Under typical room-temperature conditions (i.e., around 70 degrees with moderate humidity), if I were to leave a glass containing, say, a liquid that was 5% alcohol by volume, would the alcohol evaporate faster than the water? Would this answer change depending on whether the liquid was just ethanol and water vs. if it had other ingredients in the solution (e.g., wine)? Thanks!
Not a chemist, but I think the answer is yes. Alcohol feels cool on the skin because of rapid evaporation.
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I think the answer would be found in the vapor pressure numbers. According to a chart here for water, the vapor pressure at 20 C is 2.34 kPa, while the similar chart for ethanol shows a vapor pressure of about 5.3 kPa at 19 C.
So there needs to be more than twice as much ethanol in the air as water under similar conditions for the air to be saturated. Which I'm pretty sure means that ethanol will evaporate more quickly, though I don't know the exact equation for speed of evaporation.
Oh, also ethanol boils at 78 C compared to water at 100 C, which I'm sure contributes. It's closer to the boiling point than water is at room temperature.
You think right! In essence vapor pressure is really what determines the boiling point in the first place, when the equilibrium vapor pressure (which is a function of temperature) of a liquid matches the overall pressure of the system (say 1 atmosphere) it will boil. For example, the vapor pressure of water hits 1 atm at 100 C, so it boils.
This isn’t a peer reviewed article but check this page out if your interested in learning a bit more about it: https://www.chem.purdue.edu/gchelp/liquids/vpress.html
Yes – there's a couple of things that make a difference.
The boiling point of ethanol is lower than that of water so more molecules of ethanol will have enough energy to escape the liquid and rush off into the air as a gas than molecules of water. Over time this adds up and more of the ethanol will evaporate than the water. This means beer gets weaker as it's left out.
Another interesting thing is that ethanol will actually gather at the water surface. Not all of it, but some. I'm not suggesting that the surface of your beer is noticeably more alcoholic but the very surface of the liquid will have a surplus of ethanol molecules. This is because water hates air – this is why droplets are so round, to reduce the amount of surface for a given volume – and ethanol has one end (OH) that gets on well with water and another end that doesn't really care too much what it is pointed towards (CH3CH3). The fact that ethanol gathers at the surface might actually help more ethanol evaporate over time compared with water!
Interestingly if you were to take an alcohol with a really long CH chain at the end like dodecanol and spread that at the surface it might actually stop the water evaporating all together – it can form a film over the top and stop water escaping while the dodecanol molecules are too far from their own boiling point to evaporate themselves.