Ice Bags--Frozen Fireworks?

On the day we experimented with M&Ms and Ziploc bags of ice to learn about cold water currents, my son and daughter decided to fill baggies with water and put them in the freezer. (Note to self--no wonder I'm out of sandwich bags!)

When I cleaned out the freezer, hoping to sneak the water bags into the garbage, I was amazed by what I saw. Air bubbles had formed lines streaking out from the center of the ice, as if we'd frozen a firework explosion.

The explanation for this phenomenon has to do with the air in the water.  PhD Martin Lersch writes in his food chemistry blog, Kymos, that room temperature water has air dissolved in it, and the amount of air it can hold increases as it gets colder.  (I had to boil water to remove the air for my experiments with floating conversation hearts.)  When the water freezes, "the water can no longer keep the air dissolved and a bubble is formed."  If the ice were to freeze from the inside out, the air could escape naturally.  However, since freezer ice freezes from the outside in, and (presumably) since our ice was enclosed in air-tight Ziploc bags, the air was trapped inside the ice.  We froze the water in large amounts, which contained more air than regular ice cubes.

The result?  Lots of trapped air bubbles forming slow-motion fireworks, a beautiful reminder of Nature's inexhaustible designs.

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