Imagine a basin of calm, liquid water. Perfectly still and pure. All of a sudden it is jostled, or a speck of dust falls in, and it freezes in a moment. The water must have started below freezing, yet it was liquid. This seeming contradiction can occur when very pure water is carefully lowered below freezing, so ice crystals never have a chance to form. The result is “supercooled” water. Supercooled water may remain liquid while cooled quite far below freezing. It’s easy to make in the lab or at home, and sometimes occurs in nature.

While the water is still liquid, it seems contradictory: below freezing, yet not frozen. If water below freezing is supposed to be frozen, why is this so easy to demonstrate? It just seems wrong. Continue reading →