Here's the question: I have two nalgene bottles. The first, is notorious for super cooling water when left in the car. The other one, seemingly identical to the first, has never done it. Whats up with that?
Thank you, Jen, for the fun science question. This has happened to the SL before too! First, what is super cooling? Super cooling is when a liquid cools down to a temperature below its freezing point, but remains in the liquid state. How does it do that? Here's the thing: Freezing water needs a nucleation point to begin the formation of a water crystal (ice). This nucleation point could be a speck of dust, or a rough part (microscopically speaking) inside your water bottle. Or, if you pick up a bottle of super cooled water and slosh it around, it will freeze. The motion disturbs the molecules enough to begin crystal formation. The Mythbusters did a similar experiment with instant freezing of super cooled beer. Your seemingly identical Nalgene bottles are probably not identical. I bet if you looked at the inside surface of the super cooling bottle under a microscope, it would appear more smooth than the other one. Your bottle that normally freezes water probably has a slight surface imperfection which provides a nucleation site for ice.
Fun tidbit: the reverse is also true for water. You can also super heat water and watch it flash instantly into steam. This is why people tell you never to microwave pure water in a smooth cup. The water can't boil without a nucleation site, then when you reach in to grab the superheated cup and move it, it will flash into steam and burn you. The SL microwaved water in mugs for years (for hot cocoa) before a new (very smooth) Christmas mug did this, and exploded water all over the microwave. The SL does NOT recommend you try this at home!!!