“Tap water does the same. All cold drinks tend to have some dissolved gases in the liquid, carbonated ones even more so. Once the drink is left sitting still at room temperature without a lid, the dissolved gases come out of solution as bubbles and rise to the surface to “pop”. Once the temperature of the drink and the air has equalised and enough dissolved gas has bubbled out of the drink to balance the pressure of gas in the drink and in the air, no more bubbles will tend to pop on your tongue so the water will feel “flat”.
While Mandi’s response is good and correct, it doesn’t tell the tire story, so let’s consider some other issues.
If the water is a carbonated/effervescent product the effect of losing CO2 to the atmosphere is indeed directly relative to the taste of the water. This not only holds true for sparkling/effervescent waters, but also for all carbonated beverages. With a water such as Dasani or Aquafina however, this property is much less responsible for the taste of the water changing.
As any water sits it will absorb available substances/impurities from the atmosphere around it and the container as well. As these substances are absorbed into the water, the taste of the water will change. As an example, as CO2 from the air is absorbed, it is converted to carbonic acid which then in turn lowers the pH of the water and affects the taste. As another example,if dust, pollen, perfume, or smoke get absorbed into the water the taste will be greatly affected.
In addition, with no chlorine in the water, organisms such as bacteria, mold, viruses and algae will quickly begin to grow, also affecting the taste of the water.
Defining the perception of “flat tasting” is an ambiguous task for sure, but it is fair to say that as any drinking water sits, it’s chemistry and taste will change. It is rare that anyone finds water that has sat for a length of time, to be good tasting.