Distillation (otherwise known as distillation) is a method of separating substances based on the difference in the boiling point. In some cases, this process is also called distillation.
Let's take any liquid, such as water. Above the water surface, there is always water vapor, and the higher the water temperature, the higher the concentration of this vapor. Usually it is expressed in the so-called pressure of saturated steam - for example, for water at 20 ° C it is 2.5 kPa, that is, about 2.5% of the normal atmospheric pressure (if desired, the pressure can be translated into density according to known formulas from school physics, but operate on the pressure more clearly). This means that if the water is placed under a vacuum dome and the air is pumped out, the pressure below 2.5KPa will not be achieved - the water will evaporate and maintain this pressure. Thus, the water will cool down and as a result, will freeze, and pressure of the sated steam will change. By the way, the ice also evaporates, and not worse than liquid water.
Without pumping out the air, the water also evaporates, and the hotter it is, the faster it is. But when a certain temperature is reached, the water will begin to boil - it means that the pressure of saturated steam is equal to the atmospheric pressure and this pressure is enough to form a bubble of steam inside the liquid, which, of course, emerges. This is boiling.
If we take a solution of alcohol in water (ethanol boils at 78°C), then when heated, the alcohol will boil first, and then the water will boil... But no, everything is not quite so. Above the water-alcohol mixture, there will be steam consisting of water vapor and alcohol (gaseous ethanol). Thus, as ethanol evaporates easier, its concentration in steam will be more than in a solution. When the temperature reaches a certain level (between 78 and 100°C) the solution will begin to boil, i.e. it will displace air above it. In the distillation unit, the resulting vapors are sent to the so-called "refrigerator", where they cool down, condense and flow into the receiving flask as a distillate. The first fractions ("fraction" means literally "part") will contain more readily boiling substances than the following fractions.
During the distillation process, the vapor temperature will rise smoothly, reaching 100°C by the end of the distillation process, when only water with a miserable amount of ethanol remains in the initial solution. To which temperature to distill and how many fractions to collect - determined by the specific task.
In the distillation lab, the distillation is used for:
- Solvent purification: in particular, many solvents are obtained anhydrous by filling in a dehumidifier (sodium sulfate, phosphorus pentoxide or even sodium metal) and then distilling the solvent directly from the dehumidifier.
- Removal of solvents from the reaction mixture and purification of the reaction product, if the product is a liquid and is distilled at a sufficiently low temperature at which there is no destruction yet. Distillation under reduced pressure is often used, up to 0.1 mm Hg, i.e. 1/7000 of atmospheric pressure.
In the distillation industry, gasoline, kerosene and diesel fuel are extracted from oil, and countless compounds, including indane and adamantane, are extracted from coal tar. But the process looks a bit different from the laboratory: a distillation column is used, where the initial mixture (e.g. oil) is continuously fed and distillation products are continuously selected from different levels. Inside the column, there are many horizontal distillation plates and anything that does not condense passes over the liquid and goes higher to the next plate. At the same time, the separation of fractions is more effective - it is like ten or twenty (the number of plates can be up to a hundred!) distillation plants, working one after another.
Some substances catch fire when heated up before they can become liquid, such as caffeine or iodine. This can also be used for cleaning - the vapors of the substance condense on cold surfaces, from which they can be scraped. There is no fractionation to be talked about here, but this method, if applicable, usually produces a very pure substance.