ultrafilter, I would think 40 mL IS noticeable. However, while your math is correct, the chemistry may not be. You see, the entire problem is that volume isn’t really additive. It’s approximately so, but not in actuality, due to chemical interactions between the two things you’re adding to each other. So sometimes adding 40 mL of one thing to 40 mL of another will give you 80 mL. Sometimes you’ll get more, and sometimes you’ll get less. I seem to recall you get less with water and ethanol. But it’s been many years since I saw the light and moved from chemistry to physics, so my memory for some of the fine details has slipped.
For instance, I couldn’t remember if the term I wanted in my last post was azeotrope or eutectic (although to give myself a big pat on the back, I THOUGHT it was an azeotrope). This is only hearsay, but Caldazar, I seem to recall that the way that dry ethanol is made is distillation to the azeotrope, whereupon benzene is added to break it, and then distillation again. Benzene being a very bad thing to ingest, this is why I’ve never had any desire to try pure ethanol. But surely they don’t use something like THAT in making something for public consumption; I can’t see that being allowed. I’d think molecular sieves would be an expensive way to go about it, although I suppose reverse osmosis is a possibility. It still seems like adding an entrainer is the most obvious choice, but now I’m really quite curious about what they actually do. Ugh.
Fascinating subject.
I started a thread on that very subject the other day, but it seems to have slipped through without being noticed. If you have any more ideas on non-ideal fluids, please let us know!
Ugh. Well, I’ll give it a go, then, but you’ll have to wait until I have a chance to whip out my copy of Atkins and refresh my memory.
I was able to find that the ethanol-water azeotrope is at 96% ethanol, which means that an ideal solution with 96% ethanol and 4% water would have, say, 750 mL of alcohol and another 30 mL of water. I’m sure you’d be able to observe this, and really, while ethanol and water form a non-ideal mixture I’m skeptical that there’s so small a change in the total volume that you wouldn’t notice it as things were postulated. Here’s my best SWAG for the explanation:
Say you have 1 liter of 100% alcohol. If you expose it to the atmosphere, some of it will evaporate. This is because there is always some number of molecules that have enough energy to evaporate (i.e. ethanol has a non-zero vapor pressure at room temperature; it’s actually about 40 mmHg, according to the CRC). Thus, some of the ethanol will evaporate off at the same time as water is entering the stuff. Apparently, the amount of alchol that evaporates is roughly compensated for in volume by the amount of water that enters the solution.
IOW, of your original 1 liter bottle of ethanol, you’ll still have around 1 liter of stuff at the end, but about 40 mL of ethanol will have evaporated and about 40 mL of water will have been absorbed from the atmosphere, leaving you with a 96% solution. I haven’t done the calculations to back this up, but it seems plausible. (At this point, of course, the mixture may evaporate away still, but what’s left behind won’t change in composition.)
As has already been mentioned, opening a container of 100% ethanol and exposing it to moist air will drop the alcohol concentration to 95% fairly quickly. This is because ethanol has an affinity for water and will absorb the moisture from the air. The final volume of liquid will not be 790 mL if you started out with a 750 mL container of 100% ethanol, but rather will be something much closer to 750 mL.
The reasoning behind this is that ethanol-water mixtures are extremely non-ideal. The oxygen atoms in the ethanol molecules are strongly attracted to the hydrogen atoms in water, forming hydrogen bonds. This is the thermodynamic stability an above poster mentioned. The hydrogen bonding has the net effect of decreasing the effective volume of the mixture as the molecules are attracted to one another. Hydrogen bonds are relatively strong intermolecular forces, so the “volume-shrinking” effect will be quite large. Evaporation of ethanol does play a small part in decreasing the final volume of liquid, but ethanol doesn’t evaporate THAT rapidly, certainly not tens of mL in the span of a few seconds.
Except for extremely simple or weakly non-ideal mixtures, this volume effect is too complex to be derrived using only scientific principles, so the non-ideal mixtures are approximated using empirically-derrived models (there are several of them, each with their own strengths and weaknesses).
For industrial ethanol, the azeotrope is broken with entrainers that are typically not fit for human consumption (benzene, toluene, etc…), so I agree that azeotropic distillation is probably not the way Everclear is made. If I had to manufacture 100% beverage ethanol, I’d probably choose to use molecular sieves in a pressure swing adsorption unit myself. Using a membrane in a reverse osmosis process sounds like it would grow expensive quickly as the membrane separation unit might have to be shut down periodically and the membrane cleaned. But that’s just my WAG; as I said, I honestly have no clue what the Everclear manufacturing process looks like.
And now you know why I’m a physicist and not a chemist any more…
I’m actually rather surprised that the non-ideality is that significant, but since I avoided lab like the plague I guess I really wouldn’t have any reason to remember this…
Hmm…the most I can say is that Everclear isn’t a chemist’s term–I think you can label anything as “Everclear” and sell it. But that’s just a WAG; take it with a block of salt.
On reflection, I think I need to make a clarification on what “much closer” means to me. The non-ideality of water/ethanol mixtures is such that you’d see approximately a 25% reduction in the effective volume of the water you were adding. To continue with the 750 mL 100% ethanol example, if you added 40 mL of water to the ethanol, you’d see an approximate 30 mL increase it the total volume. Although a 25% volume reduction is very significant, “somewhat closer” is probably more accurate than “much closer”.
