Chemistry - Beer freezing after being opened

Solution 1:

When you opened the beer, carbon dioxide was lost to the atmosphere, increasing the mole fraction of water in the beer and raising its freezing point to a point above the temperature of the beer. Not all of the water froze, leaving it a slush.

Edit: There's also the possibility that the loss of CO2 from the solution is not the issue. It could just be that when you opened the beer you disturbed a supercooled solution enough to start it freezing.

2nd edit: Instead of continuing the comments I'll add here. The freezing on the outside of the can shows that the temperature of the beer is below the freezing point of water. Is the temperature of the beer lower than the freezing point of the beer? If it were, the beer should freeze instantly upon being disturbed. If the beer is above its freezing point, but the loss of CO2 raises the freezing point it should freeze gradually as it sits out.

Solution 2:

As you can see in the phase diagram for water, higher pressures decrease the freezing point.

From Chemwiki:

one is able to melt ice simply by applying pressure and not by adding heat.

When you open the beer, you release the pressure in the bottle, raising the freezing point of the beer, so it starts to freeze. Since the bottle is colder than the freezing point of water, any water condensing on the outside will freeze.

BTW, the release of $\ce {CO2}$ from the beer will actually raise the freezing point, not lower it.