The most common way to separate alcohol from water is distillation, which exploits the fact that ethanol boils at 78.5°C while water boils at 100°C. By heating a mixture to the right temperature, the alcohol vaporizes first, rises through a column, and gets collected after condensing back into liquid. But simple distillation has a hard ceiling, and getting truly pure alcohol requires additional techniques.
How Distillation Works
Distillation separates liquids based on their boiling points. When you heat a mixture of ethanol and water, the ethanol turns to vapor before the water does because it boils roughly 22 degrees lower. That vapor travels up through a column and into a condenser, where it cools back into liquid and drips into a collection vessel. The water, still in liquid form, stays behind in the boiling flask.
Fractional distillation improves on this basic idea by using a tall vertical column packed with surfaces where vapor and liquid can interact. As vapor rises through the column, it meets a trickle of liquid (called reflux) flowing back down. Each time the rising vapor contacts this cooler liquid, the mixture re-separates a little more. The vapor that finally reaches the top of the column is significantly richer in ethanol than what left the boiling flask. Industrial biorefineries use continuous distillation columns that run nonstop, pulling ethanol off the top and draining water residue from the bottom.
The 95.6% Ceiling
No matter how many times you redistill an ethanol-water mixture, you cannot get past 95.6% ethanol by mass using distillation alone. At that concentration, the mixture forms what chemists call an azeotrope: a point where the liquid and vapor have the exact same composition. The azeotrope boils at 78.2°C, which is actually slightly lower than pure ethanol’s boiling point of 78.5°C. Once you reach that ratio, boiling the liquid just produces vapor with the same 95.6% ethanol content. Condensing and reboiling it changes nothing.
If you start with a dilute mixture (say, fermented beer at 5 to 15% ethanol), careful fractional distillation will concentrate it up to 95.6% ethanol in the collection flask, with nearly pure water left behind in the boiler. But that remaining 4.4% water cannot be removed by heat alone.
Getting Past the Azeotrope
Industries that need completely dry ethanol, particularly for fuel blending, turn to molecular sieves. These are tiny synthetic pellets with pores just large enough to trap water molecules but too small for ethanol molecules to enter. Hydrated ethanol vapor passes through a bed of these pellets (type 3A sieves are the standard), and water gets adsorbed into the pore structure while dry ethanol passes through. The result is effectively 100% pure ethanol. Once the sieve bed is saturated with water, it gets regenerated by heating, and the cycle repeats.
Another approach is salting out. Dissolving certain salts into an ethanol-water mixture forces the two liquids to separate into distinct layers. Potassium carbonate, for example, triggers phase separation when added at concentrations of around 100 grams per kilogram of solution. The salt molecules attract water so strongly that they pull it away from the ethanol, creating an ethanol-rich layer on top and a salty water layer below. This method works at room temperature and requires no heat, though it leaves salt contamination in the water phase.
Freeze Concentration
You can also separate alcohol from water by freezing rather than boiling. Water freezes at 0°C, while ethanol freezes at around negative 114°C. If you cool an alcohol-water mixture enough, the water forms ice crystals while the alcohol stays liquid. Removing the ice leaves behind a more concentrated alcoholic liquid. This technique has been used for centuries to make drinks like applejack from hard cider.
Freeze concentration carries a real safety concern, though. Unlike heat distillation, where careful temperature control can separate different compounds, freezing concentrates everything that isn’t water, including methanol. Methanol is highly toxic. Even small amounts damage nerves and organs, and the compound specifically attacks the eyes and brain. Formate, the main toxin the body produces when breaking down methanol, stops energy production in cells in a way similar to cyanide. Commercial distillers carefully remove methanol during production, but freeze concentration has no such built-in safeguard.
Membrane Separation
A more modern industrial method uses specialized membranes in a process called pervaporation. A thin membrane made of silicone-based material (PDMS is common) selectively allows ethanol or water to pass through based on molecular affinity. The liquid mixture contacts one side of the membrane while a vacuum or sweep gas on the other side pulls the preferred molecules through. Operating temperatures typically range from 34 to 50°C, well below boiling, which saves energy. Pervaporation is especially useful for fine-tuning ethanol concentration after initial distillation or for processing heat-sensitive mixtures.
Legal Considerations in the US
If you’re thinking about distilling at home, the legal landscape matters. In the United States, federal law prohibits individuals from producing distilled spirits at home, even for personal use. This is a sharp contrast to home brewing of beer and wine, which is legal for adults. Operating a still without registering with the Alcohol and Tobacco Tax and Trade Bureau is a felony under 26 U.S.C. 5601, punishable by up to 5 years in prison, a fine of up to $10,000, or both per offense. Distilling with intent to avoid taxes carries the same penalties under a separate statute. Some states have their own additional restrictions. Fuel alcohol permits are available for those who want to produce ethanol strictly for use as fuel, but the application and oversight requirements are significant.