Sometimes I open a can of beer (Large 500 ml imported) and half way through it decide that, meh, I’m ready for bed. So, I stick it in the fridge and go to bed.
The next night, said can of beer is suddenly looking pretty good again. But, has the available alcohol since evaporated? Am I essentially drinking de-alcoholized beer, or does the alcohol somehow maintain a presence after, say 20 hours?
Ethanol (as a 95% ethanol 5% water azeotrope) has a boiling point of 78 C. Beer is generally around 5% alcohol (making the boiling point closer to water), but that doesn’t really matter. What does matter is that you’re keeping a can of beer with a small opening (so evaporation happens slowly anyway) in the refrigerator well below the boiling point. And even if, say, 10 mL of beer evaporated off (which would be a huge amount), you’d only be losing 0.5 mL of alcohol. I’m just surprised the beer still tastes good after being open for 20 hours.
Thanks, and who said anything about taste? 
But anyway, I’m not talking about the entire liquid evaporating, just the more volitile alcohol. I would have assumed it evaporated more readily than the H20 portion of the brew?
Here’s my poorly phrased answer (I’m not a chemist.) Alcohol has an affinity for water, so it “would prefer” to stay in the water than evaporate. So, no, the alcohol isn’t more likely to evaporate than the watery part. If you add wine or spirits to something you’re cooking, not all the alcohol will cook off.
I’ll tell you frankly, I’m not confident in the correctness of this answer. Rather than delete the whole thing, I’ll welcome a more authoritative answer.
I’m not a chemist either but here’s my WAG pending the arrival of a chemist.
Evaporation takes place at the surface because some of the molecules of the liquid have ehough velocity and the correct direction to escape into the space surrounding the liquid. As more and more of the liquid molecules fill the surrounding space some of them begin to reenter the liquid. As long as the number escaping the liquid into the space exceeds those going back into the liquid evaporation continues. Eventually equilibrium is reached and no more evaporation takes place. The pressure in the surrounding space resulting from the liquid molecules in the space is the saturated vapor pressure.
Based on the above, the lower the saturated vapor pressure the slower the liquid evaporates. This is because the lower that pressure the quicker the equilibrium point is reached where the number of molecules passing through the surface in both directions is equal.
At the temperature in your refrigerator water has a much lower saturated vapor pressure than alcohol and so its evaporation will slow down quicker than will that of the alcohol. So I would think the alcohol would evaporate quite readily relative to the water.
Cover the can.
From this chart, we can see that the vapor pressure of ethanol at 0oC is about the same as that of water at room temperature. I’ve left uncovered water containers out overnight, and I’d say evaporation is pretty trivial compared to any significant drinkable volume.
Yes, water has a low saturated vapor pressure and the air becomes saturated easily slowing evaporation. The air in a refrigerator is already at a high relative humidity so the water in the beer won’t evaporate quickly. On the other hand the equivalent relative “humidity” of alcohol vapor in the refrigerator is near zero and the alcohol will evaporate readily.
This is mainly the correct assesment. Notice though that in that graph that ethanol does have a slightly higher vapor pressure. Depending on how precise you want to be, more alcohol will evaporate than water so the concentration of alcohol will decrease slightly. But don’t make the mistake of thinking that the vapor pressure of pure ethanol will be the same as the vapor pressure of ethanol in water. The vapors will merely have a higher percent of ethanol than the solution. Whats more, the beer is sitting in a closed fridge with a small opening in the can so there is very little circulation in the can. A concentration gradient will develop so that the vapor will stay in the can thereby decreasing what little evaporation there is. I occasionally leave my wine open to breath for a day. The alcohol loss is pretty minimal.
On the other hand, if you are doing quantitative analysis of solutions of ethanol, keep the caps on your standards at all times.
For a better answer, you don’t need a chemist, you need a chemical engineer.
I missed the edit window, but it just occured to me that psycat90 might be the person with the best answer. She might actually know how much the alcohol content of wine drops when it is left out.
Uhh. Well, I am flattered you’d think of me, but I think you and athelas nailed it.
I’ll just confuse things by bringing in some winemaking info.
I don’t think any of this info would apply to half a can of beer left in a refrigerator overnight, though. It’s more about say, a barrel of wine in a cellar, being aged for a year or more.
For wine in a barrel, humidity (and temp.) is the deciding factor on how the wine will change.
If humidity is low (less than 60%), water evaporates faster than alcohol and the ethanol content of the wine increases. If humidity is high (greater than 60%), then alcohol evaporates faster than water and the ethanol content drops. About 65-68% relative humidity is required for constant alcohol content.
Like I said, I’m sure this wouldn’t apply to half a can of beer. I don’t know how the carbonation would change things, and I don’t know what the relative humidity of an average fridge is, but I kind of have the feeling it is somewhat low.
Meaning if the above *did *apply, and carbonation doesn’t affect anything, you’re actually getting *more *alcohol by volume, not less, as long as you’re not storing the beer in your veggie/high humidity/crisper drawer.
Or maybe not. Really, that’s just my WAG and I’m not a chemist or chemical engineer or winemaker.
You underestimate yourself psycat90, this is completely the opposite of what I would expect. Chemists and chemical engineers can debate all they want about how a system behaves, but only experience gives the right answer. That’s why I thought of you. I assumed that winemakers would have done the tests.
Im not sure the wine example has much to say in the present case (though it is interesting). Both the beercan and wine barrel is in a non equilibrium situation and will be substantially influenced by transport phenomena and rates. Close to equilibrium more ethanol than water will evaporate in almost any situation. I would bet that would also be the case in the fridge.
I think refrigerators have a high relative humidity because the air is cold and will hold less moisture. Take air at 70 F with 20% relative humidity and cool it down to 35 F. The relative humidity has risen to about 85%.
