The Activity coefficient for chemicals in a mixture is an indicator of what the concentration of that chemical will be in a vapor of the mixture.

In an ethanol-water mixture with less than 95% ethanol, the activity coefficient for ethanol is higher than water. Therefore, the concentration of ethanol in the vapor from the liquid will be higher than in the liquid itself. Ethanol, also known as ethyl alcohol or grain alcohol, is a flammable, colorless chemical compound, one of the alcohols that is most often found in alcoholic beverages. ... Water (from the Old English word wæter; c. ...

At 95% ethanol, the activity coefficients are the same, meaning that the vapor is also 95%. This is called an azeotrope. An azeotrope is a liquid mixture of two or more components which has a unique constant boiling point. ...

Above 95% ethanol, the activity coefficient for water is higher than ethanol, so the fraction of water will be higher in the vapor than in the liquid.

References

1. Activity Coefficient on IUPAC

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