Have you ever wondered why something as basic as water can go stale by sitting in a glass overnight? Here is exactly why...
Well, drinking water is by no means pure H2O. As a matter of fact, it has a whole collection of other molecules and ions in the amalgamation which, over time, could potentially change its taste. When your glass of water is left overnight, it also absorbs a fair amount of carbon dioxide from the air. With heat, the H2O molecules combine with the C02 molecules and form carbonic acid, thus changing its taste due to the lower pH level.
Furthermore, water purification methods usually make use of chlorine and, after drinking purified water your entire life, we have associated this tiny amount of chlorine taste as normal. But, overnight, a lot of this chlorine will evaporate out of the water solution and, too, will change its taste.
Lastly, the temperature of the water has an influence on the taste as well. Cooler temperatures slightly dissipate our ability to taste but, if a glass of water is left overnight, it warms up to room temperature and appears to taste different than the night before.
The good news is that water left overnight is perfectly fine to drink, you're just being a bit fussy!
Take a look at the video below by the YouTube channel, SciShow, on Why Does Water Go Stale Overnight?