Thursday, 13 March 2014

Thoughts on sodium acetate "instant ice"

I've just completed a practical demo with my kids that wasn't a complete success, and part of the reason for that seems to be a lack of good practical guidance online about how best to execute it. So I thought I'd better record my failure, both as reference for my future self (Hey, future self! Looking damn sexy!) and for others who might be trying the same.

The demo was the instant ice effect of sodium acetate, where you pour a liquid onto a surface and it immediately solidifies into a phallic tower. I first saw this on QI last year, and meant to get around to trying it, and then I had one student in particular who was most insistent that I should try it, after their teacher last year apparently failed at it.

The reason it's supposed to work is that sodium acetate in solution has a much higher lower melting point than pure sodium acetate, so the solution can be super-cooled, and when the stuff in solution meets a pure crystal, it triggers crystallization of the lot, sort of pulling it out of solution. The main problem I has was in getting the solution to the correct concentration. Too much liquid and it keeps the sodium acetates too far apart, and you just make the table wet. Too little liquid and it solidifies on its own, surprisingly quickly. The best how-to guide I've found for this so far was Helmenstine's write-up on, and that suggests waiting til the boiling mixture of sodium acetate and water has lost just enough water that crystals start forming on the surface. Trouble is, boiling water doesn't leave a nice clear surface, and I had to take it off the flame to settle it before I could tell crystals were forming. Not expecting this, I left it too long and the crystals there quickly grew out of control once it was off the heat.

Getting the balance just right seems to be key, because when I did finally get something like the right effect, my solution was still too 'wet' and the crystal tower was really just a crystal puddle. At the same time, as the liquid rushed out, too much sodium acetate got left behind, 'freezing' along the length of the test tube and down the neck of the funnel. That looked cool, but wasn't what I was aiming for. I'm not quite sure how to keep the solution more evenly concentrated.

Also, for cooling, I used a beaker of ice, with the test tube of solution stuck in it. That went ok, but couldn't have helped the even concentration, as the top half of the test tube was sticking out and presumably not cooling at the same rate. Silly mistake to watch out for next time.

One advantage I did have was starting out with a jar of acetic acid. This saved me all the horrible trouble of boiling off vinegar to get some.

I'd welcome any feedback on how to handle this better, if you can spot anything.