How come alcohol is measured in proof or ABV when all other drinks, like caffeine for example, measure with milligrams or milliliters or whatever?
How come alcohol is measured in proof or ABV when all other drinks, like caffeine for example, measure with milligrams or milliliters or whatever?
user
So, proof and ABV are metrics for alcohol concentration.  It’s like the proof is how you would pour a sample of alcohol on a scale to get a specific amount, but for larger amounts, it’s more convenient to use weight as a reference unit, and there are standard measurements of different volumes.  So you would pour a specific amount of alcohol into a container and weigh it to see how much proof it had.  For example, 2.5 oz of 100 proof vodka, or 1 1/3 oz of 160 proof vodka, or 8 oz of 80 proof vodka.
assistant
I get most of that, but I'm still not understanding why it's measured the way it is. Would it not be possible to say 'this 1 liter bottle of vodka has X milliliters of alcohol in it?'
user
Yeah, you could technically measure alcohol concentration in different units, like milliliters, and probably even different sizes of containers.  But the primary metric is based on the amount of alcohol that has dissolved into the beverage, which we call the alcohol concentration.
assistant
So is there a reason alcohol is measured like that as opposed to all other substances being measured in different units like milliliters or milligrams?
user
Not really.  In a scientific context, the specific measurement for ABV and proof is based on the fact that ethanol (the alcohol in alcohol) has a lower boiling point than water.  The measurement is the amount of proof or ABV in the bottle (not including the portion of the beverage that's water).  Because water boils at a higher temperature than ethanol, there's a scale of temperature measurements where you can see how much proof a liquid has (when the amount of ethanol is directly proportional to the percentage of alcohol in the liquid).  When the liquid evaporates, its boiling point is lowered by adding ethanol, and this is reflected by the change in the scale of the temperature of boiling.  I think it's easiest to understand why this works if we have a little bit of practice making measurements.  Here's a practice:
assistant
I don't see the practice..
user