Monday, I went up to the summit for work... I picked up a radio and a bottle of water from the fridge- I noticed that one of the bottles was frozen solid, the plastic was stretched by the block of ice- I didnt think anything of it, the bottle I picked up was clearly NOT frozen... I walked over to sit down for the morning meeting... I opened the bottle and took a drink... it was frozen slush. I was surprised...

I went back to the fridge and grabbed another bottle of unfrozen water- I shook it, and it started to turn coudy at the top of the bottle- as I watched tendrils of ice formed and filled the bottle with ice slush within a few seconds...

I called over some co-workers and pulled out another bottle gently and I gently rocked it to show that the water was liquid- then I banged it gently against the fridge... as we watched it turned cloudy with ice crystals near the lid and then it spread through the entire bottle over a few seconds...

I had heard about this phenomenon but I had thought it would only happen under laboratory conditions.... The water was super-cooled. Water can be cooled to below zero degrees Celcius without freezing solid if it is extremely pure and not disturbed... If the water lacks a nucleation center to start the crystalization process then it can be cooled below the freezing point and remain a liquid.

The fridge temperature had drifted below freezing- The pure water had been slowly cooled below the freezing point and did not freeze because it was lacking a nucleation center... when disturbed, it got a nucleation center and the super-cooled liquid froze to form an ice slush...

But here is where it gets REALLY neat- WHY didnt the bottle freeze solid? Why did it just make a water/ice slush? Well, when water turns from a liquid to a solid it releases heat, so as the ice formed it HEATED the surrounding water to above the freezing point... so as the water turned to ice it heated the surrounding water... so not all the water froze- some of the water was warmed up by the formation of ice...

I have never seen this happen with pure water- I have seen it happen with carbonated drinks... but I always attributed it to the pressure drop when you open a carbonated drink (PV=nRT) If pressure, P, lowers then so does the temperature, T.

It was a very COOL example of physics that really stunned a few people- myself included...

I certainly do not doubt the purity of our bottled water any more- in the entire fridge only one bottle had a nucleation center that triggered the freezing process (the frozen bottle I mentioned above) the rest were able to be cooled below the freezing point without ice crystals forming...