Alcohol evaporates from an open glass due to fundamental physical chemistry differences between the two main components of an alcoholic beverage: ethanol and water. Understanding this phenomenon requires looking closely at how liquids transition into a gaseous state, known as volatility.
Why Alcohol Evaporates Faster Than Water
Ethanol evaporates faster than water due to its inherent molecular structure and resulting weaker intermolecular bonds. Both molecules are polar and form hydrogen bonds that hold the liquid together, but water can form two bonds while ethanol forms only one. This difference gives water significantly stronger cohesive forces, making its molecules more reluctant to escape the liquid phase. Ethanol’s weaker attraction means its molecules require less energy to break free and become a gas, even at room temperature.
This tendency is quantified by vapor pressure, which is the measure of a substance transitioning into its gaseous state. At typical room temperature, ethanol’s vapor pressure is more than twice that of water. This higher pressure indicates that a larger proportion of ethanol molecules possess sufficient kinetic energy to overcome the forces holding them in the liquid. Consequently, pure ethanol has a much lower boiling point (78.3°C) compared to water (100°C). Since less energy is required for this phase change, the alcohol component is more volatile and escapes first.
Conditions That Speed Up Evaporation
The rate at which alcohol turns into vapor is significantly influenced by the surrounding physical environment.
Temperature
Higher heat translates directly to increased molecular energy. When a drink warms up, more ethanol molecules gain the necessary energy to overcome their weak intermolecular bonds and escape the surface, dramatically speeding up the evaporation rate.
Surface Area
The design of the glass determines the exposed surface area of the liquid. Wider, shallower glasses, such as a coupe or an old-fashioned glass, present a greater area for molecules to escape compared to a narrow, tall glass. Increasing the surface area directly increases the number of liquid molecules positioned at the interface with the air.
Air Movement and Humidity
Air movement, whether from a draft or a fan, accelerates the process by sweeping away the alcohol vapor that accumulates just above the liquid surface. This vapor layer, if left undisturbed, becomes saturated with ethanol molecules and slows down further evaporation. Removing this saturated layer constantly exposes the liquid to fresh, unsaturated air, maintaining a fast evaporation rate. Low ambient humidity also increases the rate by creating a greater concentration difference between the liquid and the surrounding air.
How Long Does It Take to Affect Drink Strength
Since ethanol evaporates faster than the water it is mixed with, the overall concentration of alcohol in an open drink decreases over time. This effect is most pronounced in low-alcohol beverages like wine, which typically have an alcohol content around 13% by volume. Studies have shown that a glass of wine left exposed for about six hours can lose up to 20% of its original alcohol content.
For high-proof spirits, such as whiskey or vodka at 40% ABV, the change in strength is much slower over a short period. At this specific concentration, the mixture of ethanol and water molecules forms an azeotrope-like blend, causing the two components to evaporate at rates that are closer to each other. This means that as the total volume decreases, the remaining liquid’s concentration stays roughly consistent.
For a standard drink left out for a short duration, such as 30 to 60 minutes, the change in concentration is generally negligible and unlikely to be noticed by the average person. However, the effect becomes quite significant over many hours or days, especially for lower-proof drinks left uncovered. If a spirit is left open overnight, the volume loss will be noticeable, and the overall strength will be slightly reduced.