A molecule of water is composed of 2 Hydrogen atoms and 1 Oxygen atom, this is why you see it referred to as H2O. Various atoms have positive and
negative charges which attract and repel each other. The hydrogen and oxygen atoms attract each other such that they form bonds with other water
molecules. Look at the picture on this page to help you visualize: users.rcn.com...
In liquid water, the molecules move freely around, picture them tumbling around each other, forming, breaking, and reforming bonds with other
molecules. Now when the water cools, as heat is removed, the molecules lose energy and stop moving around so much, which causes the attractions/bonds
to align them into a lattice structure, which rigidly spaces them evenly and farther apart than when they were more energetic and chaotic, thus it
takes up more space. As for where this "force" comes from, it is the force of attraction and repulsion between the hydrogen and oxygen atoms, the
molecules to the other molecules, the sharing of the hydrogen atoms with the oxygen atoms of other molecules, aka "hydrogen bonds".
It takes heat to break these bonds, which is why ice is more strongly bonded. As you heat up the ice the energy is able to overcome and break some of
the bonds, which allows the molecules to move around more and you get liquid water. Add even more heat and you can break the rest of the bonds and
there is nothing left to hold it together and it becomes vapor.
So the answer to your question of where is the energy coming from to give ice the power to break a pipe, is that due to the LOSS of energy (heat) the
water molecules slow down enough that the attraction of its hydrogen bonds forces its molecules to settle into a stable structure which is less dense
and thus expands.
It can seem counter-intuitive that the tiny force between atoms can "break a pipe", but just remember that the walls of a PVC pipe are just atoms
too, and the atoms of plastic and metal that make up the walls of a pipe are bonded together by similar bonds, but just in a relatively thin layer as
compared to the expanding interlocked lattice of hydrogen bonds of the ice. The number of interlocked hydrogen bonds just ends up having more strength
than the wimpy plastic atoms trying desperately to hang on to each other
P.S. I wrote this at 7:30am, and have been up all night with a cold and drinking Nyquil, so everything I said is probably not quite accurate, but I
was trying to phrase it in a way that was easy to envision and understand. I just realized that I typed way more than the couple of sentences I meant
to. In fact I am now noticing that just the P.S. is turning into a novel. I think I should just shut up and go to sleep now.