Why is alcohol measured in percentages?
Why is alcohol measured in percentages?
I feel like it would be useful to know exactly how much alcohol is in a can or a bottle. Also why is alcohol the only thing measured in percentages and not sugar or caffeine or medicine?
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I think alcohol can't be kept pure in normal conditions, it would turn into a gas. So it is diluted by necessity. Knowing the full volume of the thing is certainly better than knowing only the quantity of alcohol without knowing the degree of dilution.
It is not the only product to be indicated this way. All diluted chemicals are. I think syrup is also?
The boiling point of pure ethanol is 78°C / 173°F [1], so it would not turn into a gas at normal conditions.
Pure ethanol is very hard to produce because it forms an azeotrope with water in the ~96% [1] range, meaning it cannot be distilled further than that, it must be separated by other means.
Alcohol in spirits is usually diluted because of taxes on higher percentage alcohol, consumer preference, or being able to make more bottles with the same amount of ethanol. I believe using percentages for alcohol began when ye olde England was putting taxes based on the percentage of alcohol in the drink [2].
[1] - https://en.wikipedia.org/wiki/Ethanol [2] - https://en.wikipedia.org/wiki/Alcohol_proof
it would not turn into a gas at normal conditions.
It does: https://en.wikipedia.org/wiki/Vapor_pressure. In an airtight container you would have an equilibrium of alcohol vapor and liquid. In open-atmosphere, the atmosphere basically behaves like an infinitely large volume for the vapor - so the alcohol will completely vaporize (and cool the surface it is on in order to do so).
It's also trivial to demonstrate by pouring alcohol onto a surface, it disappears in seconds. Same with gasoline and numerous other liquids you've surely seen do this (another example is hand sanitizer, which is basically pure alcohol).
Being diluted doesn't really help with any of this though. Also alcohol is kept in bottles, which are usually airtight until they are first opened.
I mean sure, but you could say that about any liquid, or even any solid for that matter (with sublimation). But that’s not really what I was talking about when I responded.
It was what the GP was, though.
To take your train of thought one step further, think about gasoline. Gasoline has a high vapor pressure, maybe even higher vapor pressure than ethanol. Would you say that gasoline is a liquid at normal conditions?
If you throw gasoline out on to the pavement it will evaporate away. If you keep it in a gasoline can it will not. In a gasoline can the liquid and gas will reach equilibrium, though you'll certainly have slightly less liquid than what you started with. If the can isn't sealed then, yes, all the gasoline will eventually evaporate away - even at STP.
And, again, this is all trivial to test at home by using some hand sanitizer. Another example is your skin does not remain wet with water forever, despite human skin temperature not being 100°C. It's an everyday phenomena, I'm not sure what you're trying to argue against here. It's not my "line of thinking," it's objectively reality.
As for your distillation problem, the issue isn't that some alcohol remains in the water - it's that some water evaporates alongside the alcohol during the distillation process at the boiling point of alcohol - due to, guess what, vapor pressure. That's called an azeotrope - clicking through to that Wikipedia page might have helped.
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