Does Bottled Water Expire? Why Does It Go Bad?

Water itself doesn’t expire. When a glass sits out, it absorbs carbon dioxide from the air, forming a trace of carbonic acid that nudges the pH lower and gives it a flat, slightly off taste. The “expiration” date on a bottle is about quality, not safety: the FDA doesn’t require one, and over time the plastic can leach into the water and change its flavor.

This has happened to everyone at least once… We take out a bottle of water from the fridge, fill a glass, forget to drink it, and then leave it on a table for later. When we feel thirsty again, we might just take another bottle from the fridge and drink from that, completely forgetting about the glass of water already waiting for us on the table. Imagine that a day or two passes, and when we finally notice the glass of water the next day and drink from it, the water tastes weird.

So what actually happened to it? Did the water “go bad”? And while we’re at it, why do bottles of water come stamped with an expiration date at all? Let’s look at the chemistry.

Chemistry at Play

Water itself doesn’t have any sugars or proteins on which microbes can feast, so there is no danger of water “rotting” in the traditional sense when kept out in an open space for a long time.

However, when water is kept out in the open for an extended period of time, things do tend to change a bit. Carbon dioxide from the air slowly dissolves into the water, and a small share of that dissolved CO₂ reacts with it to form carbonic acid (H₂CO₃). Most of the gas simply stays dissolved as CO₂; only a tiny fraction (roughly 0.1–0.2%) actually converts into true carbonic acid, but that is enough to make a difference.

Carbonate

Some of the molecules of this newly formed carbonic acid go on to lose a proton or two, forming bicarbonate and carbonate ions. In turn, this nudges the pH of the water downward, making it slightly acidic (a lower pH signifies a more acidic nature). Left long enough in contact with the air, pure water settles at a pH of around 5.6 instead of a neutral 7, and it is this faintly sour, flat character that your tongue registers as a weird taste.

However, this slight change in the pH of water is not likely to hurt you, but that old glass of H₂O might not be the pleasant and refreshing beverage that you were hoping for.

More Than Chemicals

It is not only the formation of carbonate in the water that changes the taste; there are also a few other factors to consider. One of these factors is actually quite gross.

Left out long enough (think days, not hours, and in the open rather than in your fridge), standing water can also pick up unwelcome guests. Airborne dust, pollen and microbes drift in and settle on the surface, and if the water sits in warm light for long enough it can even begin to support algae (Yuck!). None of this is usually visible in an overnight glass, so don’t waste your time squinting at it. It is, however, a big reason a covered glass tastes far less stale than an open one. That settling dust from around your house is a quieter culprit on its own: a glass with a lid simply has less of the room landing in it.

Expiration Dates On Bottled Water

Bottled water is a slightly different story, but maybe not in the way you would expect. Despite the date printed on the label, the water inside does not actually “expire.” The U.S. Food and Drug Administration regulates bottled water as a packaged food yet does not require an expiration date at all, and it considers properly sealed bottled water to have an essentially indefinite shelf life. Those dates are about quality, not safety.

So why stamp a date on it? The real culprit is the bottle, not the water. Plastic is slightly permeable, so over months and years tiny amounts of compounds from the plastic can migrate into the water, and gases and odors from the surrounding air (think paint, solvents or cleaning products stored nearby) can seep in and tweak the taste. Heat and direct sunlight speed this up, which is why bottlers tell you to store water somewhere cool and dark. The water is still safe to drink well past the date, it just may not taste as fresh. If you want to dig into how compounds creep out of the bottle, we have a whole piece on plastic leaching.

Does Shaking Or Stirring Water Change Its Taste?

Here is a trick worth knowing: if that glass on your nightstand tastes flat, you do not have to pour it out. Just swirl it, stir it, or tip it back and forth between two glasses, and a lot of that fresh-from-the-tap taste comes back. The reason is the flip side of what makes water go flat in the first place.

Freshly drawn water carries a small load of dissolved gases, including oxygen, and that crispness is part of what your tongue reads as “fresh.” As a glass sits, those gases slowly drift out into the air and the water tastes duller. Cleveland Water, a municipal utility, describes exactly this scene of a glass on a bedside table tasting “flat” the next day because it is “missing dissolved oxygen,” and notes that you can put some back simply by pouring the water from glass to glass or shaking it in a bottle. So no, shaking does not make water “bad”; if anything, it can perk a stale glass back up.

Does the shaking measurably change the water? A little. In a 2020 study in the International Journal of Molecular Sciences, researchers shook water hard in the lab and found small, temporary shifts in dissolved oxygen and pH that settled back out within hours. The aeration you get from a real-world swirl is gentle and short-lived, but it is enough to nudge the flavor in the right direction.

Why Does Some Water Taste Like Nothing (Or Slightly Off)?

If you have ever sipped distilled or heavily filtered water and thought it tasted weirdly empty or even a touch bitter, you were not imagining it. Most of water’s “taste” actually comes from what is dissolved in it, not from the H₂O itself. Tiny amounts of minerals such as calcium, magnesium and bicarbonate give spring or tap water its crisp, rounded character. Strip them all out, and the result tastes flat.

The World Health Organization spells this out in its drinking-water guidance, which rates palatability by total dissolved solids (TDS). Water below about 600 mg/L of TDS is generally judged good, while water becomes increasingly unpalatable above roughly 1,000 mg/L. At the other extreme, the WHO notes that water with extremely low TDS “may also be unacceptable because of its flat, insipid taste.” In other words, you can have too few minerals as well as too many. This is why distilled and reverse-osmosis water, which is nearly mineral-free, often tastes like “nothing.”

There is a deeper layer too. A 2017 study in Nature Neuroscience by Zocchi, Wennemuth and Oka found that the mammalian tongue detects plain water using the same acid-sensing taste cells that register sour flavors, rather than through a dedicated “water” receptor. That helps explain why slightly acidified, flat water from an old glass registers as faintly “off”: your taste system is partly reading water through its sour-sensing machinery. If you are curious about the wider mechanics, here is our explainer on how we taste things.

How Long Is Water Good After You Open It?

So how long can you actually keep water around? For a sealed, store-bought bottle, the honest answer is “a very long time.” The U.S. Centers for Disease Control and Prevention (CDC) advises simply observing the printed expiration date on store-bought water for an emergency supply. Distilled water follows the same logic: sealed and stored cool, it keeps indefinitely, since there is nothing in it to spoil. Once you crack the cap, though, the clock starts, because dust and microbes from the air can drift in, especially with mineral-free distilled water that has no chlorine to discourage them.

For water you bottle yourself at home, the CDC is stricter: replace it every 6 months, and store containers somewhere cool, between about 10 and 21 °C (50 to 70 °F), and out of direct sunlight. The same cool-and-dark rule is exactly what keeps a store-bought bottle tasting fresh, since heat and light are what speed up the plastic and the air doing their work on the flavor. None of this means the water has “rotted”; it is about taste and keeping airborne hitchhikers out. (Distilled water, by the way, is not the same as boiled water, as we explain in our piece on distilled versus boiled water.)

In short, water itself does not go bad. Whether it is an open glass on your nightstand or a bottle at the back of a cupboard, the change you taste is the air and the container at work, not the water rotting. Still, it is a good idea to drink that glass right after taking it out of the fridge, before you’re distracted by the next shiny thing and forget all about the refreshment you were seeking!