Due to the fact that plain water does not contain any sugars or proteins, it can easily be contaminated by microbes. We will provide you with the scientific explanation for the unusual taste if the water which has been in glass during the night. Water which has been exposed to air for some time starts absorbing small amounts of CO2 as well as a tiny portion of it converts into carbonic acid, meaning that the chemical composition is being changed.
When the carbonic acid releases one or two portions, turning into bicarbonate or carbonate, the pH levels lower and that is how it changes its taste. Then, the water has been staying in a glass for many hours and has been exposed to bacteria, accumulating dust. This doesn’t mean that the water has gone bad, but you should better think if you want to drink this kind of water or you’ll fill your glass with fresh one.