Originally By Tony Ackland
Here's a some-what simplified explaination ....(thanks to Mike)
When you have a mixture of liquids, each with its own boiling point when pure, then the boiling point of the mix will lie somewhere in the middle, and this will depend on the relative concentrations of each liquid. Pure water boils at 100 °C, and pure ethanol boils at 78.5 °C, but a mixture of water and ethanol will boil at some point in between. The major point about distillation is that when a mixture like that boils, then the vapour given off is richer in the most volatile component, and when that vapour condenses then the resulting liquid has a lower boiling point than the mix it came from. By repeating this boiling and recondensation process up a column, using packing to hold the condensed liquid at each stage, you can separate the components more and more.
So if you have a mixture of liquids each with a different boiling point, then you heat the mixture, it will heat up until the new intermediate boiling point is reached. When you first start a distilling run, the packing in the column will be at room temperature, so vapour given off by the boiler condenses on the first cool packing it reaches. In condensing, the vapour gives up a lot of heat, and this warms that packing until the liquid on it boils again. However, this liquid is richer in volatiles than the mix in the boiler, so its boiling point is lower. When it does boil again, from the heat given off by more condensing vapour, what you get is even richer in those most volatile components. This process of boiling and condensing continues up the column and, because the condensed liquid is always getting richer in volatiles, the temperature gradually falls the higher you go. The temperature at any point is governed solely by the boiling point of that liquid mix, and any attempt to interfere with that process will disrupt the separation that Nature is carrying out automatically.
In contrast, the boiling point of the mix left in the boiler will very slowly start to rise as it is left with less and less of the most volatile components.
If you started with a mixture (fermented wash) that is mostly water & ethanol, with trace amounts of methanol, propanol, etc. then the net result will be that the most volatile components will tend to rise in greater quantity up the column than their less volatile cousins, and will be found in greatest concentration at the top. This would mean that methanol, the most volatile of the lot, will win the race and you will able to collect it and set it aside. This continues until you have collected all of the "heads" (components that are more volatile than ethanol), and you can then collect just ethanol with a trace of water. You cannot get rid of that small amount of water, as once you reach a mix of 96.5% ethanol/water, with a boiling point of 78.2 °C, then you have reached a stable mix that no amount of re-boiling and re-condensation can change (at normal atmospheric pressure).
Once you have collected the main bulk of ethanol, then the components that are less volatile than ethanol, such as propanol and the bigger organic molecules, will start to reach the top, and you will have arrived at the stage called the "tails". These "tails" may be recycled in the next batch you do, for they still contain a lot of ethanol, or a proportion may be retained as they contain many of the compounds that give a spirit a distinctive flavour, like whiskey or rum.
Note that you are not changing any part of your original brew - you're not "making" the alcohol, or converting it to something else or nasty. All you are doing is concentrating off the original brew into its various parts. There is no more methanol after you finish than what you started with. What does happen though, is that because most of the methanol comes off at once (first up), it is highly concentrated, and can damage you. You definately don't want to be sampling the first portion of distillate that you collect. But once you have thrown away this part, you have guaranteed that the remaining distillate is safe enough to partake of.