Some of what needs to be said about the principles of distillation was covered in the chapter on beverages, and there was also some mention in the chapter dealing with the construction of a still. In both these places, the distinction was made between the comparatively simple pot stills used in the manufacture of whisky and the more elaborate still with fractionating column used to remove all the impurities and leave a pure alcohol, as in the manufacture of gin and vodka. The present chapter will explain just what is involved in carrying out a fractional distillation and how you go about it, but first a few words about principles. These will let you know just why a certain procedure is being followed and. if something goes wrong, what you can do about it. There is nothing more irritating in an instruction manual than to be told arbitrarily to do something without an explanation as to why it is necessary.
At the outset it will be useful to dispose of a myth concerning distillation which is quite prevalent, so prevalent in fact that it is the basis of several small-scale stills being offered for sale. The myth goes as follows: If you have a mixture of three liquids with different boiling points, e.g. methanol (64.7° C.), ethanol (78.4° C.) and water (100° C.) it is believed that, if you raise the temperature to 64.7° C. and hold it there the methanol will boil off. Then, if you raise the temperature to 78.4° C. the ethanol will boil off. This is completely untrue. It might be approximately true for liquids which do not mix with one another, such as gasoline and water, but is totally untrue for the lower alcohols which are completely miscible with water. Being miscible they associate with one another at the molecular level and no longer act independently as individuals.
Having expunged this fallacy from our minds let's take a look at what really happens. Some of the more important chemicals we are dealing with, together with their boiling points, are shown in the table below.
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