Have you ever noticed an odd taste found in water left sitting for several hours?
Plain water, unlike food, does not contain sugar or proteins and it is easily contaminated by microbes. Even pure water can become something you wish to avoid if it has stood uncovered long enough and there is a reason for it.
It’s actually the chemistry of the water that’s changing. When exposed to air, water absorbs some CO2 and a tiny portion of that is converted into carbonic acid. When carbonic acid is released one or two protons and turns into a bicarbonate or carbonate, it lowers the pH of the water due to which it alters the taste. The small dip in pH doesn’t present a health risk. However, unsealed water gets exposed to bacteria.
Water left sitting in a glass for several hours is exposed to the bacteria, and eventually becomes polluted by dust, which in turn could give nourishment to more microbes. Harmful bacteria can bloom if given sufficient nutrients. This does not mean that the water has gone bad, but rather stagnant, so it’s always a good idea to have a fresh glass.
Instead of having to discard water, get into the habit or bottling it (in glass) when not in use and cap it, or simply place it in the fridge.