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If I pour vodka (40% abv) into an open glass, and then leave it out overnight (or for some time), what percent ABV does it stabalize at? Or does all of its alcohol content evaporate (leaving just barley-water in its wake)?

Also, as a followup, does the same behavior apply to other alcohols, like whiskey? And finally, at what rate roughly does vodka lose ABV at room temperature?

  • I don't know about room-temperature evaporation specifically, but when mulling (heating) an alcoholic beverage I understand the alcohol and water evaporate at very nearly the same rate - there may be a skew, but it is relatively little and would take a long time, and an obvious volume reduction, to change the alcohol to water ratio by even a few %. I would expect the vodka to stay at nearly 40% until the glass evaporated dry. – Megha Mar 22 at 4:33
  • Possible duplicate of Cooking away alcohol – Megha Mar 22 at 4:40
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    I'm specifically referring to at room temperature, where alcohol will evaporate rapidly, relative to other liquids like water – chausies Mar 22 at 4:48
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    @Aethenosity I'm going to go with 70 degrees. But a few degrees off shouldn't make a tremendous difference, and all I really want is a rough answer. – chausies Mar 23 at 4:14
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As a chemist I'd say that the question is really impossible to answer as given.

The gist is that for a water/ethanol mixture both components will evaporate. So if the liquid is left out long enough, the liquid will evaporate completely.

The atmosphere contains essentially no ethanol so an equilibrium between the whole atmosphere and the ethanol in a glass can never be reached. There is water in the atmosphere (aka humidity) so the evaporation of water could be retarded, or prevented entirely if the "local" humidity is 100%. But unless the "local" humidity is already 100% then an equilibrium between the liquid and the atmospheric water won't be reached either.

Pure ethanol has a higher vapor pressure and thus evaporates faster than pure water (in 0% humidity). So the gist is that as liquid evaporates the water/ethanol composition of the liquid will change. Thus the relative rate of evaporation of water and ethanol changes too.

In terms of the rate of evaporation there are a number of unspecified parameters.

  • How much volume compared to the exposed surface area? So liquid will evaporate more slowly from a tall narrow glass than a short wide one.

  • Temperature? The higher the temperature the faster the evaporation.

  • Air currents? If you use a fan to blow air across the glass evaporation will occur much faster than if a fan isn't used.

  • I just want the roughest of estimates so that I have at least some idea of the time frame one should have in mind. So can you think of an answer with the most vanilla settings? 70 degrees Fahrenheit, no wind/fan, 50% humidity, my glass is 9 cm in diameter. If you need any more reasonable assumptions, please state them. But it'd be great if you could make a rough calculation from this to give some sense of the time scale. Also, it'd be helpful if you could cite the relevant formula you're using. – chausies Mar 23 at 4:12
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No alcohol evaporates at room temperature, so 40% vodka will be 40% in the morning.

Alcohol evaporates at 174 degrees, only cooking can reduce the abv.

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    If I'm not mistaken, there exists a scientific effect that even affects water such that some content of a liquid will change to vapor form (even below boiling temperatures) because particles will randomly have enough velocity to escape. Also, I can easily speak from experience that alcohol left out tastes like water the next day – chausies Mar 22 at 4:52
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    Evaporation and boiling are two different things. The vodka let open surely loose more alcohol than water but this depend on temperature and relative humidity of the room, as well as time. Calculations are possible but a fair guess is that after one night is still substantially the same vodka. @Megha comment is incorrect as well, but it matters less because, again, we are not talking of boiling and distillation. – Alchimista Mar 22 at 9:21
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    Alcohol evaporates at any temperature above its freezing point. That is -174.6°F for ethanol, so maybe that's where you got your number? – Aethenosity Mar 22 at 16:14
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    You should read up on vapor pressure. – Kevin Mar 22 at 23:37
  • Corrige: substantially the same vodka was thinking of a bottle. With an open glass it will be a substantially less alcoholic vodka though likely still a hard spirit. – Alchimista Mar 24 at 14:24

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