A Flash of Blue: A Classic Clock Reaction
Publication No. 13131
In a flash, a colorless solution turns a dramatic deep-blue color! Amaze your students with this popular starch–iodine clock reaction.
Potassium iodate solution, 0.01 M, KIO3, 100 mL
Sodium meta-bisulfite, Na2S2O5, 0.02 g
Starch solution, 0.5%, aqueous, 100 mL
Sulfuric acid solution, 1 M, H2SO4, 3 mL
Water, distilled or deionized
Graduated cylinder, 10-mL
Graduated cylinder, 100-mL
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.
Performing the procedure above as indicated is an attention-getting demonstration that shows the classic characteristics of a clock reaction. However, this reaction is also ideal for studying kinetics—the effects of concentration, temperature, and the presence of a catalyst on the rate of reaction. For more information, please request Flinn publication no. 10245, Iodine Clock Reaction.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesDeveloping 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 IdeasMS-PS1.B: Chemical Reactions
HS-PS1.B: Chemical Reactions
Crosscutting ConceptsStability and change
Cause and effect
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.
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.
H2O + Na2S2O2(s) → 2HSO3–(aq) + 2Na+(aq)
KIO3(aq) → IO3–(aq) + K+(aq)
IO3–(aq) + 3HSO3–(aq) → I–(aq) + 3H+(aq) + 3SO42–(aq)
6H+(aq) + 5I–(aq) + IO3–(aq) → 3I2(aq) + 3H2O(l)
I2(aq) + HSO3–(aq) + H2O(l) → 2I–(aq) + SO42–(aq) + 3H+(aq)
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.