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Known since antiquity, the concentration of alcohol by the application of heat to a fermented liquid mixture is perhaps the oldest form of distillation (see distilled beverages). However, the technique is now widely used for a variety of liquids in chemistry and in the production of petroleum products, among other fields.
The device used in distillation is referred to as a still and consists at a minimum of a pot' in which the source material is heated, a condenser in which the heated vapor is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid is collected.
The equipment may effect separation by one of two main methods. Firstly the vapours given off by the heated mixture may consist of two liquids with significantly different boiling points. Thus, the vapour that is given off is in the vast majority of one or the other liquid, which after condensation and collection effects the separation.
The second method (fractional distillation) is more effective at separating liquids with similar boiling points. This method relies upon a gradient of temperatures existing in the condenser stage of the equipment. Often in this technique, a vertical condenser, or column, is used. By extracting products that are liquid at different heights up the column, it is possible to extract liquids that have different boiling points. The greater the distance over which the temperature gradient in the condenser is applied leads to easier and greater separation.
Under ideal conditions, distillation can be used to effect a near-complete separation of different components. The fractions can further be purified by further distillation.
It is also possible to separate fractions by cooling, using differences in their freezing points.
Many countries tax distilled alcohol, and preserve government income by legal restrictions on the use of a still.
Distillation was developed into its modern form with the invention of the alembic by Islamic alchemist Jabir ibn Hayyan c. 800; he is also credited with the invention of numerous other chemical apparatus and processes that are still in use today.
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