Re: Why do water pipes burst in winter?

Hi Katie,

There's a pretty good article written about this by T. Neil Davis, of the Geophysical Institute, University of Alaska Fairbanks (bursting pipes is obviously a serious issue in that part of the world). You can find it at the URL: Alaska Science Forum

There is apparently truth to the observation that hot water pipes burst much more commonly in cold weather than cold water pipes. The difference hinges upon the impurities in the water.

Water does not actually freeze at exactly 0 degrees Celsius, but will "supercool" a bit to lower temperatures before ice crystals form. Exactly how far it supercools depends upon the impurities present (molecules other than water, minerals, dust, etc.). Extremely pure water, if not agitated, can supercool by several degrees before it freezes. By contrast, water with larger impurities will begin to freeze much closer to 0 degrees. There is also a difference in how the water freezes--water which is more strongly supercooled will freeze very quickly, whereas water which freezes closer to 0 degrees freezes much more gradually.

The difference is that the impurities act as what are called "nucleation points," which the water molecules use to give themselves the preferential direction needed for ice crystal formation. You can see similar nucleation in boiling water on a stove. As it begins to boil, you will see small bubbles forming at specific points, usually associated with tiny flaws in the pan.

The process of heating water tends to reduce the size of the impurities that are present, which makes them less effective as nucleation centers. As a result, when a hot water pipe is exposed to extremely cold temperatures, the water within it will supercool more than the water in an adjacent cold water pipe. Yes, this means that the water in the cold pipe freezes first, BUT it freezes more slowly because it is not supercooled. More gradual freezing (from the edge inward) means that as pressure builds up in the cold water pipe, excess water can be pushed back out of the pipe through the still- unfrozen center. When the hot water pipe freezes, however, it freezes very quickly because it is more strongly supercooled, completely sealing off the pipe so that it cannot adjust to the pressure buildup, and so it is more likely to burst.