If I remember well, in water (even pure one), there is a chemical equilibrium between H30+ and OH- .
This equilibrium gives the PH of the water by the relation PH = - log_10([H30+]).
Water at ambient temperature having a PH of 7, that gives a concentration of [H30+] = 10^-7 mole/L.
As this is pure water, there is an equal amount of [OH-] ions.
This quantity is really small (1 mole of water is 18 grams, so this quantity is in the order of 3,6 µg of ions as 2*10^-7 moles of water gives 10^-7 mole of each ion), but is still there even if you don't have any dust in your water. The effect is a really slow corroding of what can be, thus providing the water with foreign elements.
This quantity increase with the temperature. At atmospheric pressure, it ranges from 10^-7.4690 at 0°C to 10^-6.1325 at 100°C.
So even if you prevent any dust to come in the water from the outside, the water itself will tend to make some from what it touches, hopefully it will takes some time for the effect to be noticeable.
Distilled water would however be a good idea to use when tempting things like the waterproofing experiment http://www.overclock.net/t/1540031/showme-s-motherboard-cpu-ram-waterproofing-experiment
. The use of distilled water still reduce the risk of short circuit over tap water.Edited by ldsh - 10/9/15 at 7:23am