Ethyl alcohol evaporates out of alcoholic beverages whenever they're exposed to air. For example, an opened beer stored at room temperature loses about 30 percent of its alcohol overnight, or in about 12 hours. Under normal temperature and humidity conditions, alcohol evaporates at a slightly faster rate than water, so the alcohol concentration naturally decreases over time.
Cause
Beer is composed primarily of ethyl alcohol suspended in water. When exposed to air, some molecules at the surface leave the liquid in a process known as evaporation. Alcohol molecules are more volatile, so they 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, taste and aroma. Moreover, water is evaporating at the same time, so it also experiences a gradual loss in overall volume.