Instructor Information
JCE Classroom Activity: #54
Out of the Blue
W
Mark E. Noble Department of Chemistry, University of Louisville, Louisville, KY 40292 This Activity uses color as an indication of chemical change. It also involves experimentation in testing the roles of various reactants in accomplishing the changes.
perforated
This Activity is derived from the classic blue bottle demonstration (dating from the 1950s; ref 1).W A related Tested Demonstration in this issue of the Journal provides full details and additional references (2). This version of the blue bottle has a milder formulation, with accessible ingredients and reduced total consumption of ingredients. The Activity involves a series of oxidation–reduction cycles in which the color effects are due to reduction of methylene blue by ascorbic acid, followed by oxidation back to methylene blue by O2. Copper(II) acts as a catalyst (presumably via Cu2+/Cu+). Cl– contributes, presumably as a ligand to copper. Two key points are illustrated in Part IIA of the Activity: dye reduction does not require the catalyst although it is accelerated by it; and, the oxidation step does require the catalyst. The concentration of copper is too small to affect the general color.
Integrating the Activity into Your Curriculum This Activity could be used with units on chemical kinetics and oxidation–reduction reactions.
About the Activity Students first prepare a standard formulation for the blue bottle reaction. Depending on the time allotted, the instructor may wish to indicate how many times the students should repeat the shake-and-stand sequence. Students are not told that air (oxygen) is a critical reactant. A clue is presented by noting the color across the bottom of the solution–air interface; there should be a thin layer of color due to diffusion of O2 into the upper layer of solution. To help students deduce the role of air, the instructor can prepare a bottle of the standard solution 1–2 days in advance and have it on display. The sides of the bottle slowly collapse due to air pressure as O2 is lost from the air in the bottle. All ingredients are available commercially. Ascorbic acid is available as “Vitamin C Powder” packaged by Solaray, Inc. and is also sold in bulk in natural food stores. Partly oxidized (aged, off-white) ascorbic acid or vitamin C tablets should not be used. A methylene blue suspension (5%) is available as “MethyBlu” by Aquatronics, sold in pet stores for aquariums. If MethyBlu is used, students need to shake the bottle each time immediately before use to resuspend the solid methylene blue. Copper(II) sulfate solution (1.6% Cu) is available as “Had-A-Snail” by Aquarium Products, also sold in pet stores for aquariums. Specific commercial products are cited only as examples; similar products may be available. Alternatively, the instructor can prepare the solutions (2). Iodized or non-iodized table salt can be used. Either tap or deionized water can be used, but the pH is important: a pH around 3.0 is recommended. More ascorbic acid can be added to lower the pH or baking soda can be added to raise the pH if necessary. Alternatively, adding more methylene blue may suffice (2). In Part II, students change the standard formulation and compare observations. Different teams may conduct different changes and then share their results with the entire class. In Step IIA.1, the color stays blue, because there is no reducing agent to reduce the methylene blue. In Step IIA.2, the solution turns colorless, but the blue color does not return upon shaking (since re-oxidation back to blue is quite slow without the catalyst). In Optional Steps IIB.3– 5, students vary the formulation to achieve “better” results. Two possible goals are presented: a deeper blue color and faster decoloration. Many variations favor one but disfavor the other (2). Students decide which results are “better”. Vitamin C in containers opened in a laboratory is not safe for human consumption. The bottle and measuring spoons should not be used later for food/drink. The Cu2+ and methylene blue solutions are not intended for human contact; flush liberally with water if the solutions come in contact with skin. The final solutions can be neutralized by addition of one teaspoon baking soda prior to disposal, then poured down the sink with a large volume of water.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
fold here and tear out
Background
Answers to Questions 1. Vitamin C is responsible for the blue color disappearing, although other ingredients may hasten the process. 2. The Cu2+ catalyzes the re-oxidation back to methylene blue by O2. However, until the role of air is determined by the student, it will appear that Cu2+ is the reactant for re-oxidation back to blue. 3. Air. Specifically, it is O2, but this is not directly discernible from the steps presented. 4. A deeper blue color can be most easily obtained by using more methylene blue; less vitamin C will also help. 5. Less methylene blue will decolorize faster. Increasing vitamin C and Cu2+ will also help.
