distilling is simply a way to separate different liquids dissolved in a solution. henceforth i will refer to each individual component of the mixture as a 'liquid,' and the mixture as a whole as the 'solution.' using a simple pot still such as the one in my fore mentioned instructable i will show the basic science involved in distillation.
i hope this clears up a lot of the questions out there. read up and enjoy.
Step 1: Apparatus
Step 2: Science
we were each given a jar containing a random assortment of anything our teacher felt like including--solids, liquids, even dissolved solids. we were then responsible for identifying everything in the solution based on the scientific techniques we had been taught up until this point.
the larger objects--like paperclips and nuts and bolts, etc.--were easy to filter out and identify. where this little story becomes poignant is in the clear liquid left over. this was not simply water, and we had to figure out what was in it. for this we used distillation. distilling this solution while graphing boiling points allowed us to cross reference our graphs with known boiling points and therefore identify each individual liquid in the solution, thus earning a high mark. with this knowledge, you can use a homemade still to make strong alcoholic beverages. who knew they were teaching us how to make moonshine in middle-school!
Step 3: Graphing? excuse me? i'm just trying to make booze
whether you use a pot still, or simply a test tube over a Bunsen burner (which is pretty much just a small pot still) the principle is the same. as you run your solution through the still, every 30 seconds to 1 minute record the temperature of the solution in the the pot. when the temperature remains the same for any length of time it will produce what is know as a 'plateau' on your final graph (if you were to actually graph it out). each temperature plateau will correspond to the boiling point of a specific liquid in your solution and can therefore be identified and either retained or discarded.
each type of alcohol (methanol, ethanol, etc.) boils off at a different temperature. and this is the basis for distillation. by using a little bit of scientific knowledge and a pen and paper, you can find out exactly what is coming out of your still. this is very important and should not be overlooked.
in order to have a reference for what you are getting at any given temperature, check out this boiling point calculator. it is not mine and i cannot guarantee it's veracity, but it looks good, from what i can tell. you can also google the boiling points of methanol, ethanol and water.
Step 4: The big difference
as the temperature of your pot rises, you will collect and consequently discard anything that corresponds with a boiling temperature less than that of ethanol and in turn do the same for anything boiling at a higher temperature. anything that is boiling off at a temperature corresponding to ethanol's boiling point at your particular elevation, etc., is what you are seeking. once your temperature has increased to a level greater than the boiling temperature of ethanol you can turn off the heat and stop the run. there is no reason to burn it dry in this case.
for instance, if your solution contains methanol, ethanol and water it will happen like this: once the temperature of your solution reaches around 148 °F methanol will begin to boil and come out through your tube as vapor into your condenser where it will convert back to liquid and exit into your receptacle. the temperature of your solution will not vary much beyond 148 °F until the methanol is gone and then will begin to climb again until it reaches 173 °F, the boiling point of the ethanol. the same thing will happen here. the temperature will hover around the same until the ethanol is boiled off and then on to water.
in order to get ethanol and exclude the rest you get rid of what are called the 'heads' and 'tails.' basically dump anything that boils lower than around 173 °F and everything after the temperature begins to climb again. more than one receptacle could help in this case. use one for the heads, one for what you want, and one for the tails.
after a run through your still, tossing out the heads and tails, you should have left a bit of alcohol and water worth drinking. multiple runs will distill to an even greater proof, up to a point.
welcome to the wonderful world of distillation, folks.
let me know if any of this doesn't make sense and i will correct it in the instructable. thanks for reading and i hope this helps.