======Iodine Clock======


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**Materials: **{{$demo.materials_description}}\\
**Difficulty: **{{$demo.difficulty_description}}\\
**Safety: **{{$demo.safety_description}}\\
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**Alternative titles:** Starch–Iodine Reaction Timing
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====Summary====                   
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====Procedure====  
  - Prepare solution A: 0.02 M potassium iodate (KIO₃).  
  - Prepare solution B: dissolve 4 g starch and 0.2 g sodium metabisulfite (Na₂S₂O₅) in water, then add 5 mL of 1 M sulfuric acid and dilute to 1 L.  
  - Place 100 mL of solution A in a cylinder and 100 mL of solution B in a beaker.  
  - (Optional) Chill one set in an ice bath for at least 15 minutes to observe temperature effects.  
  - Mix equal volumes of A and B, stir, and watch for the sudden appearance of the blue color.  
  - For concentration effects: dilute solution A to ½ or ¼ its concentration before mixing with solution B and compare reaction times.



====Links==== 
Make the Iodine Clock Reaction (Chemistry) - BeardedScienceGuy: 
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IODINE CLOCK REACTION - University of Glasgow: 
{{youtube>sbH6pXSpUcQ?}}\\

📄 Iodine Clock Reaction - University of Washington: [[https://chem.washington.edu/lecture-demos/iodine-clock-reaction]]\\

📄 https://www.flinnsci.com/api/library/Download/2eead53c42ed472ba2073c7141a53b52?srsltid=AfmBOooqLvLyRZPl-COd8cNznr040xwE1MI3o9tcDoL1CQb8We9awIkb - Flinn Scientific: [[https://www.flinnsci.com/api/library/Download/2eead53c42ed472ba2073c7141a53b52?srsltid=AfmBOooqLvLyRZPl-COd8cNznr040xwE1MI3o9tcDoL1CQb8We9awIkb]]\\

====Variations====
  * Show how dilution slows the reaction (½ concentration doubles the time; ¼ concentration quadruples it).  
  * Use an ice bath to slow the reaction noticeably.  
  * Compare results of warm vs. chilled solutions.



====Safety Precautions====
  * Wear safety goggles and gloves.  
  * Do not leave solution B in an open beaker for more than an hour; discard if the reaction no longer works.  
  * Handle sulfuric acid carefully; it is corrosive.  
  * Dispose of solutions down the drain with water, as the reactive chemicals are consumed and iodine is complexed.




====Questions to Consider====
  * Why does the mixture stay colorless at first, then suddenly turn blue? (Because iodine is initially consumed by bisulfite ions; once bisulfite is gone, iodine accumulates and forms a blue complex with starch.)  
  * How does concentration affect the rate of reaction? (Lower concentration slows the reaction; reaction time is proportional to dilution.)  
  * How does temperature affect the reaction rate? (Cooler temperatures slow the reaction; warmer temperatures speed it up.)  
  * Why is starch used in this reaction? (It forms a deep blue complex with iodine, making the endpoint very visible.)