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Why does water expand when it freezes?

  • 12/17/2015 9:59:00 AM
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As simple as it seems, water actually acts pretty strange. It all comes down to what holds it together. 

Water, like all other substances, expands when you heat it. When warmed, the atoms that make up a water molecule move faster, creating more space between the atoms. That means the water molecule expands.

So, when you cool the molecule, it should get smaller, right? Most things work that way, but the shape of the water molecule changes as it freezes. Normally, a water molecule has one big oxygen atom and two smaller hydrogen atoms, and it looks sort of like mouse ears.

As water turns into ice, it begins to attach to other water molecules in a hexagon-like shape that has a lot of space between molecules. That extra space means that the water is bigger when it turns to ice.

So, if you freeze a bottle of water, make sure you don’t fill the bottle completely full. Leave some extra room at the top of the bottle for the water to expand, or you could have a mess in the freezer.

That’s something to think about during your next trip out on the ice this winter!

This originally appeared in the January/February 2013 issue of Iowa Outdoors Magazine.