Asked by: Mick, Derby
A bottle that has been left in the freezer for a few hours is well below its freezing point. It stays liquid because water needs to expand in order to freeze and the pressure inside the bottle makes it harder for the molecules to rearrange themselves into ice crystals.
When you open the bottle, the sudden drop in pressure allows the ice to form and it also brings some of the dissolved CO2 out of solution. The bubbles act as ‘nucleation sites’ for the first ice crystals to form and, as the ice and bubbles rise, they accumulate more and more ice in a vaguely tree-shaped, inverted pyramid. Even just clinking the bottle against a hard surface can be enough to initiate freezing. In this case, the pressure wave creates microscopic low-pressure regions within the liquid that can act as nucleation sites.