A mixture is heated until the more volatile liquid boils. Its vapour climbs the still head, travels into a cooled condenser and turns back into liquid there. The amber dashes trace the vapour, the blue arrows show the cooling water, and the drops are the purified distillate collecting in the right flask.
Distillation is a way of separating the liquids in a mixture by boiling them off one at a time. The mixture is heated in a flask; the liquid with the lowest boiling point turns to vapour first, is carried away, cooled back into a liquid and collected on its own.
On this diagram a round boiling flask holds a liquid mixture, with a heat source beneath it. A tube called the still head leads the vapour away, and a condenser cools it back to liquid.
The amber dashes show vapour travelling from the flask, along the still head and down into the condenser. The blue arrows are the cooling water that removes heat so the vapour condenses; the blue droplets falling into the flask on the right are the collected distillate.
Distillation is one of the oldest and most useful separation methods. It turns crude oil into fuels and chemicals, makes pure water from salty or dirty water, and recovers the alcohol in spirits. Anywhere two liquids need to be told apart by how easily they boil, this is the tool.
Heat is applied to the flask. Because the liquids in the mixture have different boiling points, the one that boils most easily — the more volatile one — reaches its boiling point first.
As that liquid boils, it turns into vapour and rises. The other, less volatile liquids are still not hot enough to boil, so they stay behind in the flask as liquid.
The vapour is guided up and away through the still head, the tube that carries it out of the flask and towards the condenser, so it cannot simply escape into the air.
Cooling water flows around the condenser tube and carries heat away. As the vapour gives up its heat, it loses energy and turns back into liquid — it condenses — on the inside of the tube.
The condensed liquid drips from the condenser into the receiving flask. This collected, more purified liquid is the distillate, now separated from whatever stayed behind in the boiling flask.
Distillation separates the liquids in a mixture by boiling point. The mixture is heated until the more volatile liquid boils; its vapour is drawn off, cooled back into liquid and collected, while the less volatile liquid stays behind in the flask.
Different liquids boil at different temperatures. When the flask is heated, the one with the lower boiling point turns to vapour first, so it leaves the mixture before the other liquid does. That head start is what does the separating.
The condenser turns the vapour back into liquid. Cooling water flowing around the tube carries heat away, so the vapour loses energy, condenses into droplets and can be collected as a liquid again.
Because vapour that escapes is lost. Condensing it lets you keep the separated liquid — the distillate — and often reuse it, instead of venting it into the air.
A fractionating column gives the vapour and liquid more contact and repeated cycles of boiling and condensing, which sharpens the separation when two liquids have fairly close boiling points.
Usually not perfectly pure. It concentrates the more volatile part, and a single pass may still carry traces of the other liquids; repeating the process, or using a taller column, gives a cleaner result.