Vanishing Valentine

Introduction

Watch your Valentine vanish and then reappear with this favorite holiday demonstration. Simply shake a flask containing a colorless solution. The solution will change to a bright pink valentine color. Then, allow the solution to sit undisturbed and observe as it fades back to colorless. The cycle may be repeated many times.

Concepts

• Oxidation–reduction
• Indicators

Materials

Safety Precautions

Sodium hydroxide solution is a corrosive liquid; skin burns are possible; it is very dangerous to eyes. The dextrose and resazurin solutions are not considered hazardous; however, avoid skin and eye contact as with all chemicals. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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. Neutralize and dispose of the resulting Vanishing Valentine solution according to Flinn Suggested Disposal Method #10.

Procedure

1. Place 100 mL of the 0.13 M dextrose solution into a 500-mL Erlenmeyer flask.
2. Add 100 mL of the 1.0 M sodium hydroxide solution to the flask. Swirl the flask to mix the two solutions.
3. Add approximately 8 drops of the 1% resazurin solution to the flask.
4. Stopper the flask well and swirl the solution to mix. The solution color will be blue.
5. Allow the solution to sit and become fully reduced (colorless). Shake the flask to introduce oxygen and to obtain a pink valentine color.
6. Repeat step 5. Reintroduce oxygen when needed by briefly removing the stopper from the flask. The “Vanishing Valentine” will last well over an hour.

Student Worksheet PDF

11889_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration seven times: 700 mL of 0.13 M dextrose solution, 700 mL of 1.0 M sodium hydroxide solution, and 10 mL of 1% resazurin solution.
• The initial blue solution changes to a pink color after a short time. Allow the solution to sit undisturbed until it becomes colorless. This may take as long as 10 minutes. Show the colorless solution to the class and then place it behind your back and shake it gently. Show the students that it is now pink. The Vanishing Valentine solution will last one hour or so, depending on how often it is shaken and how much oxygen is reintroduced by opening the bottle. The color will become less vivid with time.
• The resazurin solution may have a limited shelf life. A freshly made solution has a deep blue color. The reaction may not work as well if the color of the solution has faded or changed.
• Consider performing this demonstration in a separatory funnel since its shape resembles a heart. Clear, square PETG plastic bottles (Flinn Catalog No. AP8963) also work well for this demonstration since they have a secure screw cap.
• A variation on this demonstration is the popular “Blue Bottle” experiment, which uses methylene blue indicator in place of the resazurin. The “Blue Bottle” consists of similar reversible oxidation–reduction reactions, where the colorless solution turns blue upon shaking (addition of oxygen) and the cycle repeats. Flinn provides this demonstration as the Feeling Blue Chemical Demonstration Kit, Catalog No. AP8653.
• Another variation on this demonstration is the “Stop-’N-Go Light”, which uses indigo carmine indicator in place of the resazurin. In this demonstration, upon shaking (addition of oxygen), a yellow solution changes to red which then changes to green with the addition of more oxygen. The cycle can then be repeated. Flinn provides this demonstration as the Stop-’N-Go Light Chemical Demonstration Kit, Catalog No. AP2083.

Answers to Questions

1. Describe what happened in this demonstration.

Dextrose solution and sodium hydroxide were mixed together in a flask. A few drops of resazurin solution were added, and after swirling, the solution turned blue. As the solution sat, it became pink in color, and then colorless. When the solution was swirled, it changed back to pink.

2. The color changes in this demonstration involved both reversible and irreversible reactions. Which color change do you think was due to an irreversible reaction? Which color change was due to a reversible reaction?

The blue-to-pink color change was because of an irreversible reaction, since the blue color never returned. The pink-to colorless change was a reversible reaction, as it could be repeated several times.

3. By shaking the flask, oxygen is introduced to the solution, oxidizing the colorless dihydroresorufin into pink resorufin. When the solution is colorless, why do you think there is a pink tinge at the very top of the solution?

The pink tinge appears because oxygen is mixing with the solution right at the interface. Oxygen is what is responsible for the oxidation reaction that turns the colorless solution back to pink.

4. What is an oxidation–reduction reaction?

An oxidation–reduction (or “redox”) reaction occurs when one or more electrons are transferred between molecules. Oxidation refers to a loss of electrons (and rise in oxidation state), and reduction refers to a gain of electrons (and subsequent decrease in oxidation state).

Discussion

The color changes observed in this demonstration are the result of reversible and irreversible oxidation and reduction reactions of the indicator dye, resazurin. Resazurin indicator is blue in solution, but as soon as it is added to the alkaline dextrose solution, the dextrose reduces the resazurin to resorufin in an irreversible reduction. This is evidenced by the color change of the solution from blue to a fluorescent red (or pink).

The fluorescent red resorufin molecule can then be further reduced by the alkaline dextrose (in a reversible reaction this time) to the colorless compound, dihydroresorufin.

The colorless, fully reduced form, dihydroresorufin, can easily be oxidized back to resorufin with just a gentle shake of the flask. Shaking the flask introduces enough atmospheric oxygen into the solution to reproduce the fluorescent red resorufin. This reversible oxidation–reduction step can be repeated as long as there is sufficient oxygen in the flask to reoxidize the dihydroresorufin. Notice that as the colorless solution is allowed to sit, there is a pink color at the interface between the gas (oxygen in the flask) and the liquid (the solution). This is a good example of a gas–liquid phase reaction, demonstrating that oxygen is entering the solution at the surface, causing the pink color.

During this process, the alkaline dextrose is irreversibly oxidized to a variety of products, including gluconic acid, glucuronic acid and δ-gluconolactone. The oxidizing agent is the oxygen gas and it is thought that the indicator resazurin acts as a catalyst. After repeated cycles, the reaction mixture will turn to a yellow color (rather than the expected colorless solution). The final yellow solution color is the result of the transformations and degradations of dextrose which occur in alkaline solution.

References

Special thanks to Mike Shaw, West Stokes High School, King, NC, for providing the instructions for this activity.

Shakashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers in Chemistry; University of Wisconsin: Madison, WI; 1989; Vol. 2, pp 142–146.