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Boiling Water Trick Explained

A few weeks ago, this video (made by Samantha Stewart of Yellowknife) garnered over a million views on YouTube, and was picked up by both CBC and CNN.  It shows the seemingly magical instant transformation of boiling water into snow, which can only happen at temperatures below -30 C.

Needless to say, I couldn’t wait to try this out for myself.  My opportunity came sooner than I expected; here’s me replicating the experiment from the balcony of my apartment on a typical frigid day in Ottawa.

Once the initial thrill is over (and you’ve recovered from the embarrassment of trying to explain what you’re doing to hapless passers-by below) the obvious question that arises is this: why does it only work with boiling water?

I’ve seen quite a few explanations out there in the internet world, of varying degrees of credibility.  Having perused the data and conferred with friends and colleagues, it is my belief that we’re dealing with two, and possibly three separate phenomena:

1.  We know that hot water has more energy than cold water; the molecules are bouncing around like teenagers in a mosh pit.  Throwing the water into air causes the hyper little molecules to spread out into tiny droplets.  We also know that smaller bodies of water freeze faster than large ones (a puddle freezes faster than a lake) so it makes sense that these smaller droplets quickly turn into ice crystals, that is, snow.  By contrast, cold water has less energy, so the natural adhesion (i.e. stickiness) of water keeps it all in one big blob, which doesn’t have time to freeze before it hits the ground.

2.  The second phenomenon is called “evaporative cooling”.  This is based on the theory that the average energy of a group of molecules (like a droplet) goes down much more quickly when you subtract its most energetic members.  In each drop, some of the molecules are moving so fast they can actually move into the vapour state.  With the highest-energy molecules gone, the average energy of the ones left behind is reduced, causing them to freeze even more quickly.  The contribution of this effect of is probably less than the first one, but still significant.

Evaporative cooling is a pretty important phenomenon in physics.  It’s one of the ways to make a Bose-Einstein Condensate, although that version also involves magnetism.  It’s also one of the methods used by the scientists that trapped antimatter for the first time last fall.  By allowing some antihydrogen atoms to evaporate, they ensured that the ones left behind were at a low enough energy to be trapped in their magnetic field.  (I talked to York physicist Scott Menary about this last fall, but unfortunately my version is no longer available online, so you’ll have to content yourself with my hero Bob Macdonald’s version here.)

3.  If you want to get really technical, there’s a relationship between the entropy/temperature state of the system and the form of matter in which it exhibits the lowest Gibbs free energy.  There’s a good explanation of this here, although I think the author gets left and right mixed up somewhere in the middle.  While this may contribute, I think it’s the least important of the three phenomena.

However it works, it’s terrifically fun to do, and to my mind, a perfect way to make the best of our oft-lamented Canadian climate.

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