Kitchen Chemistry

So the touch starts rapid polymerization?

If so, then what if you simply wait? Will it start somewhere by chance?

OMFG! Is this Ice-9?

It's not a polymerization -- the solution is very supersaturated, i.e. there was more solute dissoved than was stable at that temperature. When you drop in a "seed crystal" all the excess solute crystallizes out. If you look carefully at the video, when the person touches the solution, they have a small crystal on the tip of their finger.

This particular supersaturated mixture is peculiar in that the crystal that forms actually incorporates water into the matrix (the solid formed is sodium acetate trihydrate), so there is little or no solvent left after the crystallization. In most supersaturated solutions the seed crystal grows a little bit and is still surrounded by lots of liquid solvent.

This is a popular demo in chemistry class when we talk about solutions. It's also the basis for some hand-warmer packs (the ones with the little "button" that you click to initiate crystallization).

Mike

Is it necessary to use a crystal as a seed? I know from watching Alton Brown that crystal seeds will make it happen a lot easier if using a sugar solution

In making fudge, you want to use a lid so that the steam turns back to water and cleans the side of the pot.

But even without crystals, shaking it too much could cause crystals to form too.

You don't necessarily need a seed crystal. Your fingertip (or rough edges in the tray you pour it into) could act as a phase transfer catalyst. It would be a fun set of experiments to do to see what induces crystallization.

aloha
psilo

according to my recollections from my chemistry classes, a seed crystal is sufficent to induce crystallization of a super-saturated solution, just as providing some sort of rough surface, but i also recall that somehow agitating the solution, such as by shaking it, can also cause crystallization of a super-saturated solution.