Why does water become less dense when it’s frozen?

Question

Piggybacking off of a recent question asking whether drinking cold water means drinking the most water.

In: Physics

Answer 1

Water molecules are formed at an angle, like this:

H‎ ‎ ‎ ‎ H
‎ ‎ ‎ ‎ ‎ /
‎ ‎ ‎ O

That oxygen atom is hungry for electrons, so it pulls the one electron from each of the hydrogen atoms closer to its nucleus than the hydrogen nuclei. This means that there is a slight “magnetic” difference: the hydrogen atoms are more positive (because they don’t have the balancing electron nearby) and the oxygen atom is slightly more negative because it’s hogging the electrons.

In liquid form this “magnetic” difference (i.e, polarity) is negligent but you see it in surface tension. When water is frozen into a solid, this polarity forces the molecules to line up neatly and more rigidly with other slightly positive oxygen atoms attracted to other hydrogen atoms, in a lattice formation.

It’s a relatively unique property for a small molecule and it’s fundamentally important to so much biochemistry.

edit: molecule spacing ain’t right