One of  the most widely applicable and most commonly used methods of purification of  liquids or low melting solids (especially of organic chemicals) is fractional distillation at atmospheric, or some lower, pressure. Almost without exception, this method can be assumed to be suitable for all organic liquids and most of the low-melting organic solids.

The distillation apparatus consists basically of  a distillation flask, usually fitted with a vertical fractionating column (which may be empty or packed with suitable materials such as  glass helices or stainless-steel wool) to which is attached a condenser leading to a receiving flask. The bulb of  a thermometer projects into the vapour phase just  below the region where the condenser joins  the column.  The distilling  flask is heated  so  that its contents are steadily vaporised by  boiling.  The vapour passes up  into the column where, initially, it condenses and runs back into the flask. The resulting heat transfer gradually warms the column so that there  is a progressive movement of  the vapour phase-liquid boundary up the column, with increasing enrichment of the more volatile component. Because of this fractionation. the vapour finally passing into the condenser (where it condenses and flows into the receiver) is commonly that of  the lowest-boiling components  in the system.  The conditions apply until all of the low-boiling material has been distilled, whereupon distillation ceases until the column temperature is high enough to permit the next component to distil. This usually results in a temporary fall in  the temperature indicated by the thermometer.