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Why Do Freezing Pipes Burst?

freezing-pipes
(Image credit: <a href="http://www.shutterstock.com/gallery-82046p1.html">Tom Grundy</a> / <a href="http://www.shutterstock.com/?cr=00&pl=edit-00">Shutterstock.com</a>)

Bursting water pipes are a major concern when winter temperatures drop below freezing. But why do frozen pipes burst?

Perhaps surprisingly, freezing pipes don't burst because of ice expanding in place. Instead, it has to do with pressure inside the pipes.

When water freezes, its molecules crystalize into an open hexagonal form, which takes up more space than when the molecules are in their liquid form — that is, the water molecules expand as they freeze.

As the ice expands, it pushes water toward the closed faucet. This causes an immense amount of water pressure to build between the ice blockage and the faucet — eventually, the pipe ruptures under the pressure, usually at a spot where there's little or no ice.

The pipe areas upstream of the ice clog aren't typically in danger of bursting because the pressure isn't great enough. In this case, the water isn't blocked, and can always retreat back to its source.

Pipes that are the most at risk of bursting are those located outside of building insulation, or those in unheated interior areas, including basements, attics and crawl spaces.

Heating or insulating these vulnerable pipes with fiberglass or foam sleeves can help prevent them from bursting. Additionally, keeping the facet open and letting the water run — even at just a trickle — will help prevent the water pressure from building to dangerous levels.

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Joseph Castro
Joseph Bennington-Castro is a Hawaii-based contributing writer for Live Science and Space.com. He holds a master's degree in science journalism from New York University, and a bachelor's degree in physics from the University of Hawaii. His work covers all areas of science, from the quirky mating behaviors of different animals, to the drug and alcohol habits of ancient cultures, to new advances in solar cell technology. On a more personal note, Joseph has had a near-obsession with video games for as long as he can remember, and is probably playing a game at this very moment.