Triple Point of Water
Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★★ Requires a more experienced teacher
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff
Categories: Particles and States of Matter, Pressure and Fluids
Alternative titles: Coexistence of Solid, Liquid, and Gas in Water
Summary
This demonstration shows the triple point of water - the unique conditions of temperature and pressure where water exists simultaneously as a solid, liquid, and gas. By cooling water and lowering the air pressure inside a bell jar with a vacuum pump, students observe water boiling, freezing, and melting at the same time.
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:
📄 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.)