【Science Popularization】 Can Water Really Go Bad?

Leaving a glass of water by your bedside before bed, you may find that the first sip in the morning tastes rather bland instead of fresh. The situation gets even worse when you reach for a bottle of water that was opened long ago, possibly even left in the sun, to quench your thirst. Yuck, it’s disgusting! Could this make you sick? After all, you can always find an expiration date on the cap of unopened mineral water bottles.

Drinking water does not truly "go bad"—it contains neither protein nor sugar to rot, mold, or ferment. However, if water is stored in an open container, its chemical composition will change. It absorbs carbon dioxide from the air, and a small portion of the water converts into carbonic acid. Although this accounts for only a tiny fraction (around 0.13%), it is enough to lower the water’s pH value and give it a sour taste. This is not harmful at all; it just tastes unpleasant. But once water is opened, it becomes exposed to bacteria. Tap water and mineral water usually contain chlorine, which has bactericidal properties, so generally speaking, water left out in the air for a day is still safe to drink. After that, though, microorganisms will multiply rapidly. Even the cleanest indoor air is filled with tiny dust particles, which will eventually settle into an open glass of water—not to mention flies and mosquitoes that like to lay eggs on the water’s surface.

As for the date printed on mineral water bottles, consumers can completely take it in stride. While manufacturers are required to mark a shelf life on the bottles, this is by no means the date when the water expires. According to the German Mineral Water Information Center, water stored in glass bottles can be kept almost indefinitely. The same cannot be said for plastic bottles, though. After several months of storage, carbonic acid may seep out and oxygen may seep in, altering the taste of the water. Even so, it is still drinkable. The real concern with plastic bottles is that the bottles themselves may contaminate the water.

Researchers have found that hormone-mimicking chemicals in plastics can leach into the water inside. Although Germany’s Federal Institute for Risk Assessment maintains that drinking water from PET (polyethylene terephthalate—a common resin used to make plastic bottles) bottles poses no health risks, as any potential negative effects are far below the established limit values, some experts disagree. Professor Gilbert Schönfeld of Berlin’s Charité University Hospital, for one, has warned of the unforeseeable long-term consequences. To be on the safe side, plastic bottles containing beverages should never be left in direct sunlight—high temperatures and radiation can cause more questionable substances to leach out. Because plastic is more permeable than glass, PET bottles filled with drinks should not be stored near gasoline or phytotoxins. If your water starts to taste like plastic, pour it out immediately.