ASK TOM WHY: Please explain why a sealed bottle of water, kept outside in freezing conditions, doesn't freeze. But if the bottle is disturbed, like being turned upside down, the water instantly freezes.

Dear Tom,

Please explain why a sealed bottle of water, kept outside in freezing conditions, doesn’t freeze. But if the bottle is disturbed, like being turned upside down, the water instantly freezes.

— Steve Gutmann, Glenview

 
Dear Steve,

Water that is chilled below freezing (32 degrees) but remains liquid is “supercooled.” We often see the condition in clouds and fog: The droplets remain liquid, in some cases well below zero degrees. In bottled water, ice crystals form more readily when they can grow on existing ice crystals or other “rough” surfaces like impurities in the water or imperfections on the bottle surface. Lacking these, supercooling can occur. However, supercooled water is unstable, and ice crystals will immediately form if the water is disturbed. Tapping the bottle can sometimes initiate the process.

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