When a bottle or can of beer is opened and left exposed to the air, the liquid becomes an unstable mixture of water, ethanol, and dissolved carbon dioxide. Beer typically contains 4% to 7% alcohol by volume, making it a water-based solution. Exposure to the atmosphere immediately initiates several chemical and physical changes that affect the composition and flavor of the beverage over time. This prompts the question of how much alcohol remains.
The Physics of Ethanol Evaporation
The fundamental reason alcohol can leave an open beer is its volatility, which measures how easily a substance turns into a gas. Ethanol, the type of alcohol found in beer, is significantly more volatile than water due to differences in molecular structure and bonding. Water molecules form stronger, more numerous hydrogen bonds, requiring more energy to transition into the gas phase.
Ethanol’s boiling point is 78 degrees Celsius (173 degrees Fahrenheit), compared to water’s 100 degrees Celsius (212 degrees Fahrenheit). Even at room temperature, the lower boiling point means ethanol requires less energy to escape the liquid surface as vapor. This difference dictates that ethanol will preferentially evaporate from the beer compared to water.
The actual rate of alcohol loss is relatively slow, especially when the beer is kept cool. Although ethanol is more volatile, the beer is primarily water, and the two liquids interact as a solution that influences the evaporation of both. A measurable, though not complete, loss occurs only after days or even weeks of being left completely open.
Alcohol Loss Versus Carbonation Loss
The most immediate and noticeable change in an open beer is the rapid loss of carbonation, not alcohol. Beer contains dissolved carbon dioxide (\(\text{CO}_2\)) gas, which is held under pressure in the sealed container. When the seal is broken, the pressure above the liquid drops instantly to normal atmospheric pressure.
This sudden pressure drop causes the \(\text{CO}_2\) to become far less soluble, forcing it to rapidly escape the solution. The tiny bubbles of \(\text{CO}_2\) that rise to the surface carry away the gas, leaving the beer tasting “flat.” This carbonation loss occurs within hours, long before any meaningful amount of ethanol has evaporated.
The loss of \(\text{CO}_2\) is the primary reason an abandoned beer tastes unpleasant the next day. A flat beer still contains virtually the same percentage of alcohol as it did when fresh, because \(\text{CO}_2\) solubility is separate from alcohol evaporation. The negligible alcohol loss over a single night means the strength of the beverage remains practically unchanged.
Impact on Flavor and Safety
Beyond the loss of carbonation, an open beer left exposed to air begins to suffer from oxidation. Oxygen from the surrounding air dissolves into the beer and reacts with organic compounds, leading to the development of stale, off-flavors. This reaction often imparts a distinct papery, wet cardboard, or stale taste.
Oxidation can also lead to sherry-like or winy notes, particularly in higher-alcohol or darker beer styles. These flavor defects are exacerbated by warmer temperatures, accelerating the degradation of the beer’s intended profile.
Despite the drastic change in taste, a beer left open for a day or two remains safe to drink. The alcohol content is largely preserved, and the environment of the beer, with its low \(\text{pH}\) and presence of ethanol, is not conducive to the growth of harmful pathogens. Although the beverage will be flat and taste stale due to carbonation loss and oxidation, the negligible alcohol evaporation means its strength is preserved.