This Is Why Leaving Water Out Overnight Doesn't Make It Go Bad

It's always after a night of heavy drinking. You wake up with the early morning sun practically wrenching open your eyelids, spying that glass of water you told yourself to quaff before you went to bed. With a parched, sweater-coated tongue, you take a sip from it—and it's disgusting.

But why? How could a glass of pure water go from clean-tasting to eye-twitchingly metallic in the space of only a few passed-out hours?

Because water contains nothing but H20—that is, there are no sugars, fats, or proteins that can rot or go rancid—it can't really "go bad."

DNews explains that the chemistry of water does, in fact, change overnight. Blame the atmosphere: Water that's been exposed to air will taste different from water that's spent its life in a pipe. Water left out will absorb a small amount of oxygen, a very small portion of which—about 0.13 percent, says DNews—converts into carbonic acid. (Yes, that's the same stuff responsible for giving carbonated water its bite.) That carbonic acid then converts into carbonate or bicarbonate, lowering the pH of your water and turning it ever so slightly more acidic.

And that is perfectly fine for you—unless you're one of those tin foil hat-wearing celebrities who believes that we should all be eating alkaline diets to prevent cancer. (We shouldn't.)

DNews points out, however, that things are a little more serious for shellfish, which can suffer due to so-called "ocean acidification." Up to 40 percent of the carbon dioxide in the atmosphere—of which there is a shit-ton, thanks to industry—gets absorbed into the Earth's oceans, lowering its the water's overall pH. This makes it difficult for shellfish to grow and maintain their shells, endangering them in the process (The New York Times's Mark Bittman, to whom we recently spoke about urban foraging, will release an episode of his new California Matters series on this very topic in the coming weeks.)

As far as human health goes, your only concern should be how long your water has been left out in the open air. After an extended period of time, it can become a playground for dangerous bacteria, algae, dust, and even mosquito larvae—but an overnight rest isn't long enough to incubate anything.

Oh, and be ever-so-slightly vigilant about bottled water that's been exposed to heat or sunlight for extended periods of time. While the CDC has cautioned that plastic bottles can leach potentially dangerous compounds like bisphenol A (BPA), a recent University of Florida study found that those levels are generally well below government standards. But the head researcher also noted that you should maybe look askance bottled water that's been left in a hot environment for, say, a year.

If you're still worried, just remember to drink your water before you go to bed.