Nib asked
Why does ice float on water, in terms of hydrogen bonds?
Freezing locks water molecules into an open hexagonal lattice of hydrogen bonds, spaced wider than in the liquid, so ice is about 9 percent less dense.
7 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the oddity
Almost every solid sinks in its own liquid. Water is the odd one out: ice floats, and hydrogen bonds are the reason.
2the numbers
Ice is about 0.917 grams per cubic centimeter, liquid water near 1.00. Freezing makes water take up roughly 9 percent more space.
3a polar molecule
A water molecule is bent, about one hundred five degrees. Oxygen hogs the electrons, so oxygen is slightly negative and each hydrogen slightly positive.
4hydrogen bond
A positive hydrogen is pulled toward a neighbor's negative oxygen. That attraction is a hydrogen bond, and one molecule can make up to four.
5ice lattice
In ice, every molecule locks into four bonds at fixed angles. That builds open hexagonal rings with empty space in the middle, a rigid, airy lattice.
6liquid packs closer
In liquid water the bonds keep breaking and re-forming, so molecules slip into the gaps and pack closer. More mass per volume, so water sinks under ice.
7the whole idea
So, on one board: four hydrogen bonds per molecule build an open lattice, so ice is less dense than water. That is why lakes freeze from the top down.
Ice floats because hydrogen bonds hold frozen water in an open lattice, which lets lakes freeze from the top and keeps life below alive.