References, Additional Related Activities, and Demonstrations 1. 2.
Campbell, J. A. Kinetics–Early and Often. J. Chem. Educ. 1963, 40, 578–583. Wellman, Whitney E.; Noble, Mark E. Greening the Blue Bottle. J. Chem. Educ. 2003, 80, 537–540. JCE Classroom Activities are edited by Nancy S. Gettys and Erica K. Jacobsen
JChemEd.chem.wisc.edu • Vol. 80 No. 5 May 2003 • Journal of Chemical Education
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JCE Classroom Activity: #54
Student Activity
Out of the Blue Chemical reactions come in many different types and can involve many different ingredients. Chemical reactions occur everywhere across the universe, from inside every cell of your body all the way up to the grand openness of outer space. There are numerous ways of knowing when a reaction occurs. One of the most common is a change in color. This Activity explores color as a sign that a chemical reaction has occurred. Although there are actually several reactions involved in this Activity, it is possible to discover the roles played by some of the reactants by making appropriate changes to the formulation and observing the results.
Try This You will need: water, granular or powdered vitamin C (ascorbic acid), solution of copper(II) sulfate (1.6% Cu), 5% solution of methylene blue (a blue dye), table salt Be safe! Wear safety (mostly sodium chloride), clock or timer, marker or tape, and a clear, colorless 2-liter goggles. Be certain that soft drink bottle (remove label and rinse out) with its cap. bottle caps fit properly. I. Standard Solution When you are done with __1. Fill a clean, clear, colorless 2-liter soft drink bottle approximately two-thirds full your solutions, follow your of water. Mark the water level on the outside of the bottle with a marker or instructor’s directions for piece of tape. You will fill the bottle to this line each time you repeat the proper disposal. reaction. __2. Add to the bottle: one level teaspoon (5 mL) of granular or powdered vitamin C, four drops of methylene blue solution (shake or mix solution immediately before use), and one-half teaspoon (2.5 mL) of table salt. __3. Cap the bottle securely; shake to dissolve all the solids. __4. Remove the cap and add one-quarter teaspoon (1.25 mL) of copper(II) sulfate solution. __5. Recap the bottle securely. Shake the bottle for 20 seconds. Record your observations of the solution’s color, looking horizontally through the side of the bottle. __6. Let the bottle stand undisturbed for 5–6 minutes. What happens to the color? Now, observe very carefully from underneath the surface of the solution where it meets the air in the bottle: does it have a different color from the rest of the solution? __7. Repeat steps 5 and 6 several times (your instructor may specify the number of times); record your observations each time. II. Comparisons To Make Part A. What happens if you leave out an ingredient? Repeat the recipe from Part I, but with one of the following changes. What happens in each case? Record your observations. (Don’t make both changes at once.) __1. Leave out the vitamin C. __2. Leave out the copper(II) sulfate solution. Part B. (Optional) What happens if you change the quantities of the ingredients? Can you get a deeper blue color? Can you make the blue color disappear faster? Make only one change at a time (this makes it easier to figure out how each ingredient affects the outcome separately), and compare this to your prior results. How do you decide what is better overall? __3. Use two or four level teaspoons of vitamin C instead of one teaspoon. __4. Use two or eight drops of methylene blue solution instead of four drops. __5. Use one teaspoon of the copper(II) sulfate solution instead of one-quarter teaspoon.
Questions __1. __2. __3. __4. __5.
What single ingredient makes the blue color disappear? What does the copper(II) sulfate solution do? One required reactant is not in the list of required materials. What is this reactant? What changes make the blue color deeper? (Optional Part II.B.) What changes make the blue color disappear faster? (Optional Part II.B.)
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
536B
Journal of Chemical Education • Vol. 80 No. 5 May 2003 • JChemEd.chem.wisc.edu
