Final answer:
Fractional distillation is used over simple distillation when more precise separation of two miscible liquids is required, due to its ability to efficiently separate components with close boiling points.
Step-by-step explanation:
The student asked why fractional distillation might be used instead of simple distillation to separate two miscible liquids. The answer is that it allows for more precise separation of the components. This is because fractional distillation takes advantage of the differences in the volatilities of the components to achieve the separation. Unlike simple distillation, a fractional distillation apparatus creates a series of simple distillations through the use of packing material or a column which provides a larger surface area for separate sequential evaporations and condensations as the mixture moves up the column.
Thus, the key advantage of fractional distillation over simple distillation is not necessarily faster processing or lower energy consumption, but rather the ability to precisely separate liquids with similar boiling points (higher efficiency) and achieve a more complete separation of the liquids. For example, in the distillation of crude oil, the goal is to separate the mixture into components like gasoline, kerosene, and diesel fuel, which have relatively close boiling points. Raoult's law explains how the vapor above a solution of two liquids is enriched in the more volatile component, allowing selective vaporization during fractional distillation.