In terms of the relative amounts of lower-boiling and higher-boiling components, how does the temperature and composition of the liquid at the top of a fractional distillation column compare with the temperature and composition of the liquid at the bottom of that column?
A process, such as heating or boiling, used to separate volatile liquid solutions from each other into simpler/fractional substances.
Individual liquids separates based on the difference of their boiling points. A simple diagram is provided...
The fractional distillation column is set up with the heat source at the bottom. As the distance from the bottom increases, a temperature gradient is formed in the column; As the mixed vapor ascends the temperature gradient, some of the vapor condenses and revaporizes along the temperature gradient.
It is coolest at top and hottest at the bottom. The liquid boiling in a lower temperature evaporates faster than the other. Each time the vapor condenses and vaporizes, the composition of the more volatile component in the vapor increases. This distills the vapor along the length of the column, and eventually the vapor is composed solely of the more volatile component.
The most volatile component of the mixture exits as a gas at the top of the column. The vapor at the top of the column then passes into the condenser, which cools it down until it liquefies. The liquid with higher BP separates at the bottom.
NB : Sometimes two components cannot be separated completely. The vapor at the top of the column may be an azeotrop rather than the vapor of the pure component. In these cases mixtures form azeotropes, where the mixture boils at a lower temperature than either component. For example, a mixture of 96% ethanol and 4% water boils at 78.2 °C; the mixture is more volatile than pure ethanol. For this reason, ethanol cannot be completely purified by direct fractional distillation of ethanol-water mixtures.
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