A Flash of Blue

Demonstration Kit


In a flash, a colorless solution turns a dramatic deep-blue color! Amaze your students with this popular starch–iodine clock reaction.


  • Clock reactions
  • Catalysts
  • Kinetics
  • Rates of reaction
  • Indicators


Liquid starch, 5–7 mL*
Potassium iodate solution, 0.01 M, KIO3, 100 mL*
Sodium meta-bisulfite, Na2S2O5, 0.02 g*
Sulfuric acid solution, 1 M, H2SO4, 3 mL*
Water, distilled or deionized, 95 mL
Beaker, 250-mL
Graduated cylinder, 10-mL
Graduated cylinder, 100-mL
Stirring rod
*Materials included in kit.

Safety Precautions

Potassium iodate solution is an oxidizer. It is moderately toxic by ingestion and a body tissue irritant. Sodium meta-bisulfite is a skin and tissue irritant. Sulfuric acid solution is corrosive to eyes, skin and other tissues and moderately toxic by ingestion. 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.


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. Dispose of the resulting solution according to Flinn Suggested Disposal Method #12a.


  1. Mix 5–7 mL of liquid starch plus 95 mL of distilled or deionized water in a 250-mL beaker. Stir.
  2. Add 0.02 g of sodium meta-bisulfite to the starch solution. Stir to dissolve.
  3. Add 3 mL of 1 M sulfuric acid solution to the beaker and stir.
  4. Quickly, but carefully, add 100 mL of 0.01 M potassium iodate solution to the beaker. Stir to mix.
  5. Observe the appearance of the deep-blue color which suddenly appears (after about 10–15 seconds).

Student Worksheet PDF


Further Extensions

Performing the procedure above as indicated is an attention-getting demonstration that shows the classic characteristics of a clock reaction. However, when combined with the procedures outlined below, this reaction is ideal for studying kinetics—the effects of concentration, temperature, and the presence of a catalyst on the rate of reaction.

  1. Time the above reaction by measuring the time between the addition of the potassium iodate solution and the appearance of the blue color. Begin timing as soon as the potassium iodate solution is added. 
  2. Vary the concentration of the potassium iodate solution and measure the time until the appearance of the blue color for each concentration studied. Graph the data to determine the effect of concentration on the reaction rate. The data should show that as the concentration of potassium iodate is decreased, the rate of reaction decreases. 
  3. Vary the temperature of the potassium iodate solution and measure the time until the appearance of the blue color for each temperature studied. Graph the data to determine the effect of temperature on the reaction rate. The data should show that as temperature increases, the rate increases, and as temperature decreases, the rate decreases also.

For enough material to perform several kinetic studies of this clock reaction seven times, consider purchasing AP4601, Iodine Clock Reaction: Effect of Concentration, Temperature, and a Catalyst on Reaction Rates Chemical Demonstration Kit.

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Developing and using models
Planning and carrying out investigations
Constructing explanations and designing solutions
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

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.
MS-PS1-4. Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
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-PS1-5. Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.

Answers to Questions

  1. Describe what happened in this demonstration. Include all the chemicals used.

    Sodium meta-bisulfite and sulfuric acid were added to a solution already containing liquid starch and water. Potassium iodate was then added, and in about 15 seconds a deep blue color was produced.

  2. The reaction that occurred in this demonstration involved several steps. Write a balanced chemical equation for each step listed:
    1. Sodium meta-bisulfite produces hydrogen sulfite ions in water

      H2O(l) + Na2S2O5(s) → 2HSO3(aq) + 2Na+(aq)

    2. Potassium iodate decomposes

      KIO3(aq) → IO3(aq) + K+(aq)

    3. Iodate ions react with hydrogen sulfite ions

      IO3(aq) + 3HSO3(aq) → I(aq) + 3H+(aq) + 3SO42–(aq)

    4. Iodide ions react with iodate ions in the presence of hydrogen ions

      6H+(aq) + 5I(aq) + IO3(aq) → 3I2(aq) + 3H2O(l)

    5. Before the iodine combines with starch to produce the color change, it reacts with any excess hydrogen sulfite ions to form iodide ions

      I2(aq) + HSO3(aq) + H2O(l) → 2I(aq) + SO42–(aq) + 3H+(aq)

  3. Define a clock reaction. What part of this reaction triggered the “alarm?”

    A clock reaction is a reaction with an initial period of no change, followed by an abrupt change of some sort, usually in color. The alarm is the term for the sudden change of the reaction. The production if iodine, and iodine’s subsequent reaction with starch to produce a dark blue colored complex, is the alarm in this reaction.


This reaction is a classic example of a clock reaction. A clock reaction is a reaction characterized by an initial period with no noticeable change, followed by a sudden change, commonly in the color of the solution. The time period during which no noticeable change occurs is called the clock period, and the sudden change is called the alarm. What actually triggers the alarm varies from clock reaction to clock reaction.

In this reaction, potassium iodate and sodium meta-bisulfite react to form iodine. The starch solution serves as an indicator of the end of the reaction, forming a deep-blue colored starch–iodine complex in the presence of iodine. The chemical pathway for the formation of iodine is complicated and not completely understood, but the following mechanism serves as an outline.

Step 1: Sodium meta-bisulfite contributes hydrogen sulfite ions, HSO3, while potassium iodate contributes iodate ions, IO3, to the solution.

H2O(l) + Na2S2O5(s) → 2HSO3(aq) + 2Na+(aq)
KIO3(aq) → IO3(aq) + K+(aq)

Step 2: The iodate ions react with the hydrogen sulfite ions to produce iodide ions, I.

IO3(aq) + 3HSO3(aq) → I(aq) + 3H+(aq) + 3SO42–(aq)

Step 3: In the presence of hydrogen ions, H+, the iodide ions react with excess iodate ions to produce iodine, I2.

6H+(aq) + 5I(aq) + IO3(aq) → 3I2(aq) + 3H2O(l)

Step 4: Before the iodine can react with the starch to produce a dark-blue colored complex, it immediately reacts with any hydrogen sulfite ions still present to form iodide ions.

I2(aq) + HSO3(aq) + H2O(l) → 2I(aq) + SO42–(aq) + 3H+(aq)

Step 5: Once all of the hydrogen sulfite ions have reacted, the iodine is then free to react with the starch to form the familiar dark-blue colored complex.

I2(aq) + starch → dark-blue colored complex

The deep-blue color of the complex is due to the presence of the pentaiodide anion, I5. By itself, the pentaiodide anion is unstable; however, it is stabilized by forming a complex with the starch. The appearance of the deep-blue color in solution indicates that all of the reactants have been consumed and the reaction has gone to completion.

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.