Quick Freeze (Demonstration)
(Reference: Bare, J.D. J of Chem Ed. 1991, 68, 1038.)
 

Pressure   :   Freezing point depression
 

Purpose:
Illustrate the idea of freezing point depression and dissolved gases
 

Safety:
Safety goggles should be worn by instructor

 
Materials:
10 oz club soda in clear glass bottle
1L beaker
Salt
Thermometer
Crushed ice
 

Procedure:
1. Remove labels from club soda and cool in fridge for at least 5 hours  before demo.
2. Place layer of ice on bottom of beaker
3. Sprinkle salt over ice
4. Place bottle of club soda in center of beaker, then continue to layer ice and salt around it.
5. Place thermometer in beaker, close to soda to get an accurate temp of the soda.
6. Soda must reach –8 °C and stay for 10 minutes.  Don’t allow the temp to go lower than –8 °C
7. After 10 minutes remove bottle and observe, DON’T shake!  Remind students that pure water would have frozen at this temperature.
8. Open bottle and notice how quickly the soda solidifies
 

Explanation:
Depending on the concentration of a solute, the freezing point of a solution will decrease when it is added, this is a colligative property.  Water is the solvent in both of these solutions.  The sodium chloride is the solute in the ice bath (which decreases the water temp from 0 °C to –8 °C) and the carbon dioxide is the solute in the bottle of soda, allowing it to remain lower than 0 °C without freezing.  The freezing point of the salt/water solution is lowered because the solutes affect the ability of the water to crystalize.  The bottle of water contains all the CO2 gas, once it is opened the concentration of this solute decreases, causing the water to freeze because it is still so cold.
 

Clean-up:
All solutions can go down the drain.
 
 

Ginger Chateauneuf, 2000.
 
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