I apologize for creating yet another thread on Alcohol concentration, but reading another thread made me think of a related phenomenon that I’ve never had adequately explained.
The strength of (drinking) alcohol is normally referred to in percent by volume. Naively one would think of a 40% solution as being two parts alcohol to three parts water.
I do however distinctly remember reading that if one were to mix two parts ethanol with three parts water, the resulting solution would occupy slightly less than five parts. This effect was supposed to come from the different bond types, and the fact that ethanol is ‘partialy non-polar’.
Supposedly this was what Mendeleyev (of periodic-table fame) wrote his doctoral thesis about (According to this web-site the English translation of the title was “On the Combinations of Water with Alcohol”. - It doesn’t say much more about the thesis though.)
Now we’re getting to the GQ:
[ul]
[li]Is this effect real?[/li][li]How big is it?[/li][li]If it is real, how is the alcohol content measured? Does 40% mean that the alcohol (if separated from the water) would occupy 40% of the volume of the diluted solution? Or that the amount of alcohol is [sup]2[/sup]/[sub]3[/sub] of the volume of water? Or something else?[/li][/ul]
I’ve considered starting a thread about this question. Yes, the phenomenon is real, but I have never been able to figure out how big an effect it is. Volumes are additive only for “ideal solutions”, and water/alcohol is a “non-ideal solution”. One of my thermodynamics texts says something about “apparent molar volume” but I can’t seem to make heads or tails of it.
It has been almost a yean, and plenty of knowledgeable people have joined us since I asked this question. I have done some additional research, but I have found nothing firm.
Let me just pop this question to the top again, to see if any of our newcomers have any ideas.
I hope our esteemed mods don’t consider this undue bumping…
I know if you add 1 mole of water (18 ml) to a large volume of ethanol, the volume of the resulting solution will only increase by 14 ml. Chemists say that the “partial molar volume” of water in ethanol is 14 ml.
Some substances, like magnesium sulfate, actually have negative partial molar volumes, so if you put them in water the water will shrink.
As for “percent by volume,” it means the volume of the solute divided by the volume of the solution times 100. So if you have 100 ml of whiskey that’s 40 percent alcohol by volume, and you could separate out the alcohol, you would have 40 ml of alcohol, and presumably more than 60 ml of water.
Aha, thank you Bob, for the best answer in almost a year…
Still no cites, but at least another person sharing the same gut feeling, with what appears to be some solid science to back it up.
You have to be impressed at the quality of this board, even though it can be a bit slow at times