Probably most of you have already noticed that the water that has been sitting for a few hours in a glass has a rather strange taste. However, not many know whether this means that it has “gone off” or something else has changed its flavor.
Well, there is a scientific explanation for the unusual taste of the water which has been in a glass during the night.
In contrary to food, plain water does not contain sugar and proteins, which means that it is easily contaminated by microbes. According to Discovery News, water that has been exposed to air for some time absorbs small amounts of CO2 and a tiny portion of it is converted into carbonic acid, in fact, its chemical composition is being changed.
Namely, when the carbonic acid releases one or two protons, and coverts to a bicarbonate or carbonate, this lowers the pH of the water, and leads to an alteration of its taste.
Apart from the change in taste, this water that has been in a glass for several hours has also been exposed to bacteria, and has accumulated dust.
To conclude, this water that has stayed in the glass for some time has not gone bad, but if you consider the facts given before, it is not advisable to drink that glass next time you are thirsty; instead, drink a fresh one.