Figure citation: Techniques in Organic Chemistry by Mohrig, Hammond, Schatz. ISBN 978-1429219563.
Imagine you have a big stack of differently colored marbles. For whatever reason, you want to sort your marbles into different piles based on their color. So you begin to go though and pick out the colors one by one. If you have a small number of marbles this won't take long. But now imagine your marbles are about a billionth of a meter in diameter and there are so many of them it would make Avogadro, er, I mean your head spin. Well, it wouldn't be very practical to go through with a very small tweezers, picking out all the marbles that are the same color. Indeed, color loses its meaning at those scales anyway. So we have to use some other method to sort them, hopefully a method that doesn't involve much manual sorting.
I'll return to the realm of not insulting your intelligence now.
Distillation is a method of separating two or more liquids by taking advantage of their differences in vapor pressure. Vapor pressure is the pressure exerted by the gas above a compound. All compounds have vapor pressure, even solids. Granted, the vapor pressure of steel is much less than that of, say, water. Given this fact, it would be fairly easy to separate water from steel, yes? Vapor pressure is exerted by ice. This means ice can evaporate, and in fact this is what causes freezer burn. But what if we have two liquids that have relatively similar vapor pressures (but still different) and we can't just use some sort of filter to separate them?
Let's say we have two liquids A and B. A and B have a vapor pressure at room temperature of 22Torr and 40Torr, respectively (Torr is a unit of pressure). Let's say we have exactly the same amounts of A and B in a container. This means that the composition of the liquid is 50%A and 50%B, but the vapor ABOVE the liquid has a composition of 22/(22+40)=35%A and 40/(22+40)=65%B. There is significantly more of compound B than compound A in the vapor! By the way, this is probably a horrendous oversimplification of the proportions, and I COULD flip though my O-chem book to find the correct relation, but I'm sure you get the general idea. Now, if we can collect this vapor and turn it into a liquid, we would have a liquid of composition 35%A and 65%B. We would be one step closer to separating our compounds.
In fact, we CAN collect the vapor and turn it into a liquid by cooling the vapor down so that it condenses. And as luck would have it, as we remove vapor, it is replenished constantly by the liquid below. Unfortunately, this process will be EXTREMELY slow because the vapor pressures are so low and very little vapor can escape from the liquid in the container in a given amount of time. It could take days or weeks to separate them. If only there was a way to raise the vapor pressure... Well there is. And we do that by adding energy, in the form of heat, to the system. By adding heat, we can raise the total vapor pressure within the container to exactly 760Torr, but no further. This is because 760Torr is the pressure of the Earth's atmosphere, and it is at this point that the liquid in the container begins to boil.
Now we can collect the vapor above the liquid and it will be replenished VERY quickly. The boiling will actually push the vapor out of the container, and as long as we channel the vapor through something cold, thereby reducing the vapor pressure once again, we can condense it and collect it as a liquid with a high concentration of compound B.
This is what a distillation apparatus does, and one distillation step is referred to as a "simple distillation". A mixture of liquids is placed in a pot that is heated to boiling. The temperature of the vapor is measured by a thermometer at the top of a vapor column, as an indication of relative purity. Condensate (distillate) is usually collected over a certain temperature range, which is indicative of the purity of the result. The vapor travels through a condenser that cools the vapor, returning it to a liquid state that is a different composition from the starting liquid.
If we do this over and over again, we'll eventually get a liquid that is nearly pure B, and almost no A. Note that purification actually takes many repetitions of distillation. Well, that's kind of a pain. What if we want to completely purify something very quickly?
That's where fractional distillation comes in. A fractional distillation column is a device wherein MANY distillations occur in ONE step. The fractional column is a long tube that is packed with a material high in surface area but low in volume. Glass beads, chambers separated by plates, and various shredded materials work well as packing. As vapor travels up the column, a temperature gradient is created (higher temp on bottom, lower temp on top). This causes vapor to condense on the packing as it rises. The condensed material is higher in purity than the starting material. Then more vapor travels up the column, heating this condensed liquid to its new boiling point and vaporizing it, with the vapor being one step higher in purity than the last step. As the composition skews further toward B, the boiling point drops, reinforcing the temperature gradient as the material travels up the column. This occurs many times as vapor travels up the column, with the lower vapor pressure liquid separating out and dripping back down the column and flowing back to the original container. Fractional columns are often referred to as reflux tubes for this reason.
The use of a perfect fractionating column would result in very sharp jumps in temperature readout on the thermometer. As each pure compound is exhausted in the container and the next lowest boiling compound climbs the column, the temperature will rise quickly, indicating the change in compound.
Performing a distillation using a fractional column is referred to as "fractional distillation" and results in a highly purified liquid.
This project allows for the use of both types of distillation, since many raw products to be distilled contain organic solids that can contaminate the packing of a fractional column, and MUST be simply distilled first to remove these impurities.