Chemiluminescent Elephant’s Toothpaste

Introduction

A new twist on an old favorite—mix together hydrogen peroxide, dishwashing liquid, yeast and a light stick in a tall cylinder and stand back. Elephant toothpaste is back with “glowing” reviews!

Concepts

• Catalysts
• Decomposition reaction
• Chemiluminescence

Experiment Overview

Hydrogen peroxide decomposes through a catalyzed reaction to form water vapor and oxygen gas. Light stick chemistry is introduced, for an “enlightening” effect.

Materials

Safety Precautions

Hydrogen peroxide, 30%, will act as an oxidizing agent with practically any substance. Hydrogen peroxide is severely corrosive to the skin, eyes and respiratory tract, a very strong oxidant, and a dangerous fire and explosion risk. Do not heat this substance. Although the dishwashing liquid and yeast are considered nonhazardous, do not ingest these materials. Do not stand over the reaction; steam and oxygen are produced quickly. Wear chemical splash goggles, chemical-resistant gloves and a chemical resistant apron. Follow all laboratory safety guidelines and wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. If the procedure was carried out without any glass from the light stick being transferred into the graduated cylinder, the leftover product mixture may be washed down the drain with plenty of water according to Flinn Suggested Disposal Method #26b. If glass was introduced into the graduated cylinder, the mixture should be filtered or decanted to remove any possible broken glass pieces. The glass particulate should be placed into the broken glass container. Once the glass is removed the remaining solution may all be washed down the drain with plenty of water according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

In a small beaker, mix ⅛-ounce (half packet) of yeast into 30 mL of deionized water. Note: Prepare just prior to demonstration.

Procedure

1. Place a 100-mL graduated cylinder on a plastic tray that is several inches deep.
2. Measure 20 mL of 30% hydrogen peroxide into the 100-mL graduated cylinder. Caution: Take special care when handling 30% hydrogen peroxide. Contact with skin may cause burns.
3. Add 10 mL of dishwashing liquid to the graduated cylinder containing the hydrogen peroxide.
4. Stir the solution with the stirring rod.
5. Have students record their observations (little or no reaction occurs).
6. Activate the light stick by bending it until a cracking sound is heard.
7. Mix the contents inside the light stick by mild shaking.
8. Carefully cut open the light stick at one end and add the liquid contents to the 100-mL graduated cylinder. Note: The light sticks contain a glass ampule which, if possible, should be left in the light stick—only the solution is needed for this demonstration.
9. Stir the solution in the 100-mL graduated cylinder until the light stick color is uniform throughout the mixture.
10. Place the yeast solution next to the plastic tray containing the 100-mL graduated cylinder and then darken the room.
11. Quickly but carefully add the 30 mL of yeast suspension into the graduated cylinder.
12. Step back and instruct students to record their observations (thick, glowing foam erupts from the cylinder).

Student Worksheet PDF

12031_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration as written seven times: 140 mL of 30% hydrogen peroxide, 70 mL of dish soap, 4 packets of yeast and 7 light sticks.
• This demonstration produces foam. If a demonstration tray is not available, it is possible to do the demonstration in the laboratory sink. Cleanup is relatively easy due to the presence of safe products and the generous amount of detergent. Note: If any glass from the light stick went into the graduated cylinder, it must be disposed of properly. See the Disposal section.
• The cylinder will get hot, so let it cool before handling. Use only a borosilicate glass graduated cylinder.
• This demonstration can be easily and safely scaled up for larger audiences. A 500-mL or 1-L Pyrex^® graduated cylinder works well in this case.
• Other catalysts that will catalyze the reaction are potassium iodide, KI, sodium iodide, NaI, and manganese(IV) oxide, MnO₂. The Flinn Chemical Demonstration Kit, Old Foamey, (Catalog No. AP2085) used sodium iodide as the catalyst without the chemiluminescence.
• To demonstrate that oxygen is indeed one of the products, immediately following the demonstration, light a match or wooden splint and blow it out. Ideally, the center of the splint will still be glowing. Hold the splint very close to the foam bubbles produced—the match should reignite due to the production of oxygen.
• Show students the effect of concentration on reaction rate for the decomposition of hydrogen peroxide with the Flinn Chemical Demonstration Kit, Sudsy Kinetics, Catalog No. AP4866.

Answers to Questions

1. Describe what happened in this demonstration before and after the addition of the yeast solution.

   Hydrogen peroxide, dishwashing solution, and the contents of a light stick were added to a 100-mL graduated cylinder. A small amount of yeast was added, which caused thick glowing foam to erupt from the cylinder.

2. Write the chemical equation for the decomposition of hydrogen peroxide.

   2H₂O₂(aq) → 2H₂O(g) + O₂(g)

3. Why does the dishwashing solution foam?

   The water vapor and oxygen gas get trapped in the dishwashing liquid, causing it to foam.

4. What was the purpose of the yeast? Was it consumed during the reaction?

   The yeast served as a catalyst, which is a substance that speeds up a reaction but is not consumed during the reaction. Thereby, the yeast was not consumed during the reaction.

5. What is chemiluminescence?

   Chemiluminescence is light energy that is released from molecules that have gained chemical energy.

Discussion

This demonstration evolves a considerable amount of heat as shown by the steam coming off the foam as it is produced. The reaction, therefore, is exothermic. The action of a catalyst is demonstrated. The catalyst is the yeast which speeds up the decomposition of the hydrogen peroxide. The decomposition of hydrogen peroxide produces steam and oxygen gas (see Equation 1). The oxygen gas and water vapor cause the dishwashing liquid to foam. The light stick mixture is added to the reaction mixture to introduce the concepts of chemiluminescence and light stick chemistry.

A light stick is a chambered vessel containing two sections. The outside of the light stick is semipliable plastic and inside the plastic tube are two different reactants. To prevent the chemical reaction from occurring until desired, one reactant is put inside a glass ampule. Individual commercial formulations vary but the two reactants are hydrogen peroxide and a phenyl oxalate compound such as trichlorophenyl oxalate (TCPO) with a florescent dye. To activate a light stick the light stick is flexed (see Figure 1). The outer plastic tube material is semipliable but the inner glass ampule is not and breaks. The distinctive noise heard upon activating a light stick is the inner glass breaking. Upon mixing, the trichlorophenyl oxalate reacts with the hydrogen peroxide producing trichlorophenol, carbon dioxide, and energy (see Equation 2). The energy released in the reaction is from the decomposition of an unstable, high-energy intermediate, C₂O₄. The dye molecules harness this reaction’s energy elevating the dye molecules to an excited state. The chemical energy produced in the reaction is then transformed into light energy. The dye releases the energy by the emission of light—chemiluminescence. Chemiluminescence is light energy that is released from molecules that have gained chemical energy. Dyes used in commercial light sticks have a light emission wavelength in the visible range so that they may be seen. In the military, light sticks use dyes that may release light emissions in the visible and infrared ranges.

References

Special thanks to John J. Fortman (retired), Wright State University, Dayton, OH, for providing the idea for this activity to Flinn Scientific.

Fortman, J. J.; Burns, R.V. A Luminescent “Elephant’s Toothpaste” Demonstration. Chem 13 NEWS, 1997, May, p 8.