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As you cool a system, you are removing energy, yet as water transitions to a solid, it expands, exerting sufficient force to rip through metal, for example in residential copper water pipes that freeze. Where does that energy come from?

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    $\begingroup$ Wow, cool question! Welcome to physics.SE! $\endgroup$ – Ben Crowell Apr 11 '18 at 15:29
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The change from liquid to solid releases some energy as (stronger) bonds are made between the water molecules - latent heat of fusion. The expansion pushes back the surroundings and work is done at the expense of some of the released energy when the bonds are being made.

The internal energy (potential energy) of the water decreases as heat is abstracted from the water and the water (ice) does work on the surroundings in expanding.
As the bonds are formed the molecules are closer together than their equilibrium separation ie there is “elastic” potential energy stored in bonds and when the containing vessel is ruptured those compressed bonds expand just like a compressed spring releasing energy when it expands.

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    $\begingroup$ Put another way, it sounds like it requires more energy to make the phase change than one might expect. The heat of fusion for water is 80 calories / gram). You couldn't take 100 grams of water at 1 degree C and remove 100 calories and expect it to spontaneously burst its container, you'd need to remove more like 8000 calories from the system for that to happen. Still, that doesn't seem like it would exert enough force to burst copper tubing ~30000 psi. $\endgroup$ – drone6502 Apr 11 '18 at 17:20
  • $\begingroup$ @drone6502 When you put a book on a table the table deforms by a very small amount and exerts a force on the book. So small deformations of a solid does not mean small forces are exerted just think about the expansion gaps which need to be in place between rails and between sections of a bridge and what would happen if those gaps were not there. $\endgroup$ – Farcher Apr 11 '18 at 17:31

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