======Halloween Clock Reaction======


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**Difficulty: **{{$demo.difficulty_description}}\\
**Safety: **{{$demo.safety_description}}\\
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**Alternative titles:** Old Nassau Reaction
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====Summary====                   
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====Procedure====  
    * See method in links below.



====Links==== 
Halloween clock reaction - Hegelrast: 
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Halloween Clock Reaction - ChemEx LK: 
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📄 Halloween Reaction or Old Nassau Reaction - Anne Marie Helmenstine, Ph.D.: [[https://www.thoughtco.com/halloween-or-old-nassau-reaction-604253]]\\

====Variations====
  * Dilute the solutions to slow the reaction and make the timing of color changes longer.



====Safety Precautions====
  * Mercury(II) chloride is highly toxic and must be handled with extreme care; wear gloves, goggles, and a lab coat, and conduct the experiment in a fume hood.  
  * Dispose of mercury-containing waste according to institutional hazardous waste guidelines—do not pour down the sink.  
  * Sodium metabisulfite is an irritant; avoid inhaling dust or vapors.  
  * Potassium iodate is an oxidizer; store away from reducing agents and flammable materials.  
  * Use small-scale quantities only and keep students at a safe viewing distance.




====Questions to Consider====
  * Why is this reaction classified as a “clock reaction”? (Because there is a timed delay before the dramatic color change occurs.)  
  * What chemical species is responsible for the orange color? (Mercury(II) iodide, HgI₂, precipitate.)  
  * What causes the final blue-black color? (Iodine forming a complex with starch.)  
  * How does changing the concentration of reagents affect the timing of the reaction? (More dilute solutions slow it down; more concentrated solutions speed it up.)