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.)