Experiments on a Tuna Can

By: 24hourprof , 5:41 PM GMT on December 19, 2012

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Whoa Nellie! I guess I caused a stir by announcing that 32 degrees Fahrenheit is not "freezing." To repeat, only pure water freezes at 32 degrees (at a pressure of one atmosphere). Moreover, finding pure water is rather difficult. Yes, even distilled water contains some impurities (contamination by particles in the air, etc.). Even the water from the reverse-osmosis system that my wife and I use regularly for cooking and drinking isn't pure.

Craig Bohren, now a retired professor from Penn State, performed an experiment which I duplicated for my textbook. I took an empty tuna can, greased it with oil, and then placed drops of tap water on the can (see photograph on the left below). Please note that I first boiled the water in an attempt to remove any dissolved air. At any rate, I placed the can in my freezer (about -11 degrees Celsius, which roughly equals 12 degrees Fahrenheit) and waited ten minutes...tick, tock, tick, tock...


(Left) Water drops on the bottom of a tuna can. (Right)After ten minutes in my freezer (-11 degrees Celsius), four drops resisted freezing. Courtesy of A World of Weather: Fundamentals of Meteorology

I removed the can, and, lo and behold, there were several drops that did not freeze (see photograph on the right above). Back into the freezer. A couple more drops froze. Even after another ten minutes, two drops refused to freeze. I can only deduce that the drops lacked freezing nuclei. I plan to repeat the experiment using reverse-osmosis water. I'm expecting more drops to resist freezing.

And so it is with tiny water drops that exist in high, cold clouds. Despite the temperature being well below 0 degrees Celsius, they resist freezing because they are bereft of freezing nuclei. At temperatures below -40 degrees Celsius, however, all bets are off, and the tiny drops spontaneously freeze.

A short blog, I admit, but I hope I presented sufficient empirical evidence that 32 degrees Fahrenheit is not the freezing point of water.

Here endeth the lesson.

Lee Grenci

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4. WunderAlertBot (Admin)
4:56 PM GMT on December 20, 2012
24hourprof has created a new entry.
3. AETHOMAS
12:38 AM GMT on December 20, 2012
Very Cool. :) I am in 8th grade, and I just studied that in school! :)

Thanks
Member Since: February 6, 2010 Posts: 15 Comments: 243
2. georgevandenberghe
7:32 PM GMT on December 19, 2012
Raindrops don't usually supercool to -10C. My sentence was not clear. I meant to imply only cloud droplets supercool this much.
Member Since: February 1, 2012 Posts: 17 Comments: 1626
1. georgevandenberghe
7:28 PM GMT on December 19, 2012
Water in nature usually contains enough solid impurities to function as freezing nuclei that it freezes at close to the melting point but not enough solute that the melting point is depressed more than a small fraction of a degree. The droplets in the demonstration were very clean and supercooled. However raindrops do supercool a lot and cloud droplets even more.. generally to at least -10C but rarely below -20C.
The raindrop supercooling issue leads to a very tough forecasting question when there is precipitation and a melting layer aloft. Will the precipitation at the surface be ice pellets or supercooled freezing rain. The consequences are very different but, at least for me, it's not a trivial ROT question because it depends on depth of the cold layer at the surface, presence of freezing nuclei in the drop, and perhaps turbulence also. (the other really tough one in cold saturated foggy air with no melting layer is do you get freezing drizzle or snow)

An interesting demonstration of supercooling can be made by cooling a plastic water bottle in a freezer for an hour or two so its temperature is around -5C. It will remain liquid. When you take it out gently and tap it,
an ice front will form somewhere in the bottle and quickly spread through it until latent heat of fusion raises the bottle temperature to the melting point. This is quite impressive and even impresses middle schoolers.

Finally I often use the solute effect to make a field freezer. I take 30 pounds of ice cubes and two pounds of salt and mix it all up in a cooler. This will produce temperatures of -10C in the cooler in about fifteen minutes (only the surface melt layer dissolves the salt otherwise I'd need a lot more salt). I've demonstrated both the solute effect and supercooling this way to cub scouts way out in the sticks who want a way to preserve the coolness of water bottles or even popsicles.
Member Since: February 1, 2012 Posts: 17 Comments: 1626

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About 24hourprof

Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.

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