Activity coefficient
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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.
The activity coefficient of a species "i" in a mixture is proportional to the ratio of the fugacity of species "i" calculated as a mixture component to the fugacity of species "i" calculated as a pure substance.
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.
At 95% ethanol, the activity coefficients are the same, meaning that the vapor is also 95%. This is called an azeotrope.
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.
For more details on this subject and activity coefficients in ionic solutions , see activity and the Debye-Hückel equation
