Why does beer only freeze when you open it?

Beer bottles


I like really cold bottled beer. But when I put it in the freezer, why is it liquid until I take the cap off and then freezes?
Claire, Holloway


** Definitive **
Name: Daniel, Chiswick
Qualification: Scientist
Answer: Liquid can only freeze as crystals. The crystal needs something to hold on to. In a bottle, the liquid has nothing to seed on. When the cap comes off, it bubbles and the crystals have something to seed on.
Name: David
Qualification: Chemistry at school
Answer: It's due to pressure. When it's in the bottle, it's pressurised. When the cap comes off, that is reduced and it freezes.
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