Why is vodka 40 percent alcohol? Is this arbitrary? No — and the reasons are linked to the specific properties of alcohol and water. All of them rather amazingly lead to the “40 percent alcohol by volume’’ label we know so well.
Most liquids shrink as they cool, until they freeze. At about 39 degrees Fahrenheit, however, water does something strange: It starts to expand. When it freezes, water’s solid form, ice, takes up more space. If you put a full bottle of water in the freezer, the water will expand and explode the bottle. (This can be dangerous, so please take my word for it.)
But if you mix 40 parts of alcohol and 60 parts of water, the alcohol acts as an antifreeze, allowing the mixture to stay liquid below zero. So you can leave vodka in a freezer and the bottle won’t burst.
This 40/60 mixture has other fascinating properties. You can pour vodka into an open glass at room temperature and the alcohol and water will evaporate in such a way that it will leave the relative concentrations unchanged. If you come back later, you’ll still have vodka — just less.
The Russian czar Ivan the Terrible (1546-1584) introduced a monopoly on vodka production, making the government responsible for quality. The law apparently allowed barkeeps who watered down vodka to be beaten to death by their customers. But how to check the alcohol content? At 40 percent or more alcohol, you can set fire to the fumes. Add a little water and you can’t anymore.
One more bit of magic: As you add alcohol to water to achieve that 40 percent balance, the mixture becomes more viscous, because there are enough alcohol molecules to start forming spaghetti-like chains. This contributes to the smoothness of vodka and its almost syrupy consistency when cold. Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to firstname.lastname@example.org or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.