Ask A Scientist General Science Archive

 
Subject: NEWTON BBS  -General 
name         Will Jordan E.
status       student
age          14

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.
--------------------------------------------------------------------
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
=====================================================