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Suppose I open the drinks cabinet, and pour some of the strong stuff into a saucepan.

Now the boiling point of ethanol is lower than water, so if I boil the mixture for a while, all the alcohol will surely evaporate long before the water does.

But how quickly does the alcohol evaporate? Is it all gone by the time the water starts to boil? Or does some remain unless you boil the mixture for a long time? (How long?) Or maybe the alcohol is gone by the time the mixture starts to get slightly warm?

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Certainly water and ethanol and all mixtures thereof will completely evaporate without boiling given enough time.

Water and ethanol form a postive azeotrope that is 95.6% ethanol and 4.4% water and boils at 78.2 degrees C.

If you boil a mixture of water-ethanol that has less than 95.6% ethanol, the concentration of water in the liquid phase will increase, but it will only asymptotically approach pure water as all the liquid boils away. There will not be a time when all the ethanol is gone and you are left with a finite amount of pure water.

If you boil a mixture of water-ethanol that has more than 95.6% ethanol, the concentration of ethanol is the liquid phase will increase.

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  • $\begingroup$ ...so I think what you're saying is that there will always be some alcohol left. (?) Are we talking amount "enough alcohol that you could detect it with sensitive analytical equipment" or "enough alcohol that you can still taste it"? (Bearing in mind that any mixture intended to be drunk will usually have an initial concentration far, far lower than 95%.) $\endgroup$ – MathematicalOrchid Feb 1 '15 at 16:07
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    $\begingroup$ The remaining alcohol content in food after cooking has been previously investigated. While some of the alcohol evaporates quickly, a macroscopic amount can remain even after hours of simmering. You can find more information in this question at Seasoned Advice, the cooking-oriented Stack Exchange. Feels good to link something from there! $\endgroup$ – Nicolau Saker Neto Feb 1 '15 at 16:28
  • $\begingroup$ @NicolauSakerNeto Interesting. So it seems both water and alcohol evaporate simultaneously (though at very different speeds) as you heat. Also, it seems my concoction probably contains A LOT of alcohol still... $\endgroup$ – MathematicalOrchid Feb 1 '15 at 17:22

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