The Reappearing Pumpkin

Introduction

A slight variation of the classic “Blue Bottle Experiment” allows a pumpkin face to disappear and then reappear over and over again.

Concepts

• Oxidation–reduction
• Reaction kinetics

Background

The classic blue bottle experiment has been described and used for many chemical education purposes. The reaction in the blue bottle experiment is an oxidation–reduction reaction. The bottle contains sodium hydroxide, glucose, water and an indicator (methylene blue). Glucose is a reducing agent and in an alkaline solution will reduce methylene blue to its colorless form. Shaking the bottle allows the oxygen in the air to oxidize the methylene blue to its blue form (Equation 1). Upon standing it is reduced back to its colorless form as shown in Equation 2:

MB = reduced form methylene blue (colorless)
MB_ox = oxidized form methylene blue (blue)
CH = glucose
X^– = oxidation products of glucose : arabinoic, formic, oxalic and erythronic acids

In the demonstration, the dark blue color due to the oxidized form of methylene blue is used to mask the appearance of the pumpkin face. Yellow food coloring is added to the solution so the reaction now oscillates between an orange and a dark bluish color. When the methylene blue is reduced back to its colorless state, the pumpkin face “magically” reappears.

Materials

Safety Precautions

Sodium hydroxide is a corrosive solid, may cause skin burns and is very dangerous to the eyes. Methylene blue is slightly toxic. 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. The solution may be disposed of according to Flinn Suggested Disposal Method #10.

Prelab Preparation

1. Use a black or dark blue marker to draw a pumpkin face onto the outside of the demonstration flask.
   {13880_Preparation_Figure_1_Pumpkin face on outside of flask}
2. Mix the necessary chemicals in a separate beaker using the following recipe:

   Sodium hydroxide solution, 42 mL
   Glucose, 10 g
   Methylene blue solution, 1 mL
   Distilled water, 1 L
   Stir until all of the ingredients have dissolved.

3. Pour the liquid into the pumpkin-faced flask until the flask is about 90% full.
4. Allow the flask to sit undisturbed until the solution turns clear.
5. Slowly add drops of yellow food coloring until the solution turns an orange color. (This is likely to be 10–20 drops of food coloring.) If the solution turns blue, just wait until it turns colorless or orange again.
6. Test the bottle to be sure everything works. Place a rubber stopper into the top of the flask. Shake the bottle vigorously. The solution should turn dark blue (if it does not, then add more methylene blue). The pumpkin face should now be “invisible” from several feet away. Allow the bottle to set undisturbed. The solution should turn to orange and the dark-colored face outline should become visible.

Procedure

1. Shake the flask vigorously out of the view of students. It should be a dark blue and the happy pumpkin face should not be visible.
2. Set the bottle on a demonstration spot where students can see the bottle.
3. As students observe the solution, it should change color and the pumpkin face should become visible.
4. Repeat the demonstration again. Shake the bottle and make the face “disappear” and then let it set and “reappear.”
5. Discuss the demonstration and its chemistry.

Student Worksheet PDF

13880_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to set up seven flasks worth of solution. One flask is likely to “work” for several hours and can be used for more than one class period. The kit contains: 400 mL of sodium hydroxide solution, 70 g glucose, 1 bottle of yellow food coloring and 20 mL of methylene blue.
• A round Florence flask or a volumetric flask works best for this demonstration since the round shape looks more like a pumpkin than a regular flask.

Answers to Questions

1. Describe what happened in this demonstration.

   A dark blue solution in a flask was shaken vigorously. The blue color changed to an orange, and a black “pumpkin face” was visible on the flask. After the solution was allowed to stand, the color changed back to dark blue and the face disappeared.

2. Oxygen will oxidize methylene blue, once it is in its reduced form. What color is methylene blue in its oxidized state? What color is it in its reduced state? Hint: Food coloring was added to the solution to produce the orange color.

   Methylene blue is blue in its oxidized state and colorless in its reduced state. Though the solution appears orange the solution has been reduced, the orange color is simply due to food coloring. Therefore, methylene blue’s “natural” color in its reduced form is colorless.

3. 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).

References

Cook, A. G., Tolliver, R. M., Williams, J. E. J. Chem Ed., 1994, 71, 160–161.