols present in the atmosphere. It takes about eight times the presence of moisture for this to occur without the foreign matter present so almost all atmospheric water is not entirely pure.

The aerosols tend to be acidic in nature. Sulfur from volcanoes and under refined fossil fuels react with the water and oxygen and form sulfuric acid. Present also are carbonic acid and a slew of others. These bind the oxygen atoms and give rain a slightly acid ph. Around industrial centers and large cities (down wind) the increase in ph can be dramatic, almost as acidic as lemon juice! Take a walk through an old cemetery and try to read some of the old marble (Calcium oxides) markers! That is the result of chemical reaction, not mechanical weathering.

At any rate this acidic solution hits the compounds which make up the crust of the earth. These elements are mostly bases as they are either chemically inert or weak chemical reactors. Iron, calcium, manganese, magnesium and so on are base because of the abundance of oxygen which is weakly bound to them. The acid hits and ions are transferred and the water becomes harder as these minerals go into solution. In chemistry water is known as the Universal Solvent. Any thing, ANYTHING dissolves in water. This includes platinum, gold, silver and glass. Given enough time, water dissolves anything. Hence water tends to become base or hard once it reaches land. The exceptions are, as noted above in other posts where it reaches forest detritus, peat and other types of organic deposits. Here the ph tends to remain on the acidic side of the equation.

There is really much more science to this subject but that, in a coconut shell is the basics.