Distillation | Intermolecular forces and properties | AP Chemistry | Khan Academy

Let's say that you have a solution where the solvent is water and the solute is what we would consider drinking alcohol or ethanol. So, this is our solution right over here. Let's say that it is 10 percent ethanol, which is drinking alcohol, and let's say the rest of it is water.

What we want to do is figure out a way that we can get something that has 15 or 20 or 30 percent ethanol. How would we do it? Well, the answer lies in the fact that ethanol and water have different boiling points. For example, the boiling point of ethanol is approximately 78 degrees Celsius, while the boiling point of water is 100 degrees Celsius.

So, a mixture of ethanol and water is going to have a boiling point someplace in between. You could imagine we can keep heating up this solution. We're going to go beyond the boiling point of ethanol, but before we get to the boiling point of water, this solution is going to boil.

When it's boiling, because ethanol has a lower boiling point, the vapor mixture is going to contain more than 10 percent ethanol. If you could condense that vapor, you would then have a solution with higher than 10 percent ethanol. That is exactly what distillation is all about.

You might distill with a contraption that looks like this. At first, it looks complicated, but then when you really understand what's going on, it's actually pretty straightforward. It's exactly what I just described.

What we would do is we would put the original solution right over here, and then you would heat it up. Oftentimes, you're going to heat it up in an oil bath. That's what you see in this yellow right over here, because the oil won't evaporate readily, and it can distribute the heat around in a nice, even way.

Then, if you were to heat this up so that the solution boils, remember that's going to be at a boiling point someplace in between 78 degrees Celsius and 100 degrees Celsius. You're going to, for sure, have some water molecules vaporizing, but at that temperature, which is beyond ethanol's boiling point, you're for sure going to have a higher proportion of ethanol molecules vaporizing.

Then, they're going to be in vapor form, and so they could enter this chamber right here. What this chamber is, is essentially what's called a condenser. Oftentimes, you're going to have separate from the vapor, but you're going to make cold water go around this tube right over here. Oftentimes, a glass tube. Sometimes, you might chill it with cold air instead.

But you have water coming in—cold water—and it's going around the tube. It's not mixing with the actual air inside the tube, and then the water comes out of the tube. What it does is it lowers the temperature in that tube.

So, you can imagine the vapor that comes in, which contains both ethanol and water, but the proportion of the vapor that is ethanol is going to be higher than the proportion in our original solution. It's going to condense back into liquid form, and then that stuff is just going to pour back down into what you could call this collection chamber right here.

The collection chamber is surrounded by a very cold, oftentimes ice bath, so that you minimize vaporization. Depending on how well you do this, maybe this sample right over here might be twenty percent ethanol or thirty percent ethanol. There will still be some water that comes up here and condenses, and then comes back down.

Now, if you wanted an even purer sample of ethanol, you could keep doing this. You could take what you collect here and then heat that up to its boiling point, which will now be a little bit lower than the first solution because it has a higher proportion of ethanol.

Then you would have the same process go over and over again, and so every time you do that, you would get a purer and purer sample of ethanol. If you don't want to do that because it's tiring, it takes a lot of work, and actually, you end up with a lot of lost material through all of this mechanism over here, there's something you can do called fractional distillation.

What fractional distillation is, is getting multiple vaporization and condensation cycles all at once, so that you get a more pure sample. The way that you get those multiple vaporization and condensation cycles is to give the vapor in this tube right over here something to condense on.

That condensate—some of it will revaporize, and some of it will drip back down. The part that re-vaporizes is likely to have a higher proportion of the distillate of the thing that has the lower boiling point. You can have multiple cycles of this condensation and re-vaporization inside this column, and so at the end, you're more likely to have a more pure sample of your distillate.