Chemistry Activity
Fractional Distillation
Purpose To separate a mixture of two miscible liquids (liquids that mix in all proportions) with different boiling points. Your mixture will consist of water and 2-propanol (isopropyl or rubbing alcohol). | | Menu Purpose
Background
Safety
Procedure
Data Analsysis
Implications and Applications
Concluding Questions |
Background
Distillationis a physical process used to separate mixtures that contain at least one liquid. Distillation works because each substance in the mixture has its own unique boiling point. So, as a mixture is heated, the temperature of the mixture rises until it reaches the temperature of the lowest boiling substance in the mixture. The lowest boiling substance boils away, of course. Meanwhile, the othercomponents of the mixture remain in their original phase—either solid or liquid—until the lowest boiling component has all boiled off. Only then does the temperature of the remaining mixture rise and other components are boiled off.
If the original mixture contains a solid and a liquid, the liquid will boil off as the mixture is heated. As the liquid boils, it turns into a gas. If we can remove that gasfrom the original container and then cool it, the gas will condense into its original form—a liquid. The solid from the original mixture would then be in one container and the original liquid in another container. The separation would be complete.
This is the simplest kind of distillation. In fact, it is nature’s method for purifying water. You may have studied the water cycle in an earthscience class. As the heat from the sun warms the waters of the earth, the liquid water turns into water vapor, leaving behind any dissolved impurities. The water vapor is then cooled by the atmosphere—that is, it is condensed—and returns to the earth as a liquid, otherwise known as rain.
If the original is a mixture of liquids, each boils off at its boiling point and is cooled in turn to form afraction of the original mixture. This is known as a fractional distillation. Crude oil, a mixture of many substances including a number of liquids, is refined in this way.

ethyl alcohol, or ethanol
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Distillation often follows fermentation. Fermentation is used to produce alcoholic beverages. Grain is fermented to make beer, while grape juice is fermented to make wine. Beer and wine nevercontain more than around 12% alcohol, because any higher concentration kills the yeast that produce the alcohol. To make stronger drink, distillation is used. Wine, beer, or fermented brews made from corn, sugar cane, or potatoes can be purified by distillation. Since ethyl alcohol boils at only 83°C while water boils at 100°C, the alcohol will boil off, leaving the water behind. The alcohol vaporsare then condensed and collected. Distillation can produce liquors that range from 40-95% ethyl alcohol.
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A confiscated moonshine still
shows the basic components
of a distillation apparatus. |
Although fermentation occurs naturally, most distillations are artificial processes. Distillation was mentioned by Aristotle (384-322 B.C.) as a method of purifying water, and Pliny the Elder(23-79 A.D.) recorded one of the earliest references to a rudimentary still, the apparatus used to perform a distillation. In its most basic form, distillation requires:
* a container for the original mixture—called a retort.
* a condenser to cool the vapors
* a receiver to collect the distillate
This can be seen in the picture of a moonshine still on the right. Thisconfiscated still is similar to the apparatus to be used in this activity. The tank in the center of the picture is the retort where the impure liquid is boiled. The pipe extending from the top of this retort is the condenser. This is where the vapors from the boiling liquid cool and condense back into a liquid. The tank in the bottom right corner of the picture is the receiver. After the vapors…