When an ideal mixture of two miscible liquids is heated to boiling, the solution boils at a temperature between the boiling points of each component. If these liquids have very different boiling points, when the mixture starts to boil, the vapor is rich with the molecules of the more volatile component. This phenomenon is often used to separate mixtures using simple distillation, where a mixture of two miscible liquids is heated and the vapor is then condensed back to liquid and collected.
As the vapor rich with the more volatile component is collected as the distillate, the liquid phase becomes rich with the molecules of the less volatile component. However, this technique requires the solution to be heated at least to the boiling point of the more volatile compound and often beyond that.
In the case of temperature-sensitive organic compounds, this high temperature could lead to the organic molecules decomposing into something else. So, how can we separate these types of compounds? First, let's take a step back.
Recall that the pressure of a vapor in equilibrium with its condensed phase is called vapor pressure. The components of a mixture of liquids each have their own vapor pressure, which we call their partial pressure. We know that a solution boils when the total vapor pressure of the solution is equal to the atmospheric pressure. The total vapor pressure is equal to the sum of the partial pressures of the components.
For a mixture of miscible liquids, meaning that any combination of the liquids forms a homogeneous solution, the partial pressures are calculated from the vapor pressures of the pure compounds multiplied by their mole fractions in the solution. However, for a heterogeneous mixture of immiscible liquids, meaning that the liquids are insoluble in each other, the partial pressures are simply the vapor pressures of the pure compounds.
Since each component of the heterogeneous mixture contributes to the total vapor pressure independently of the other components, the mixture boils when the total vapor pressure, which is the sum of the partial pressures, is equal to the atmospheric pressure. This occurs at a lower temperature than the individual boiling points of each component because the total vapor pressure increases with temperature much faster than you would expect for even the most volatile component.
We can harness this phenomenon to perform steam distillation, which is used to isolate a temperature-sensitive organic compound that decomposes under high heat and is insoluble in water from non-volatile substances. The steam distillation setup is similar to a simple distillation setup with the addition of a water reservoir to replenish water throughout the process.
As the mixture boils, both the water and the organic compound of interest are vaporized. The water and organic compound vapors travel into the condenser, are condensed to liquid, and collected. The immiscible liquids are separated afterward. Only water and non-volatile materials are left in the mixture in the flask.
In this lab, you will set up and perform a steam distillation experiment to extract essential oil from the non-volatile components of an orange peel. You'll then use liquid-liquid extraction to extract the essential oil from water into an organic solvent.
At the end of this lab, students should know...
What is the vapor pressure of a substance?
The vapor pressure of a pure substance is the pressure exerted by the gaseous phase on the liquid or solid phase in a closed container.
At what point does a liquid boil?
A liquid boils when the vapor pressure of the liquid is equal to the atmospheric pressure.
What is the difference between a miscible and immiscible solution?
A miscible solution contains substances that can form a homogeneous solution, whereas an immiscible mixture is composed of substances that cannot be mixed, and therefore form a heterogeneous mixture.
How is the total pressure of an immiscible mixture defined
In an immiscible mixture, the sum of the individual pure component vapor pressures equals the total vapor pressure of the solution.
How does the steam distillation setup differ from a simple distillation setup?
The steam distillation setup has a water reservoir, which lowers the boiling point of the mixture by forming an immiscible mixture with the organic compounds.
Source: Lara Al Hariri at the University of Massachusetts Amherst, MA, USA
- Preparation of the Laboratory
Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.
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To begin, put on the appropriate personal protective equipment, including a lab coat, safety goggles, and gloves.
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Set out the following glassware and equipment that the students will need to build the distillation apparatus (we suggest that students work in pairs):
2Lab stands 1Stir plate 1Heating mantle 1Temperature controller 1500-mL round-bottom flask with cork stand 1Distilling head 1Condenser 1Connecting tube 1Separatory funnel 1Claisen adapter 1Thermometer adapter 1Thermometer 1Small spatula 1Stir bar 4Joint clips 2Pieces of rubber tubing 1Tube of vacuum grease 2Standard clamps 1Ring support 1Conical funnel 1250-mL round-bottom flask and cork stand 1100-mL graduated cylinder 2150-mL beakers 1125-mL Erlenmeyer flask 1Filter paper 1500-mL beaker -
You will need one blender to share between all of the groups. Place it in a central location along with a 10-mL graduated cylinder and a bottle of deionized water.
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Next, assemble the steam distillation apparatus for the students to refer to during the experiment. First, place the two stands next to each other in the hood.
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Then, put the stir plate on the base of one of the stands. Set the heating mantle on top of the stir plate and plug it into the temperature controller.
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Clamp the 500-mL round-bottom flask to the stand above the heating mantle.
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Locate the Claisen adapter and connect it to the flask.
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Clamp the condenser to the second stand, orienting it so that it is nearly horizontal.
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Connect the thermometer adapter to the distilling head and gently insert the thermometer until the bulb is in line with the side arm of the distilling head.
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Then, add the distilling head into the outer arm of the Claisen adapter and connect the condenser to the distilling head.
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Attach one piece of rubber tubing to the inlet of the condenser, which is the port furthest away from the distilling head.
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Attach the other end of the tubing to the tap water faucet in the hood. Connect the second piece of rubber tubing to the other port, which is the outlet, and place the end of the tubing in the drain.
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Now, get the separatory funnel and make sure that the stopcock is closed. Insert the separatory funnel into the Claisen adapter.
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Finally, attach the connecting joint to the end of the condenser and attach a 250-mL round-bottom flask to the connecting joint. Secure each joint with a plastic clip.
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Now that you have set up the demo apparatus, prepare the oranges. Each group will need 3 long peels on a watch glass, so obtain as many oranges and watch glasses as needed.
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Peel all of the oranges and place 3 peels on each watch glass. Allow the peels to dry for two hours.
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Next, obtain a bottle of magnesium sulfate and ensure that there is enough for each lab group to have a small spatula full, or about 2 mg each.
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You will also need a bottle of diethyl ether. Each group will need 40 mL, so ensure that there is enough for the class size.
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Finally, make sure there is a dedicated container for non-halogenated waste.
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