Super Cooled Water (Denise Randol)
Author Denise Randol
Principle(s) Illustrated
1. Heat Transfer
2. Nucleation: Heterogeneous
Standards
Chemistry 7a,
Questioning Script
Materials
· Large deep ceramic dish
· Ice Cubes
· Coarse or Rock Salt
· Small Glass Cup
· Distilled Water
· Thermometer
Procedure
1. Put ice in dish
3. Fill with ice cubes
4. Sprinkle liberally with rock salt or some other coarse salt
5. Put distilled water in a small glass cup- no higher than the top level of ice in dish
6. Add more salt to ice
7. Insert thermometer into water glass
8. Wait 20 minutes or until water reaches -3˚ C
9. Carefully remove thermometer
10. Carefully remove cup of water from ice
11. Drop a small piece of ice into glass of water
12. The water should freeze instantly
Prior knowledge & experience:
Students have experience with freezing water in a water bottle. If a bottle of water is left in a freezer, the students have observed the water inside the bottle changing to a frozen state and expanding. If the bottle is not yet frozen, the cooled water will stay in a liquid state if poured from the bottle.
Root question:
What will happen when an ice cube is added to the glass of cold water?
Will the water change? If yes, in what way?
Post Demonstration:
How can cooled water turn instantly to ice?
Why didn’t the ice cube float on the water?
The distilled water is without the usual minerals found in tap water. It has a lower freezing temperature than water with impurities. When the water comes in contact to some impurities (such as an ice cube or frozen surface), then the freezing temperature changes to 0, allowing ice to form.
Unopened distilled water bottles change when shaken or tapped on hard surface, introducing a gas bubble into the water.
Target response:
Students will guess that dry ice or another super cooled substance/chemical must be added to the water.
Students might think the salt has caused a change.
Common Misconceptions:
All fresh water freezes the same, at 0˚ C or 32˚ F. Water freezes over time when exposed to freezing temperatures.
Photographs and Movies
After students offer explanations for results, show videos of super cooled water in bottles that change when poured a frozen surface or tapped on table. These experiments show no use salt and bottles of unopened water changing.
