Finding the Freezing Point of Water

Most recent answer: 08/05/2012

Q:
I read the page about super cooled water on this site. What puzzles me is that can water ever freeze without a nucleating agent? If nucleating agents are always required then distilled water should never freeze at temperature around freezing point( unless we shake it). How do we decide that the freezing point of water is 0 degree Celsius, when it can actually stay in liquid state at temperatures around -40degree centigrade in the form of super cooled water. Please help.
- Gamini (age 18)
India
A:
It's easy to find the freezing point of water. Prepare an ice-water mixture. Isolate it from heat exchange, maybe with styrofoam. The ice will grow or shrink until the temperature is just at the freezing point. No nucleation is required since both phases are already there. Measure the temperature with a thermometer.

Mike W.



(published on 08/05/2012)

Follow-Up #1: Is a nucleus needed to freeze water?

Q:
If we start lowering the temperature of water from say 20 degree Celsius to freeze it. There is only one phase at 20 degree Celsius. Will the water require a nucleating agent to freeze when 0 degree is reached?
- Gamini (age 18)
India
A:
Yes, at or even a bit below 0 °C water won't freeze in any reasonable length of time without some sort of special nucleation site. Without such sites water has to cool to around -38 °C to freeze via self-nucleation in a reasonable time.

Mike W.

(published on 08/06/2012)