Does Water Ever Expire?

Have you ever left a glass of water out overnight, and it tasted terrible the next day? Does water go bad?


Why does my bottled water have an expiration date? Does this stuff actually go bad?

Hi everyone, Julian here for DNews. Have you ever left a glass of water out overnight, and the next day took a drink and noticed it tasted funny? Does that mean the water is going bad? What’s happening here?

Well, the water itself doesn’t have any sugars or proteins like food, so microbes aren’t consuming it and rotting it. However, that’s not to say the chemistry of the water hasn’t changed. By being exposed to air, the water absorbs some CO2, and a tiny portion of that, about 0.13%, is converted into carbonic acid. Some carbonic acid will then lose a proton or two, forming bicarbonate and carbonate, respectively. This lowers the pH of water, making it slightly acidic and changing the taste.

So does that mean it’s unsafe? Probably not, no. Unless you were a shellfish, carbonic acid forms about 4 million times more bicarbonate than carbonate, and shellfish need carbonate to build their shells. The bias towards bicarbonate production means they have less to build their shells from. Not only that, but the higher acidity can actually dissolve shells.

Since the industrial revolution, the ocean surface has become slightly more acidic. So if you love oysters, there’s another reason to care about CO2 in the atmosphere. Anyway, if you’re a human, the small shift in pH will not be what hurts you. The problem is water in an unsealed container has been exposed to bacteria. Most tap and bottled water have chlorine additives, which will keep the microbes from multiplying for a day or two. But after that, they can go crazy. Water left outside can also start to grow algae and host mosquito larvae, making you sick and gross to think about, but not as gross as thinking about the dust inside your house landing in your water! It can be purified with chemicals, like iodine or chlorine, or filtered and boiled, and it’ll be good as new.

Again, it’s not the H2O breaking down and going bad. It’s just hosting other nastiness that spoils the taste. Our ancestors didn’t benefit from germ theory or chemical purification, so when their water became unsanitary, they had to come up with a different solution. When they set out for long voyages, European sailors had stores of water in wooden barrels. After a few weeks, though, it would start to grow algae, so sailors also brought along a beer to get around this. And when the beer ran out, they switched to that old pirate favorite, rum. The alcohol in the beer and rum killed off bacteria, but the side effect was drunken sailors early in the morning.

In fact, British sailors actually got a daily rum ration until 1970. Now though, we can store water indefinitely without having to worry about its safety, provided it’s stored correctly. The CDC says commercially bottled water is the safest, but it should still be stored out of direct sunlight. Some plastics release a hormone disruptor called bisphenol-A, or BPA, into the water when heated. Plastic bottles are also permeable, so water should be stored away from pesticides and gasoline.

Aside from that, though, you don’t have to worry about the expiration date on bottled water. That’s a holdover from an old New Jersey law that has since been repealed because there’s no scientific evidence to support it. Gotta love it when science informs lawmaking. So now the question is, how much water do you actually need to drink? Trace covers that here. Have you gotten sick from drinking water? Did you switch to rum? Or do you have any other questions you want us to field?

Ali Kaya


Ali Kaya

This is Ali. Bespectacled and mustachioed father, math blogger, and soccer player. I also do consult for global math and science startups.

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