The substances that will be absorbed depend on:
• The size of the molecules in the impurities (they must be smaller than the
• The density of the impurities
• The amount of impurities in the alcohol
• The boiling point of the impurities
The impurities must be small enough to fit in the carbon pores. An impurity with a higher boiling point is more easily absorbed, and adheres better than one with a lower boiling point. If the carbon becomes saturated, an impurity with a higher boiling point can eject a lighter impurity and take its place. This happens most easily on the carbon surface, where the impurities are loosely attached, but can even happen inside the carbon pores. This is why we never filter the same alcohol more than once through the pipe: the result would be worse.
Room temperature works well, whereas purification in cold temperatures works less effectively, or not at all.