What Happens to the Alcohol Concentration When Bottles of Beer Are Left Open?

Water and alcohol will evaporate at different rates from an open container.

Evaporation will occur whenever a liquid is exposed to open air. Under normal temperature and humidity conditions, alcohol evaporates at a slightly faster rate than water, so the alcohol concentration will slowly decrease over time.

Cause

Beer is composed primarily of ethyl alcohol suspended in water. When exposed to the air, some molecules at the surface will leave the liquid in a process known as evaporation. Alcohol molecules are more volatile, so they will escape at a faster rate than water molecules.

Factors

The rate of evaporation is affected by many factors, such as the amount of surface area exposed and the temperature and humidity of the surrounding air. If an opened bottle of beer is left in a refrigerator overnight, the amount of alcohol that evaporates will likely be negligible, due to the low temperature and humidity.

Bottom Line

Leaving a bottle of beer open for an extended period of time can have many negative effects (such as loss of carbonation), but loss of alcohol content is usually trivial. Moreover, water is evaporating at the same time, so the main effect is a gradual loss in volume rather than alcohol content.

References

• Straight Dope Message Board: Ethyl Alcohol Evaporation
• Educational Electronics: Heat--Part III