Crystallization of sodium acetate - GIF

[nova_robotics]

I recall an activity from a LONG ago physics class where we melted some white solid material, I do not remember what it was, then it cooled but did not solidify. When we touched it with a toothpick and it hardened almost instantly.

Is that what is going on here? or is that a saturated solution waiting for someplace for the crystals to form?

Might very well have been the same stuff.

This stuff has been heated to melt it, then cooled off again. Going by the temperature it should have solidified, but it can't due to a lack of nucleation points. The crystals can't grow without nucleation points, so it just stays a liquid. It's like having super pure water, in a very clean and smooth container. If you're careful, you can cool that way below 0C/32F and it will stay liquid. Now it's a supercooled liquid just waiting for some nucleation point or shock. When the material gets the nucleation point it needs to allow crystals to take hold and begin forming, the whole thing crystallizes and turns to a solid.

What you can't see from the video is this thing releases a ton of energy when it crystallizes. As soon as you put that poker into the liquid, or even give the container a good flick, it will get HOT. There's a ton of potential energy here that all gets released when the material goes through a phase change (latent heat of fusion), which is why this material can be used as a thermal battery. You just throw them in boiling water to recharge them. Very cool.