The Yellow and Blue Switcheroo

Demonstration Kit

Introduction

Your students will think you are the King or Queen of Chemistry when you do this amazing demonstration. Three colorless solutions are mixed to produce a yellow solution that suddenly turns blue and then yellow again. The solution will oscillate between yellow and blue for several minutes. If you have never seen or done an oscillating reaction, don’t pass this one up—it’s fit for a king or a queen!

Concepts

  • Oscillating reactions
  • Reaction mechanisms

Materials

(for each demonstration)
Hydrogen peroxide, H2O2, 8.6%, 40 mL*
Potassium iodate solution, KIO3, 0.2 M, 40 mL*
Starch–malonic acid–manganous sulfate solution, 40 mL*
Beaker, 250-mL
Graduated cylinders, 50-mL, 3
Stirring rod or magnetic stirrer with stir bar
*Materials included in kit.

Safety Precautions

Hydrogen peroxide solution is an oxidizer and a skin and eye irritant. Potassium iodate is an oxidizer; the soluton is acidified and contains sulfuric acid. Sulfuric acid is severely corrosive to eyes, skin and other tissue. Starch–malonic acid–manganous sulfate solution is a strong irritant, moderately toxic and corrosive to eyes, skin and respiratory tract. The reaction produces iodine in solution, in suspension and as a vapor above the reaction mixture. The solid iodine is toxic by inhalation. Iodine in solution is irritating to eyes, skin and respiratory tract. Perform demonstration in well-ventilated room. Avoid all body tissue contact. 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 reaction mixture may be reduced according to Flinn Suggested Disposal Method #12a.

Procedure

  1. Using a 50-mL graduated cylinder, measure out 40 mL of 8.6% hydrogen peroxide solution and transfer it to a 250-mL beaker.
  2. Using a clean 50-mL graduated cylinder, measure out 40 mL of the 0.2 M potassium iodate acidified solution and add it to the beaker. Stir using a stirring rod or magnetic stirrer.
  3. Using the third 50-mL graduated cylinder, measure out 40 mL of the starch–malonic acid–manganous sulfate solution. Add this solution to the beaker and stir.
  4. Bubbles will begin to appear. In a short period of time, the solution will turn yellow, then blue, and finally colorless. The entire process repeats itself over and over again. The yellow to blue to colorless oscillations will continue for about 10 minutes.

Student Worksheet PDF

14127_Student1.pdf

14127_Teacher1.pdf

Teacher Tips

  • The reaction can also be done using 3% hydrogen peroxide, although the color changes will not be as sharp. Therefore an 8–9% solution is recommended for this demonstration.
  • A magnetic stirrer can be used to stir the solution throughout the entire demonstration or used to simply mix the solutions well at the beginning and then simply enjoy the oscillations.
  • Use only distilled or deionized water. Chloride ions from tap water can contaminate the reaction and stop the oscillations.

Correlation to Next Generation Science Standards (NGSS)†

Science & Engineering Practices

Analyzing and interpreting data
Engaging in argument from evidence
Obtaining, evaluation, and communicating information

Disciplinary Core Ideas

MS-PS1.B: Chemical Reactions
HS-PS1.B: Chemical Reactions

Crosscutting Concepts

Stability and change
Cause and effect
Patterns

Performance Expectations

MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
HS-PS1-2. Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
HS-PS1-6. Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.

Answers to Questions

  1. Describe what happened in this demonstration.

    Hydrogen peroxide, potassium iodate and starch-malonic acid-manganous sulfate solution are all added to a beaker. The colorless solution starts to bubble and soon it turns yellow. The solution then turns blue and then colorless, and the process repeats itself for about 10 minutes.

  2. This reaction can be divided into two component reactions. Write the equation for each reaction described.
    1. Iodate ions reacting with hydrogen peroxide and hydrogen ions to form the intermediate molecule HOI.

      IO3–(aq) + 2H2O2(aq) + H+(aq) → HOI(aq) + 2O2(aq) + 2H2O(l)

    2. The reaction between HOI and malonic acid.

      HOI(aq) + CH2(CO2H)2(aq) → ICH(CO2H)2(aq) + H2O(l)

  3. The color of the solution depends on the concentrations of iodine (I2) and iodide ions (I–). What color is the solution when:
    1. I2 is high, I– is high

      The solution is blue.

    2. I2 is high, I– is low

      The solution is yellow.

    3. I2 is low, I– is high

      The solution is clear.

Discussion

This oscillating reaction is known as the Briggs-Rauscher (BR) Reaction and was developed by Thomas S. Briggs and Warren C. Rauscher of Galileo High School in San Francisco. The reaction mechanism is very complex. During the reaction, oscillations occur in the concentration of iodine and iodide ions. The yellowish color is attributed to the rise in I2 concentration; the blue-black color of the starch–iodine complex results from the rise in both I– and I2 concentrations. The colorless solution is caused by the decline in I2 concentration and the continued rise in I– concentration.

The blue-black starch iodine complex is amylose-iodine. Amylose is the linear starch fraction which is com posed of chains of 1,4 linked α-glucose units as shown above. The color of the complex, blue-black, comes from the pentaiodide anion, I5– formed when I2 and I– concentration are elevated. Though normally an unstable anion, it becomes stable as a part of the starch complex.

{14127_Discussion_Figure_1}
The overall BR reaction is:

IO3–(aq) + 2H2O2(aq) + CH2(CO2H)2(aq) + H+(aq) → ICH(CO2H)2(aq) + 2O2(aq) + 3H2O(l)

This reaction consists of two component reactions that create an intermediate molecule HOI.
{14127_Discussion_Equation_1}
{14127_Discussion_Equation_2}
The two reactions are themselves very complex, consisting of ten steps. Iodine (I2) and iodide ions (I–) are produced as intermediates in various steps of these reactions.

In the proposed reaction mechanism, the concentration of HOI rises and falls, triggering oscillations in the I– and I2 concentrations in solution. When I2 and I– concentrations are high, the solution is blue; when I2 is high and I– is low, the solution is yellow; and when I2 is low and I– is high, the solution is clear.

The oscillations continue until either malonic acid or iodate ions are consumed. A detailed explanation of the reaction is included in the attachment.

References

Special thanks to Jim and Julie Ealy, The Peddie School, Hightstown, NJ, who provided us with the instructions for this activity.

Briggs, T. S. and Rauscher, W. C. J. Chem. Ed., 1973, 50, 496.

Kolb, D. J. Chem Ed., 1988, 65, 1004.

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press: Madison; 1985; Vol. 2, pp. 248–256.

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†Next Generation Science Standards and NGSS are registered trademarks of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.