What invisible bond makes ice float and water so hard to boil?

The hydrogen bond. It forms when a hydrogen atom, already attached to a strongly electronegative atom, is also drawn toward a lone pair of electrons on another electronegative atom nearby. It's weaker than a covalent or ionic bond, but stronger than van der Waals forces, and water is unusually good at making it. When water freezes, these bonds arrange its molecules into an open structure that is less dense than the liquid, so ice floats; and because so many of them must be broken, water boils at a surprisingly high temperature for such a small, light molecule.

Water is unusually good at forming these bonds. Its oxygen atom has two lone pairs and two hydrogen atoms, meaning each water molecule can link up with as many as four neighbors at once. In liquid water, that web keeps shifting: a hydrogen bond between two water molecules lasts on average only about 10 picoseconds. But in ice, hydrogen bonding strongly affects the crystal structure, locking molecules into an open hexagonal lattice. Because that lattice spreads molecules out more than liquid water does, the density of ice ends up lower than liquid water at the same temperature, and the solid simply floats, unlike almost every other substance, where the solid form is denser and sinks.

The same bonding explains water's high boiling point, melting point and viscosity compared to similar liquids: breaking all those bonds between molecules takes real energy. Scientists worked the idea out over a few years. Linus Pauling credited T. S. Moore and T. F. Winmill with the first mention of a hydrogen bond in 1912, while the description of hydrogen bonding in water came in 1920 from Latimer and Rodebush, who credited unpublished work by their colleague Maurice Loyal Huggins on "a hydrogen kernel held between two atoms." The same weak bond turned out to hold the two strands of DNA together.