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

Distillation is 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 other components 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 gas from 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 earth science 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 a fraction 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

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 never contain 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 vapors are then condensed and collected. Distillation can produce liquors that range from 40-95% ethyl alcohol.

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. This confiscated 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 condense, they flow into the receiver where they are collected.

Miriam the Prophetess is considered the mother of the protoscience of alchemy. Also called “Maria the Jewess,” she is believed to have lived in the first century A.D. Writing 500 years later, the Egyptian alchemist Zosimos credits her with inventing the kerotakis, an early forerunner of the modern still. It had characteristics of what we know as a double boiler, and in France the double boiler is still called a bain-marie—Maria's bath.

Early historical accounts of distillation focus on the distilling of wine. The ancients refered to the hard liquor made by distilling wine as aqua vita or eau ardent. By the 11th century distillation was known in Europe, and brandies and whiskies became popular. Soon after, the Chinese learned of the process, and called brandy burnt wine. But the word “brandy” originated with the Dutch, who called it brandewijn, which means “burnt wine.” A recipe for distilling wine is recorded in the medeival English cookbook Curye on Inglysch, and several methods of producing what was likely flaming brandy have been found in early 15th century manuscripts. In a work printed in London in 1602, Hugh Platt describes “Secrets in Distillation.”

Today, among the many uses of distillation are the preparation of alcoholic beverages, the refining of petroleum, and the desalination of salt water. In this activity you will distill a mixture of two liquids. That is, you will perform a fractional distillation.

General Safety Guidelines

  1. Wear protective goggles throughout the laboratory activity.
  2. 2-propanol (rubbing alcohol) is generally safe; however, it is flammable, toxic, and requires care in handling.
  3. Use open flames with caution since the alcohol and some alcohol-water mixtures are flammable.
  4. Place the alcohol-water mixtures in a designated container.

Procedure

2-Propanol

Carefully examine the mixture that is to be separated. Cautiously smell the mixture and note any odor. Dip a piece of filter paper in the mixture to wet one end and try to light it with a match. Carefully measure the density of the mixture using a graduated cylinder and balance. Test the solubility of sugar in a small amount (about 1 ml) of the liquid. Record all observations.

Distillation apparatus

  1. Set up a distillation apparatus as shown in above. Check the following details:
  2. The top of the thermometer bulb should be just below the side arm of the distilling flask, ensuring that the entire thermometer bulb will be bathed in the rising vapor as the liquid mixture is heated.
  3. The side arm should extend beyond the cork in the top end of the condenser, and the condenser tip should extend beyond the cork in the adapter. The distillate cannot pick up impurities from the corks when set up this way.
  4. Both the distilling flask and the condenser must be firmly clamped in place.
  5. To prevent having a closed system, the lower end of the adapter should not be connected to the receiver by a cork. This arrangement prevents pressure from building up and causing an accident.
  6. Begin circulating water from the lower end of the condenser prior to heating the distilling flask.
  7. Add two or three boiling chips (small marble chips or porcelain chips) to the distilling flask to prevent superheating and bumping.
  8. Have your teacher check the setup before applying heat to the flask.
  9. Label and number four large test tubes (20- x 150-mm) for collecting fractions. Place Tube 1 under the condenser outlet.
  10. Remove the thermometer with its cork. By means of a long-stem glass funnel, add about 20-30 ml 2-propanol-water mixture to the distilling flask.
  11. Gently heat the flask until the liquid begins to boil. Take temperature readings every half-minute and record these along with the time. Continue heating just enough to keep the liquid boiling while continuing to take temperature readings. Continue boiling the liquid until the temperature remains the same for about 3-4 minutes.
  12. Graphing the data from Step 4 provides the information needed to decide about the temperature range to collect separate fractions. Do the graphs before continuing. Repeat the procedure (Steps 3-4) but this time use the data collected in Step 4 to collect separate fractions of distillate in tubes #2, #3, and #4. You should be able to detect the presence of at least three separate fractions, the last one being essentially pure water.
  13. Cautiously smell each fraction and record your observations. Test the flammability of each fraction by dipping a small piece of paper in each and trying to ignite the wet paper with a match. Use a balance and graduated cylinder to measure the density of each fraction. Test the solubility of sugar in a small amount (about 1 ml) of each fraction. Record all observations.
  14. Thoroughly wash your hands before leaving the laboratory.

Data Analysis

  1. Can you tell just by looking that the clear liquid is a mixture?
  2. Does the paper dipped in the liquid burn?
  3. What is the density of the mixture prior to distillation?
  4. What is the density of each fraction?
  5. Does sugar dissolve in each fraction?

Implications and Applications

  1. What do you conclude about the composition of each fraction?
  2. On the basis of the tests performed on the original solution and each fraction, what can you conclude about the composition of each fraction?
  3. What do you think would be the result if you redistilled each fraction into three fractions of equal volume?

Concluding Questions

  1. What are the differences in properties that may be used to separate the components of a mixture?
  2. What purpose is served in separating the components of mixtures?
  3. How do components of a mixture affect the properties of a mixture?
  4. How does a mixture differ from a compound?
  5. How does a homogeneous mixture differ from a heterogeneous mixture?
  6. What are some household products that are the result of separation techniques?