Taking a sip of water that has been sitting in a glass overnight is often met with a flat, lifeless, or subtly unpleasant taste. What began as a refreshing drink from the tap transforms into something noticeably “stale” after just a few hours of exposure to the open air.
This shift is not a sign that the water has gone bad in a harmful sense, but rather a clear indication that a series of subtle chemical and physical changes have taken place. These alterations occur as the water attempts to reach a new state of equilibrium with its surrounding environment, affecting its dissolved gas content, chemical purity, and microscopic biological makeup. The combined effect of these environmental interactions is what ultimately changes the water’s flavor profile, producing the distinct taste we perceive as unappealing.
The Escape of Dissolved Gases
The initial perception of water tasting “flat” or less refreshing is largely a result of changes in its dissolved gas content. Tap water, especially when cold and fresh from the faucet, contains dissolved gases like oxygen, nitrogen, and a certain amount of carbon dioxide, which contribute to its crisp, satisfying mouthfeel. When this water is left uncovered, the temperature rises to match the room, which significantly reduces the water’s capacity to hold these dissolved gases.
The dissolved oxygen and nitrogen begin to escape, or off-gas, into the air, causing the water to lose its characteristic aeration and acquire a noticeably duller taste. Simultaneously, the water begins to absorb atmospheric carbon dioxide (\(text{CO}_2\)). As \(text{CO}_2\) dissolves, it reacts with the water to form a small amount of carbonic acid (\(text{H}_2text{CO}_3\)). This reaction causes a slight decrease in the water’s \(text{pH}\), making it marginally more acidic. This subtle change in acidity alters the overall flavor, contributing a faint, slightly sour, or metallic note.
Airborne Absorption of Environmental Compounds
Water is often called the “universal solvent” because of its capacity to dissolve more substances than any other liquid. This characteristic extends to absorbing gases and compounds from the air around it. When a glass of water is left out, its exposed surface area acts as a collector, readily absorbing volatile organic compounds (VOCs) and other airborne molecules from the immediate environment. These VOCs are carbon-containing chemicals that easily evaporate into the air and are abundant in a typical indoor setting.
Common household sources of these airborne contaminants include cleaning products, fresh paint, new carpet, cooking fumes, and plasticizers leaching from nearby containers. The water absorbs these compounds, and they subtly alter its taste, leading to flavors that can be described as medicinal, metallic, or chemically “off.” The longer the water sits exposed, the greater the concentration of these environmental compounds becomes, intensifying the unwanted flavor.
The Role of Microbial Growth
For water that has been sitting out for a longer period, generally beyond 12 to 24 hours, the taste change becomes less about chemistry and more about biology. Tap water is not sterile; it contains trace amounts of harmless bacteria which are initially suppressed by the residual chlorine added during the treatment process. As the water sits at room temperature, this chlorine dissipates into the air, eliminating the primary defense against microbial proliferation.
The warmer temperature of standing water provides an ideal environment for the remaining bacteria to multiply rapidly. This accelerated growth leads to the production of metabolic byproducts. Two compounds, geosmin and 2-methylisoborneol (\(text{MIB}\)), are the primary culprits for the truly unpleasant taste. These compounds are responsible for the unpleasant earthy, musty, or moldy flavor sometimes detected in old water, even at extremely low concentrations. If the water was consumed from the glass at any point, introducing oral bacteria and organic material from backwash accelerates the entire biological process, leading to a much faster and more pronounced shift in flavor.