Many people notice a “stale” or “flat” taste when drinking water that has been sitting out. This subtle change in flavor often prompts curiosity about what happens to water when exposed to its environment. While water doesn’t truly “spoil” like food, scientific processes alter its composition and taste.
The Chemical and Biological Changes in Stored Water
Freshly poured tap water contains dissolved gases, particularly oxygen and carbon dioxide, which contribute to its crisp, refreshing taste. Over time, these gases dissipate into the surrounding air, making the water taste less vibrant or “flat.” This loss is a primary reason for perceived staleness.
As water sits exposed to the air, it absorbs carbon dioxide from the atmosphere. This reacts with the water to form carbonic acid, which slightly lowers the water’s pH, making it more acidic. This increase in acidity can contribute to a subtle change in taste, often described as bitter or stale.
Water, even from a treated municipal source, contains naturally occurring microorganisms. When left exposed, especially in warmer temperatures or with light, these microorganisms can multiply. While usually not harmful in short periods, their growth can contribute to off-flavors and odors.
The container can also alter water’s taste. Materials like plastic can leach trace chemicals, such as antimony or BPA, into the water over time. This leaching, often accelerated by heat and prolonged storage, imparts an undesirable flavor.
Temperature significantly influences these chemical and biological changes. Warmer water loses dissolved gases more quickly and accelerates both chemical reactions and microbial growth. Cold water often tastes more refreshing because cold temperatures help retain dissolved gases and inhibit microbial activity.
Safety Considerations for Drinking Stale Water
For most people, drinking tap water left out for a few hours or overnight is generally safe. Taste changes are primarily aesthetic, resulting from gas exchange and minor chemical shifts, not significant health risks. Water does not contain sugars or fats that would cause it to “rot” like food.
However, certain factors can increase safety risks. If water has been exposed to airborne contaminants like dust, pollen, or insects, or if the container is unsanitized, impurities can be introduced. Bacteria from your mouth can also transfer if you drink directly, potentially multiplying over time, especially in warmer conditions.
Water left out for several days or weeks, particularly if uncovered or in unsanitary conditions, might accumulate significant bacterial growth or contaminants. Tap water contains chlorine that initially inhibits bacteria, but this dissipates when exposed to air, reducing its protective effect after one to five days. If water develops a visible film, unusual particles, or a strong, unfamiliar smell, discard it.
Tips for Keeping Your Drinking Water Fresh
To maintain fresh taste, store water in clean, airtight containers. This prevents absorption of ambient odors and gases, and protects against airborne dust and other contaminants. Glass or stainless steel containers are often preferred as they do not leach chemicals into the water like some plastics can.
Refrigeration is an effective way to keep water tasting fresh. Colder temperatures slow the dissipation of dissolved gases, keeping the water crisper, and significantly inhibit microorganism growth. Storing water in the refrigerator helps maintain its initial quality for longer.
Using a water filter can improve the initial taste of tap water by removing impurities like chlorine, sediment, and certain minerals that can contribute to off-flavors. While filters enhance initial taste, filtered water should still be stored properly for optimal freshness, especially since it may lack chlorine to inhibit bacterial growth.
Regularly refreshing the water in your glasses or pitchers is a simple practice to avoid staleness. Replacing old water with fresh ensures optimal taste and composition. This also reduces the chance for significant microbial buildup.
Avoid storing water in direct sunlight. Sunlight, particularly UV rays, can degrade plastic containers, accelerating chemical leaching. It also promotes the growth of algae and bacteria, which can affect both the taste and safety of the water.