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Name:  Will Jordan E.
Status: student
Age: 14
Location: N/A
Country: N/A
Date: 10/16/2004


Question:
How can you make super-cool water in a classroom, my teacher really wants to know and I can get extra credit for finding a solution.


Replies:
If by supercooling you mean the strict definition of cooling a liquid down to below its natural freezing point without letting it freeze - water is going to be particularly hard to supercool. Crystal formation depends on the molecules in the liquid state finding a surface to form crystals on - these are called nucleation sites. Water has enough of other substances and particles mixed in it, that it crystallizes rather easily. Moreover, even pure water is easy to crystallize because even what we think of as smooth surfaces are rough enough for water to form crystals on. However, if you can get really pure water and place it in a really clean glass container, you might be able to supercool it. Have you ever placed a small, unopened bottle of pure water in the freezer, taken it out before it crystallized, but by shaking the bottle or squeezing on the plastic container - induced crystallization?

If on the other hand, you (and your teacher) are using a looser definition of supercooling - that is, being able to bring the temperature of water down to less than 0 deg-C without it forming crystals, then there are many tricks to do this. The one that I like best is taking advantage of a principle called "Freezing Point Depression" - it is a principle that states that all solutions will freeze at a much lower temperature then the solvent it was made from. Thus, if you dissolve something in water (like salt or sugar), it will not freeze at 0 deg-C.

Roberto Gregorius


Will,

Select a small, clean drinking water bottle. Drill its cap so as to allow insertion of a thermometer. Seal the thermometer at the cap with a suitable sealant -- silicone, perhaps. Fill the bottle with distilled water. Screw on the cap and allow the water to stand undisturbed for a day. Without shaking the bottle, place it on a freezer so you can see the freezing zone of the thermometer. Allow the bottle to stand in the freezer for several hours. Check it from time to time -- do not move the bottle or agitate it in any way.

If all goes well, the water will cool to below its freezing point (32 F, 0 C) as evidenced by the thermometer reading of (perhaps) -2 C. The water inside the bottle has supercooled.

Remove the bottle from the freezer, shake it gently for no more than one second, and sit it on the lab bench. The water will suddenly start to freeze solid.

You will note that the temperature will rise as the water freezes. Extra credit: How can this be? Why does the temperature rise while the water is freezing?

Regards,
ProfHoff 918


Wow!! Did you uncover a widely studied, complicated, and not fully understood topic. Here are just a few of the "hits" I got after only a couple of minutes on the search topic "supercool water procedure".

http://polymer.bu.edu/hes/articles/ms98.pdf

http://ej.iop.org/links/q11/TGywDBleltg5k5kYtWOnzw/cm3_45_r01.pdf

http://symp15.nist.gov/pdf/p697.pdf

http://polymer.bu.edu/hes/articles/sbcmmsss98.pdf

Water water everywhere and it gets mysteriouser and mysteryiouser all the time.

Vince Calder



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