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It's often believed that the strength of alcohol is measured in Percent Proof. This is supposed to be something you might see on the label of a bottle of drink. The higher the percent proof, the more alcohol the drink has in it. But what percent proof is pure alcohol? And, how does this compare to Percent Vol?!
Well, the first thing is, it's not "percent proof". If you look closely at the label, you'll see that what you thought was "70 percent proof" is printed as "70º proof". So, it's actually 70º proof (70 degrees proof), not "70% proof" (70 percent proof). It's true, you know: Look for an old bottle and if you can find one, see, and notice it is no-many º proof.
The measure of degrees proof has an old-fashioned tradition. In the old days, it wasn't that easy to get a brew checked to see what percentage alcohol was in it, so instead there was a simple test where some gunpowder was soaked in the spirit and then set fire to. If it burned, then it was at least 100º proof. I don't know how they measured other degrees of proof, but I know that 100º proof was calibrated by the method of soaking some gunpowder in strong liquor and setting fire to it.
In the British system of measurement, pure alcohol was 180º proof. In continental Europe, pure alcohol was 190º proof, and in the USA, pure alcohol was 200º proof. Clearly such mismatches of booze measurement needed mending, as it needed to be consistent across the world. Otherwise it would be if folk were still using cubits to measure buildings, measuring between their elbow and finger tip, rather than having a pure calibrated metre, yard, etc. The obvious choice with alcohol concentration is the %vol. This is the percentage of the stuff (by volume) that is alcohol (C2H5OH). On the %Vol scale, 100% Vol is always pure alcohol.
Many types of available strong liquor are 37.5% vol alcohol or 40% vol alcohol. That is the percentage of the drink by volume that is ethyl alcohol (ethanol). It's not done "by weight", and if it was then there would be issues with the fact that alcohol has a lower density than water.
If you like to climb the highest mountain, have the fastest computer, etc, you may also want to see if you can get the strongest spirit, pure 100% vol alcohol. Would it be drinkable? Well, in practice, one of the strongest spirits available commercially in bottles is Brugal 151 which is 151º proof (in the USA system), a spirit manufactured in Puerto Plata in the Dominican Republic. It's 75.5% pure alcohol, so gives a taste of what pure 100% alcohol would taste like. The locals advise tourists against drinking it as it's like "gasolina"! However, it's legal, and you can drink it if you want. I can tell you from personal experience, yes, it is possible to drink it neat.
There are some types of distilled spirit that are even closer to the pure alcohol. However, even with the purest still distilling repeatedly, there's a limit which is about 95% vol. If you want to get alcohol more pure than that you need to desiccate it to remove the residual water. Sodium works, provided you can stop it blowing up. Various chemical companies have 99.99% vol alcohol available. In the law hidebound UK you're not supposed to drink it, but you still have to pay the customs duty on it!
Beware, though, because if you drink pure 100% alcohol, you will be dessicated by the stuff and dehydrated as the pure spirit extracts water from your body. I've experienced something I term a "travelling discontinuity" in the gut. You'll need to (pre-load) drink some tea or orange juice to stop yourself being nobbled by it. However, you can still make a valid claim to be able to drink pure alcohol.
If you acquire some pure alcohol from a chemical supplier, you can also drink it by diluting it 60:40 with pure orange juice. The result has vitamin qualities of orange juice but the alcohol strength of good quality vodka.
In many ways, the %vol (percent volume) is a more realistic measure of alcohol concentration in alcoholic drink than the º proof (degrees proof), as everywhere in the world agrees on a common scientific definition. Quite simply, if you've got a pint of drink and it's 50% vol, then you've got half a pint of pure alcohol and half a pint of water-based stuff which gives it the flavour as well as an aquatic virtue that gives it some sympathy with your body (which for the human is mainly water).
There is a slight anomaly with this, because if you get half a pint of water and half a pint of alcohol and mix them, the result oddly only takes up the space of 0.96 pints. There is a 4% discrepancy. The reason the mixture takes up less volume than it might if volume were conserved, it is that the molecules to-some-extent fit inbetween each other. Someone explained this well by this analogy: The best way to think of it is rocks and sand. If I have a cup completely filled with rocks, then add sand, the top level doesn't increase, because the sand can fit in the spaces between the rocks.
I'm not sure how the official assay of alcohol deals with this, but I would guess that a drink which says 50% vol on the bottle actually contains 50% by volume of fluid in the bottle, which would then infer that if you could somehow separate it into two vessels, one containing the alcohol and the other containing the aqueous portion, the alcohol would be the larger portion.
See, it's not quite "quarts in pint pots", but it is 1.92 pints made by mixing together a pint plus a pint, producing 96% of a quart!
This is an entirely separate issue from the fact that water and alcohol are different "by weight" than "by volume". The specific gravity of alcohol is 0.789
If you are looking at alcoholic drink on the shelves of a supermarket, be careful what you buy as some alcohol has diluted alcohol in it and isn't what it seems!
This site encourages responsible drinking rather than puritanical abstention or reckless boozing. Choose your drink and have fun!
High concentration alcohol, as well as being drink, can also be used as Fuel, which in contrast to oil, will never run out. Hence such expressions as those said by voices in my head as a paranoid schizophrenic, "Brazil is one of the world's leading alcohol-producing nations".