FLINN LAB www.flinnsci.com Catalog No. AP6384 The Reappearing Pumpkin Chemical Demonstration Kit Introduction A slight variation of the classic “Blue Bottle Experiment” allows a pumpkin face to disappear and then reappear over and over again. Chemical 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: O2 + MB → MBox Equation 1—Shaking Equation 2—Standing CH + OH– → C– + H2O MBox + C– → MB + X– MB MBox CH X– = reduced form methylene blue (colorless) = oxidized form methylene blue (blue) = glucose = 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 (for each demonstration) Sodium hydroxide solution, NaOH, 6 M, 42 mL* Blue or black marker Glucose, C6H12O6, 10 g* Beaker, 1000-mL Yellow food coloring, 10–20 drops* Distilled water, 1000-mL Methylene blue, 1 mL* Stirring rod Flask, 1000-mL (Florence or volumetric, round) Rubber stopper to fit flask *Materials included in kit. 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 Material Safety Data Sheets for additional safety, handling, and disposal information. ©2020 Flinn Scientific, Inc. All Rights reserved AP6384PUB 091707 Preparation 1. Use a black or dark blue marker to draw a pumpkin face onto the outside of the demonstration flask. 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. Disposal Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing the disposal of laboratory waste. The solution may be disposed of according to Flinn Suggested Disposal Method #10. 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. –2– © 2007 Flinn Scientific, Inc. All Rights Reserved. AP6384PUB • 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. Connecting to the National Standards This laboratory activity relates to the following National Science Education Standards (1996): Unifying Concepts and Processes: Grades K–12 Constancy, change, and measurement Content Standards: Grades 5–8 Content Standard B: Physical Science, properties and changes of properties in matter Content Standards: Grades 9–12 Content Standard B: Physical Science, structure and properties of matter, chemical reactions Answers to Worksheet 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). Reference Cook, A. G., Tolliver, R. M., Williams, J. E. J. Chem Ed., 1994, 71, 160–161. The Reappearing Pumpkin—Chemical Demonstration Kit is available from Flinn Scientific, Inc. Catalog No. AP6384 GP3095 AP8653 AP5929 Description The Reappearing Pumpkin—Chemical Demonstration Kit Flask, Florence, Pyrex® Feeling Blue Vanishing Valentine Consult your Flinn Scientific Catalog/Reference Manual for current prices. –3– © 2007 Flinn Scientific, Inc. All Rights Reserved. AP6384PUB Name:_____________________________________ The Reappearing Pumpkin Demonstration Worksheet Discussion Questions 1. Describe what happened in this demonstration. 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. 3. What is an oxidation/reduction reaction? AP6384PUB © 2007 Flinn Scientific, Inc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased The Reappearing Pumpkin, Catalog No. AP6384, from Flinn Scientific, Inc. No part of this material may be reproduced or transmitted in any form or by any means, electronic or mechanical, including, but not limited to photocopy, recording, or any information storage and retrieval system, without permission in writing from Flinn Scientific, Inc.
