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.
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