Wednesday, September 9, 2009


This apparatus is what a chemist uses to distill things. There is a long cylindrical tube connected to a flask that sits on a heat source. The tube connects to another, similarly shaped tube that sticks out from its side, and is pointed downward. This tube, called the "condenser" has water running through a cavity between the inside and outside of the glass -- sort of like having a glass tube within a slightly larger glass tube. This tube ends in a spout, where some sort of receptacle is placed for collection. In the picture above, the receptacle is a graduated cylinder with a red plastic bottom.

What occurs macroscopically in a distillation is pretty common to everyday experience: You add heat to some liquid, and the liquid evaporates up the tube and eventually travels through the condenser, where the water quickly cools the vapor, and drips out of the spout and into the receptacle. In particular, this is how liquor companies obtain higher concentrations of alcohol. When you make alcohol, the alcohol is fully dissolved in water -- like beer, or wine. The trick to higher alcohol content lies in... Chemistry!

So, suppose a beaker full of recently made alcohol -- it will be clear, and from appearances look to be the same liquid. This is because alcohol is miscible in water, which is the opposite of what happens when you mix oil and water. No matter how much water and alcohol you mix together, they will always freely intermingle. So, you're left with a beaker of water and alcohol molecules:

Behold the power of paint! The blue atoms with two red atoms coming off of them is a water molecule. The other one is a molecule of the drinking variety of alcohol. It has a blue atom as well because both water and alcohol have Oxygen in them.

How would you separate these?

A quick look at ethanol's MSDS sheet tells us that the alcohol has a boiling point of 78 degrees Centigrade. Water's boiling point is 100 degrees centigrade. Attempting to boil the mixed liquid seems like a good idea. And, in fact, this is how alcohol and water are separated -- first the alcohol evaporates and is collected, then the water will stop evaporating. If you want to keep them separate, you stop the distillation once you have collected the majority of your alcohol. How does one tell when that happens?

You'll notice in the photograph a thermometer. If we plot a graph of the amount of liquid collected on the x-axis versus the temperature of the vapor (which corresponds to the liquid's temperature) on the y-axis, you'll see something like this:

I chose this image on purpose because it displays the two types of distillation on the same graph -- simple and fractional. They both have roughly the same shape, only fractional distillation has a much larger spike in its temperature. We'll come back to this sh0rtly.

Note also that alcohol, which evaporates first, has a lower boiling point than the water. Also note that the temperature in the graph climbs as the distillation occurs. This is because the vapor evaporating has an increasing number of water molecules, which require a higher temperature to vaporize. So, you know that you have collected as much alcohol as you can when you reach a mid-point on the graph, which you can determine experimentally by running the whole distillation once through.

Also note in this graph that the fractional distillation has a much sharper jump in temperature. This is because, initially, you are evaporating mostly alcohol and leaving most of the water, but then suddenly you only have water. In the simple distillation, the rate of change of the ratio of alcohol to water is much more gradual (prepositional phrase glory, right there). That is because...


The fractional distillation simulates doing a simple distillation hundreds of times over! Well, I'm uncertain about the actual factor, but it does simulate it going over and over again. The photograph above shows a set up for fractional distillation. If it were a simple distillation, the flask carrying the mixture wouldn't be connected to a long vertical tube, but would be next to the condenser. In the vertical tube are placed several glass beads. As the vapor rises, it condenses on the beads (since the beads are cooler than the vapor), and the heat from more vapor gradually warms up the bead until the condensate evaporates again. This occurs time and time again, with some of the liquid pouring back down into the initial flask. Each time this occurs, the mixture becomes a little more concentrated in the chemical with the lower boiling point -- in this case, the drinking alcohol. With a simple distillation, this occurs only once, but the beads essentially simulate many simple distillations in a row.

Now, an oddity here -- you'll notice from the Paint drawn beaker diagram above that the alcohol molecules are actually larger than the water molecules. The molecular weight of alcohol is, roughly, 46 grams per mole. Water's molecular weight is 18 grams per mole. Yet, despite having more mass (thereby giving the impression that it will need more heat, which can be roughly thought of as energy, to turn into a gas), the alcohol has a lower boiling point. Stay tuned for this explanation next time! Whenever next time is. This is a busy semester.