Simple question with no detailed answer online, surprisingly. I just wanted to know the molecular process associated with freezing.
The process of crystallization of a solid from any liquid is actually a pretty complex business. I sat in on a course Franz Rosenberger gave on just this process several years ago. Every day he brought in pages from a bookm he was writing on the topic. He appears not to have published the book – I can’t find it on Amazon. More’s the pity.
what happens is that a thin layer of material in the liquid self-organizes in the form of the crystal as it approaches the solid/liquid interface. In the case of water, under normal conditions the forces causing this aren’t teriffically strong, and the solid is actually only seiregular. Under different conditions of temperatrure and pressure, liquid water tends to preferentially form cryastals (they used to have a great exhibit on this at the Boston Museuim of Science, but it stopped working a few years ago and hasn’t been repaired, and has been taken down). Growth from vapor to solid tends to produce regular crystals, and you can get hexagonal solid and pillars, as well as snowflakes. (There is an exhibit about the shapes of cystals you can get from vapor growth now peresent at the MOS, but it doesn’t get into the dynamics).
I am not sure what level of answer you require? On the simplest level the molecules stop moving (except some vibration). This is because temperature (heat) is the “force” (bad terminology sorry) that keeps them moving*. They may also arrange themselves into crystal structures at the same time as they can get maximum interactions with their neighbours that way.
On a deeper level the question is still not well understood, and may of the kinetics and exact mechanisms still being studied.
*Heat is just a term we use for substances whose molecules have lots of kinetic (moving) energy
thanks for the info!