A glass of water left sitting out for some time often tastes different, losing its refreshing quality. This change in taste is not merely a perception; it results from specific scientific interactions between the water and its environment. Understanding these processes explains why your water might taste “stale” after sitting on your bedside table overnight.
The Science Behind the Stale Taste
The altered taste of water left out is partly due to chlorine dissipation. Tap water is typically treated with chlorine to disinfect it and prevent bacterial growth within the distribution system. When exposed to air, this chemical gradually evaporates, reducing its distinct “tap water” taste that some people notice. As chlorine leaves, the taste can become “flat” or less crisp.
Water also absorbs carbon dioxide (CO2) from the surrounding air. Water and air constantly exchange gases, and CO2 readily dissolves into water, reacting to form carbonic acid. This slightly lowers the water’s pH, imparting a subtly different, sometimes sour taste.
While less common for short periods, microorganism growth can also influence water’s taste. Airborne particles and microbes can settle into an uncovered glass. Over longer durations, especially in warmer conditions or if contaminated by backwash, bacteria or algae can grow. These activities can contribute to a musty or earthy flavor, though usually not the primary cause of typical “stale” taste overnight.
Is Stale Water Safe to Drink?
For most individuals, drinking water left out for short periods, such as overnight, is generally safe. Taste and odor changes are primarily aesthetic and pose no health risk. Minor chemical alterations, like chlorine loss and carbon dioxide absorption, do not make the water harmful.
Safety concerns arise with prolonged exposure or contamination. Uncovered water left for days can accumulate dust and microorganisms, increasing bacterial levels. Saliva contamination from repeated sips introduces bacteria that multiply, especially in warm environments. While your own mouth bacteria are unlikely to cause illness, sharing water can transfer different bacteria, potentially leading to health issues.
To maintain freshness and safety, use covered containers, like screw-top bottles, to protect against airborne contaminants. Regularly cleaning water bottles also prevents bacterial buildup. Storing water in a cool, dark place deters microbial growth and prevents chemical leaching from plastic containers exposed to heat or sunlight.