Well, first, in a pristine enviroment, rainwater should
be equivalent to drinking 'RO' or distilled water. It evaporates off the earth, condenses in the heavens and falls back down as gentle rain and purified water for us to use....(aaahhhh what a pretty picture)...
However there are these nasty little things called VOCs (volatile organic compounds) and other things with low boiling points that like to evaporate off the ground with the water and end up 'in' the rain (due to man's foul treatment of our lovely planet).
Acid rain being one such thing that is caused by contaminated water. So unless you know the chemical content of your local rain water, I would not drink it thinking it's pure... Technically it is 'soft' and mineral free, good for rinsing things. But I would worry about VOCs in it and as the rain passes through heavily polluted air, it tends to 'pick up' particulants and drags it back down to earth. Which does cleanse the air, but again, not worth drinking, 'straight up'.
You can boil water, this will kill off any biological nasties, but you are left with any inorganics like heavy minerals and any other contamination that didn't evaporate off.
Boiling it, and then collecting the distillate that re-condenses into water is the ony way to insure it's pure, as the minerals and other heavy particulants do not migrate with the steam. If you are just boiling it, and not filtering or distilling it you are only getting partial protection.
Just sitting an open container on your counter to 'air off' the chlorine (if this is the only thing in your water you are concerned about) is a good idea, but it depends on the amount of chlorine in your local water supply (or how much you added, if you are treating unknown water supply) as to how long to do so.
Generally: You want the water to smell of chlorine if you are treating it for 'biologicals'. Then you want it to 'sit or boil it off' until is has NO smell.
I would think 12 hours is sufficient for most chlorinated water (sit method). Or just boil your water for a few minutes, then let cool. Shake up the water in a seal container to incorporate air back into it, as it tastes flat after boiling.
NOW...If you are boiling your water for worry about pathogens, then boil for 10 minutes to kill EVERYTHING (add an additional 1 minute for every 1000 feet above sea level)
You can also 'solar pasturize' it to 150 F (65 C) for about 6 minutes and all the germs, viruses, and parasites that cause disease in humans are killed, including cholera and hepatitis A and B. (Again, does nothing about any inorganic contamination. You would need to filter it through a carbon filter to remove this stuff.)
I'm not sure what your application and/or needs are here, but even a Britta counter top 'filter pitcher' from Target/Walmart would satisfy the 'carbon' filter treatment. There are some 'jury rigged' ways to do this, if you don't want to buy one.For example:
You found a puddle of water and need to drink it because of a 'bad' situation.
You collect the water in container of choice.
You filter it through some cloth to get rid of any REALLY big stuff (leaves, frogs, bugs...etc).
You then add chlorine bleach (see table) to kill off any biological pathogens or you boil/pasturize it for the recommended time. (Maybe do both, if there were critters in the water, kwim?)Then I would do the following to ensure inorganic compounds are removed and the 'chlorine':
Run the water through any carbon based 'Britta like filter' you have, then drink/cook with it.
OR, I would line a strainer with a large coffee filter. Then I would layer some charcoal (like the kind they sell for fish tank filter use at a pet store) to about 1 inch depth. (It's cheap), then pour your boiled and/or chlorinated water through it.
Now it's ready to drink/cook with. If you are cooking over an open fire for same 'bad' reasons..you could take bits of the burned up wood that are now mini 'charcoal' bits, and use that if all else fails...
*LIQUID CHLORINE BLEACH USAGE
- any brand - must be 5.25% or 6% sodium hypochlorite (like Ultra Clorox) and contain NO soap, fragrance or phosphates. Measuring by drops is more accurate and the preferred method.1 GALLON / 4 LITERS WATER
Add 16 drops (1/4 tsp. / 1.25ml) chlorine
5 GALLONS / 19 LITERS WATER
Add 80 drops (1 tsp. / 5ml) chlorine
Hope that answers your question. This is erm...a ....erm...'hobby' interest of mine. Let me know if it makes sense, ok?