April 2016

# Cold finger distillation

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Robyn Ford, Dallas Independent School District, Dallas TX

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## Introduction

We’ve gotten more mileage out of this simple lab than anything else we’ve ever done! We must admit it is easy and fast — with no disposal issues. We basically are separating water from a colored solution by boiling and collecting the water on a cold finger — created with inexpensive aluminum foil filled with ice.

Of course, we use Dr Pepper — it’s a Texas thing — but any colored aqueous liquid solution will work. Dr Pepper gives a distinct difference between caramel-colored solution and collected water. As the water is removed from the soda, the soda may darken, it is impossible to detect with our eyes. Orange juice will visibly darken when the water is separated. You can expand the lab to include colorimetry with this activity as a post-lab.

In the following analysis, we focused on the density of a solid (aluminum) and of two liquids (colored solution and distillate). This activity can also be used to illustrate a variety of high school curriculum topics. It could be used as a simple introduction to lab observations, such as the color of the original solution, the smell of the solution versus the distillate and the volumes of the original solution and distillate. One observation that will surprise students is that amount of distillate collected, combined with the remaining darker solution, will be greater than the amount of original colored solution. Ask students to explain this discrepant event. The explanation for this is that water vapor inside the large beaker condenses resulting in the increased volume of the distillate.

It is also a great review of the concept of chemical and physical change. Since students will observe a color change, you will have the opportunity to discuss whether a color change always indicates a chemical change. You could also have students calculate the percentage by volume of water that you extracted from your sample.

## Prelab questions

1. Define solute.
2. Define solvent.
3. Define solution.
4. Define distillation.
5. Differentiate between a chemical and physical change.

## Materials for each group

- Ruler
- Hot plate
- 100 mL and 800 mL beaker (small and large beakers)
- 100 mL graduated cylinder
- Stopper large enough to form a stable platform for the smaller beaker
- Aluminum foil (large enough to make cone; 10" × 12" edges as straight as possible)
- Common colored liquid solution (caramel-colored soda, orange juice, etc.)
- Ice (crushed is preferred)

## Safety

- Always wear safety goggles when in a chemistry lab.
- Exercise caution when using a heat source.
- Students should wash their hands thoroughly before leaving the lab.
- When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

## Procedure

1. Review density: Obtain a piece of aluminum foil. Determine its thickness given a density of aluminum = 2.65 g cm-3. Make sure you measure the length and width in cm before you weigh it!
2. Obtain two beakers, one small and one large.
3. Obtain the mass of the smaller beaker.
4. Obtain the mass of an empty graduated cylinder.
5. Obtain 50 mL of a colored solution.
6. Obtain the mass of the 50 mL solution inside the cylinder and add it to the bottom of the larger beaker that is setting on the hot plate.
7. Make a "cold finger" with the sheet of aluminum foil shaped into a cone with a sharp point — no holes! See above diagram for set-up.
8. Place the stopper on the bottom of the big beaker to be used as a platform for the smaller beaker. (Screw tops can be used as platforms if flat so small beaker is steady.)
9. Place the cold finger on top of the larger beaker with the point pointed toward the small beaker. Pinch the foil around the top of the larger beaker in order to prevent most of the steam from escaping.
10. Heap crushed ice on top of the aluminum cone. (You can use ice cubes — fill to the max.)
11. Heat until the ice mound has melted (Do not add more ice!) and then let the setup cool until you can separate the small and large beakers with your fingers.
12. Record observations as to the physical and chemical properties and changes observed.

Here orange soda was used as a starting solution. The orange soda is starting to darken and the steam is starting to condense.

## Analysis

1. Review the concept of density while your distillation is proceeding.

   Thickness of Aluminum Foil

   Length of aluminum foil __________ cm

   Width of aluminum foil __________ cm

   Area of aluminum foil __________ cm2

   Mass of aluminum foil __________ g

   Density of aluminum = 2.65 g cm-3

   Calculate the thickness of the aluminum foil.

2. Density of 50.0 mL colored solution

   Mass of filled 100 mL graduated cylinder __________ g

   Mass of empty 100 mL graduated cylinder __________ g

   Mass of colored solution __________ g

   Provide a picture of your filled graduated cylinder with the colored solution.

   Calculate the density of the colored solution.

3. Density of collected distillate

   Mass of smaller filled beaker (cooled!) __________ g

   Mass of smaller empty beaker __________ g

   Calculate the estimated density of the distillate collected in the smaller beaker.

   How do the densities of the distillate and colored solution compare?

   Attach a picture of your system of the two beakers on the hotplate before and after the ice has melted. Did the color of the solution change?

4. Identify chemical and physical changes.

Initial setup with Dr Pepper as the starting solution.

## Answers to analysis

1. Thickness of aluminum foil: 0.00236 cm
2. Density of the colored solution: 1.10 g/mL
3. Density of the distillate: 1.02 g/mL

   Since the distillate is water, we would expect the density to be 1.00 g/mL.

4. Physical changes:

   When heated, liquid was changed to a gaseous state. When cooled, steam condensed to form a liquid.

   No chemical changes occurred in this experiment.

Editor’s note: This lab came to our attention on ChemEdX website in an activity called “Thank you Lady from Texas” by Chad Husting, Sycamore High School, Cincinnati, Ohio.

Chad wrote about this activity presented by Diana Mason at ChemEd 2015 at Kennesaw State, Kennesaw, Georgia.

Robyn and Diana also have a version of this lab at American Association of Chemistry Teachers, AACT website.

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