Here’s a YouTube video that describes how it is done. Watch:
It does a good job of explaining some of the physics behind how we can achieve cooling water to some temperature below 273 K without forming ice.
Things to ponder:
At one point, the guy says you should not leave the water bottle in the freezer for too long (say, more than a few hours) — freezing happens while the bottle is still inside the freezer.
After you take the bottle out of the freezer, you should handle it very, very carefully (i.e., gently). Towards the end, he shows several clips of “unsuccessful experiments” in which water froze before he could catch the process on video!
At one point, he also talks about why water with dust particles in it would not work for this experiment.
You could also take a look at this really beautiful video that shows the effect of ‘dust’ introduced into very thin films of water (soapy water film trapped in a wire-frame).
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Update (21 January 2015). Okay, two more videos. The first video, from Scientific Tuesdays, also does a good job of demonstrating the “instant” freezing of undercooled water. Note the use of a (salt+ice) mixture; also note the warning that you should be careful to not agitate the water while taking the beaker out of the (salt+ice) cooling mixture.
The second video shows some of the beautiful patterns formed by ice as it grows into supercooled water:
Okay, I can’t resist posting a third video: this one is not quite about freezing, but about formation of snowflakes, including the notion that no two snowflakes are alike.