======Triple Point of Water======


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**Alternative titles:** Coexistence of Solid, Liquid, and Gas in Water
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====Summary====                   
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====Procedure====  
  - Cool water with ice and pour into a watch glass until half full.  
  - Place the watch glass on an insulating stand inside a bell jar.  
  - Seal the jar onto the rubber mat base.  
  - Connect the bell jar to a vacuum pump via a drying trap filled with calcium sulfate (Drierite).  
  - Turn on the vacuum pump and reduce the pressure. At about 28 mm Hg, water begins to boil.  
  - Continue evacuation for several minutes. Ice will form on the surface while liquid water boils below it.  
  - Discuss the observed coexistence of solid, liquid, and gas.



====Links==== 
Triple Point of Water - UCSC Physics: 
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📄 Triple Point of Water - UCSC Physics Demonstration Room: [[https://ucscphysicsdemo.sites.ucsc.edu/physics-5b6b-demos/triple-point-of-water/]]\\


====Variations====
  * Use a video camera for projection in large lecture halls.  
  * Repeat with other substances (e.g., carbon dioxide using dry ice) to compare triple points.  
  * Plot observed behavior on a phase diagram to connect the demonstration with theoretical models.



====Safety Precautions====
  * Operate the vacuum pump carefully and ensure connections are secure to prevent implosion.  
  * Use a water vapor trap to protect the vacuum pump from excess water vapor.  
  * Handle glassware with care to avoid breakage under reduced pressure.




====Questions to Consider====
  * What is the triple point of water? (The unique temperature and pressure at which water’s solid, liquid, and gas phases coexist—0.1 °C and 0.006 atm.)  
  * Why does water boil under reduced pressure even when it is cold? (Lowering pressure reduces the boiling point of water, allowing it to vaporize at near 0 °C.)  
  * Why does ice form at the same time water is boiling? (Evaporation removes energy, cooling the liquid until it freezes, while pressure keeps boiling ongoing.)  
  * Why is the triple point important in science? (It provides a fundamental reference point for defining temperature scales and understanding phase behavior.)