Boiling Water in a Vacuum Chamber

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: Room Temperature Boiling

Summary

When water is placed in a vacuum chamber and the air is pumped out, the reduced external pressure lowers the boiling point. The water boils at room temperature, and because the energy for vaporization comes from the liquid itself, the remaining water cools noticeably.

Procedure

Place a small beaker of room-temperature water inside a vacuum chamber.
Seal the chamber and begin pumping air out to reduce the internal pressure.
Observe that as pressure drops, bubbles begin to form and the water boils without heating.
Continue pumping and note that the boiling slows as the liquid cools down.
Optionally, measure the water’s temperature before and after to observe cooling.

Links

Boiling Water at Reduced Pressure - Demo with Mr Pauller - Lab Hamster:

Boiling cold water In a Vacuum Chamber - MrGrodskiChemistry:

Variations

Try the demonstration with warm water and compare how vigorously it boils compared with room-temperature water.
Place a thermometer in the water to show how its temperature drops during boiling.
Repeat with ice water to demonstrate that water can boil and freeze simultaneously under very low pressure (approaching the triple point).
Compare with the syringe version of reduced-pressure boiling for a smaller-scale demo.

Safety Precautions

Use only glassware designed for vacuum use; thin glass may implode.
Ensure the vacuum chamber is intact and free from cracks before use.
Keep hands and faces away from the chamber during operation.
Follow manufacturer guidelines for the vacuum pump and chamber.

Questions to Consider

Why does water boil at lower temperatures under reduced pressure? (Because boiling occurs when vapor pressure equals external pressure, and lowering external pressure makes this happen at a lower temperature.)
Why does the water become colder as it boils in the vacuum? (Because energy for vaporization is taken from the liquid, lowering its temperature.)
How does this relate to high-altitude cooking? (At higher altitudes, lower atmospheric pressure means water boils at a lower temperature, so cooking takes longer.)
What is the triple point of water, and how does this demonstration approach it? (The triple point is the unique temperature and pressure where water can exist as solid, liquid, and gas simultaneously.)