The highest proof alcohol you can buy is Everclear, at 190 proof. That’s nothing! Let’s get together and make an alcohol that’s 200 proof! Except we can’t possibly do that. There’s a physical limit to how pure alcohol can actually get, and we’ll tell you why.

Top image: Auchentoshan Distillery, photo by Alan Jamieson/Flickr.

Ethanol, the business molecule of alcohol, is more volatile than water. Given any set of conditions, it will be more likely to fly away than water molecules. This includes higher temperatures. Heat up a mix of ethanol and water, and more of the ethanol will go away. This proved a bane to liquor makers, until someone stumbled on the secret of distillation. Heat up a mixture of ethanol and water to a point where the alcohol boils but the water (except for the stray molecule or two) does not, and you can make a liquid that’s pure water and collect a steam that’s pure ethanol.

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Few people actually wanted pure ethanol, though. Though a little extra kick was good, only the hardcore drinkers demanded pure alcohol — and they weren’t going to be repeat customers. So it took a relatively long time for people to realize that no one could make pure alcohol or pure water from ethanol and water.

This is because ethanol is not an ordinary mixture, it’s an azeotrope. Instead of boiling purely and separately at two different temperatures, its vapor will form a certain proportion. Steam from alcohol is 95.57 percent alcohol. Get a pot of 95.57 percent ethanol boiling and the steam will be 95.57 percent ethanol right down until the last drop evaporates. That’s the limit.

It seems like it should be enough. (There is, reportedly a Bolivian beverage, Cocoroco, that’s 96 percent ethanol. It’s not legal, but it does exist.) However, leave it to people to try and change it. Someone found a solution. Benzene, when added to a mixture of water and ethanol, will allow more alcohol to steam upwards. What a disappointment that benzene was found to be carcinogenic. Well, we still can dream.

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[Source: Solvent Recycling, Proof: The Science of Booze by Adam Rogers.]