There are two kinds of bonds to consider. The one that joins 2H[sup]+[/sup] with O[sup]2-[/sup] is strong, and involves large amounts of energy. It's not in play here. The second kind is the Hydrogen bond which is due to a weak force (Van der Waals) between water molecules. When water freezes and thaws, it's this Hydrogen bond which will break and reform under cooling and heating. It's not H[sub]2[/sub] and O (?) changing phase, only H[sub]2[/sub]O. Note: H[sup]+[/sup], O[sup]2-[/sup], H[sub]2[/sub], O[sub]2[/sub] and H[sub]2[/sub]O are different chemicals, with entirely different properties. If you look at a table of melting points for various compounds and compare them to the bond energies involved in the formation of those compounds, you'll notice there is no correlation.