During fractional distillation of ethanol/water mixture, why can't pure ethanol be obtained (i.e. distillated contains a little water)?
Ethanol and water can be separated by fractional distillation (pure water distills at some points and pure ethanol at some points) until the mixture in the distilling pot reaches approximately 95% ethanol-5% water. This 95/5 mixture is called an azeotrope or azeotropic mixture. Azeotropes are unique for the mixture and they are part of the non-ideal nature of solutions. At any rate. at the 95/5 point, that is what distills. The only way to prevent the 5% water distilling at the same time as the 95% alcohol, is to add something to the distilling flask that will hold back the water. In such cases, absolute alcohol (pure alcohol without the water) is prepared. It may be interesting to you that if we take absolute alcohol (with no water), pour it into a beaker, allow the beaker to stand in an open setting, the pure alcohol will pick up moisture from the atmosphere until it reaches the 95/5 level. Here is a site that you can read more about azeotropes.
http://www.chemguide.co.uk/physical/phaseeqia/nonideal.html
During the fractional distillation of an ethanol/water mixture, it is difficult to obtain pure ethanol because of a phenomenon called azeotropic behavior. Azeotropy occurs when the vapor composition of a mixture is the same as the liquid composition, causing a constant boiling point for the mixture.
In the case of ethanol and water, there is a specific composition (approximately 95.6% ethanol and 4.4% water by weight) where the two components form an azeotrope. This means that when this mixture is vaporized and condensed, the resulting liquid will have the same composition as the original mixture. This azeotropic mixture has a boiling point of around 78.2 degrees Celsius.
During fractional distillation, the liquid mixture is heated, and the vapor rises up through the fractionating column. As the vapor ascends, it gradually cools down due to contact with the cooler surfaces of the column. Cooler vapors condense and fall back down, while the lighter components continue to rise. This process allows for the separation of the different components of the mixture based on their boiling points.
However, when reaching the azeotropic composition of ethanol and water, the vapor will consist of both ethanol and water in the same ratio as the starting mixture. Even with repeated distillation, it is not possible to eliminate the water completely because the azeotrope acts as a constant boiling point mixture, and it is difficult to separate the ethanol and water completely.
To obtain anhydrous (water-free) ethanol, alternative methods such as molecular sieves, dehydration agents, or azeotropic distillation with other substances like benzene are utilized. These methods help break the azeotropic behavior and remove the remaining water content from the ethanol, resulting in a higher purity product.