Does rain ever fall as pure water?

No. Water is an excellent solvent and rain always contains dissolved
gases from the atmosphere. Even in a remote, pollution-free region,
rainwater will still be slightly acidic because carbon dioxide in the air
reacts with water to form carbonic acid. Rainwater isn’t even pure
when the raindrop forms, because each drop precipitates around
a speck of dust, or an airborne bacterium.