Why does water expand when it freezes?

+44 votes
asked Mar 10, 2015 in Science by hing loong (540 points)
I’ve accidentally put a sealed water bottle in the freezer before, forgetting to pour some of the water out first. The result was an exploded mess! It seems strange that water at the liquid state would take up less space than when it’s a solid. But it’s the same amount of water, then why does water expand so much when it freezes?

1 Answer

+10 votes
answered Mar 11, 2015 by Rebekah (1,270 points)
The shape of the water molecule is the reason why water expands when it freezes. Water expanding instead of shrinking when freezing is weird for a substance because usually when something is cooled, the speed of the molecules slows down. They vibrate less and therefore take up less space.

The water molecule kind of looks like a Mickey Mouse head though, with the hydrogen atoms being the ears and the oxygen atom being the face. Because the oxygen has a slightly negative charge and the hydrogen atoms have a slightly positive charge, water molecules tend to stick together and form bonds. The weird shape of these molecules causes them to create a very open structure though when they link together. It forms a lot of holes and gaps, meaning there’s a bunch of extra space taken up. When water undergoes the freezing process, it creates a lot of these bonds. So all these extra bonds take up extra space and cause water to expand when it freezes instead of shrinking.
commented Mar 14, 2015 by Alysia (700 points)
I know why does water expand when it freezes now. Thanks!!
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