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First off, salty means that there is a certain amount of dissolved material in the water.
This dissolved material will affect the smell and the taste.
Now to begin with, rainwater is NOT pure. Rainwater gets its compositions largely because
a raindrop usually forms around a grain of dust, and it can also dissolve the gases in
the atmosphere. Often carbon dioxide, but certainly some pollutants as well. As a result
of where the rain is forming, rainwater compositions vary geographically.
For the most part though, rainwater is safe to drink (also known as potable)
As soon as the rain hits the ground it starts to flow. It will either flow into the pores
in the ground becoming groundwater, or it will flow off a surface and we call it run-off.
Most of the fresh water in the world becomes ground water. As water moves through the ground,
whatever it encounters, if it can be dissolved, will be picked up by the water, and if it
can't be dissolved it might be carried by the water. This would include bacteria, organic
waste, pollutants, chemicals... and so on. BUT if the water stays in the ground, and
travels far enough many of the harmful elements will be "filtered out" by sand, coal, and
clay which act as a filter. If you were to then drill for that ground water, you might
have a pretty safe source of drinking water.
The surface run-off picks up sand and dirt, and silt and clay and transports it downstream
-- along with everything else it encounters only the surface runoff does not get filtered
and that's why a river will get murkier and murkier the longer it runs. This is generally
NOT a good place to be getting your drinking water, but it's still not as salty as the
ocean.
Imagine all of the water that runs all over the world running into the ocean, with all
of its substances. Some organisms that live there will eat the organic material, other
organisms might break down the pollutants, other chemicals will eventually settle out
on the bottom or turn to limestone, but much of the minerals that have been dissolved over
the route to the ocean stay, and the more water that runs in, the more minerals appear.
But here's the thing, the rain water comes from evaporated water (which as we all know
comes from off the land and sea) and all those salts that are in the ocean can't evaporate
with the water -- they remain behind in the ocean... becoming more and more concentrated.
The oceans are salty, because they're old, and there are lots of salt in them that was
left behind.
The process of evaporating water and leaving behind the salts is called distillation. The
sunshine will do this naturally, but if you want to do this artificially it's going to
take a LOT of energy... something that historically humans have not always had a lot of access
to.