Definition
Fractional distillation is a method for separating a mixture of liquids that blend together, by using differences in their boiling points (the temperature at which a liquid turns into a gas). When one round of boiling isn't enough, a tall column makes the vapor evaporate and condense again and again, so each liquid can be drawn off with precision.
The Secret of the Droplets on Your Pot Lid
If you've ever boiled soup, you've seen clear drops of water on the inside of the lid. The salty broth stays in the pot. Only pure water floats up first as steam, hits the lid, and turns back into liquid. Boiling a liquid into gas and then cooling it to collect pure liquid is called distillation.
But some liquids are hard to separate with a single pot. Water and alcohol mix completely, and both evaporate easily. Water boils at 100°C (212°F), while alcohol starts boiling at about 78°C (172°F). If you heat a mix of the two, it boils at a temperature somewhere between those two points. The vapor that rises has more alcohol in it, but plenty of water comes along too.
So if you boil once and collect the vapor, you still get an impure mix with water in it. The fix is to make the path the vapor travels very long and fill it with obstacles. That idea is the beginning of fractional distillation, which separates a mixture in many stages.
The Magic Tower That Sorts Things as They Rise
A fractional distillation setup has a tall fractionating column, or a giant distillation tower, attached on top. When you heat the mixture at the bottom, vapor starts to rise. The bottom of the tower stays hot, but the higher you go, the cooler it gets.
Substances with a high boiling point only make it partway up. They meet the cooler air, quickly turn back into liquid, and drip down. Substances with a low boiling point stay gaseous even as the temperature drops, so they keep rising all the way to the top.
Inside the tower, this happens over and over. Vapor rises, cools, and falls. Then it meets the hot air below and turns into gas again. Thanks to this loop, the higher you go, the purer the gas is in low-boiling-point material. At the top, you cool it down and collect it separately.
A Closer Look: Crude Oil and Your Daily Life
The raw materials for plastics, gasoline, and kerosene all start with this technology, which splits crude oil into parts. Crude oil (unrefined petroleum) is black and fresh from the ground. It's like a messy batter made of countless hydrocarbons and other compounds. When you heat it in a giant distillation tower, it separates into layers by boiling point.
The lightest part, with the lowest boiling point, is liquefied petroleum gas (LPG). It comes out at the top. Below it, gasoline, kerosene, diesel, and heavy oil separate out in order. At the very bottom, asphalt is left behind as the residue that never boils. With a single round of heating, you harvest many useful oils layer by layer.
To be more precise, an azeotropic mixture is hard to separate fully with fractional distillation alone. This is a mix whose parts have almost the same boiling point, or that boil together at the same temperature at a certain ratio. For example, no matter how many times you repeat fractional distillation on water and ethanol, you can only raise the ethanol purity to about 95.6% by mass.
Common misconceptions
When you boil a liquid mixture, only the substance with the lower boiling point turns into gas first, 100% of it.
The vapor from a boiling mixture just has more of the low-boiling-point part in it. Some of the high-boiling-point part comes along too. That's why one round of distillation can't separate things perfectly, and you need fractional distillation, which repeats evaporation and condensation many times.
If you keep repeating fractional distillation, you can separate any liquid mixture to 100% purity.
A mixture that boils together at the same temperature at a certain ratio, like water and ethanol (an azeotropic mixture), can't be fully separated by ordinary fractional distillation alone.
Where you meet it
Oil refining heats crude oil and splits it layer by layer into petroleum gas, gasoline, kerosene, diesel, asphalt, and more.
A water and ethanol solution is passed through a fractionating column to get high-purity ethanol.
Air is cooled to nearly -200°C (-328°F) to turn it into a liquid, then slowly warmed to separate nitrogen, argon, and oxygen.
In one sentence
Fractional distillation is a technique for cleanly separating substances with different boiling points. Inside a tower, it repeats evaporation and condensation so each part can be drawn off on its own.