This simple demonstration uses water and ice to show that two phases in equilibrium at a fixed pressure always have the same temperature.

2 liter beaker filled with crushed ice
thermometer
wooden dowel to stir the ice/water mixture
1 liter of distilled water
Fischer burner, striker, and 8 ft gas hose
ringstand with ring to support beaker.
Wire gauze to support beaker on ring

This demonstration should accompany a discussion of phase equilibrium and basic phase diagrams. The key point to make is that as long as pressure remains unchanged, solid and liquid phases in equilibrium must keep the same temperature.

Begin by showing that the crushed ice and the water have different temperatures. Mix the ice and water and stir. Measure the temperature of the mixture. Discuss what happened (ice absorbed heat from the water and melted). Add heat to the ice/water mixture with the burner (don’t melt all the ice!). Stir and remeasure the temperature. Discuss the fact that it is still the same.

Don’t set yourself on fire with the burner. Don’t immerse yourself in the icewater; hypothermia is serious. Pour the water and ice down the drain. It’s one of the few things OSHA allows.