Wednesday, September 25, 2013

Making "hot ice" (sodium acetate) crystals

Before we talked about rocks we talked about minerals, and when you are talking about minerals you have to talk about crystallization. I had seen a photo tutorial on how to make "hot ice" (the stuff that comes in those little hand-warmer packs; you break a disc inside and the whole packet crystallizes and turns warm) and it looked pretty simple, so I thought we'd try it. Then I read a couple other tutorials and all of a sudden it sounded like it wasn't simple at all, in fact it was fraught with uncertainty and peril, so I thought we wouldn't try it. We had several other crystallization activities, so I didn't really think it was necessary. . . and yet. I kept thinking about it, thinking how much the children would like it, IF we could get it to work.

Finally I found this video tutorial. He does a great job of making it seem possible, and giving you ideas for troubleshooting, but I also really appreciated how he said, "This is tricky to get right. You probably won't get it right the first couple times you try it." 

So, trying to set expectations as low as possible, I explained the demonstration to the children, and asked if they wanted to try it, knowing it might be frustrating and it might not work. They wanted to. So we began.
First you mix baking soda and vinegar. Carefully!---since you know what happens when those two mix.

You mix till all the baking soda is dissolved and the liquid is clear. Then you heat the solution over the stove.

He explains all this in the video, but basically you are trying to concentrate your vinegar---since household vinegar is quite weak; only 5% pure vinegar or so. It makes the house smell STRONGLY like vinegar, so open a window and don't stick your nose right down by the pot, or your eyes will be watering like crazy. This concentration process can take quite awhile. I think it was about a half hour for us. You are watching for a thin, slippery film to form on top of the liquid. Again, he explains this in detail in the video, but this (above) is about what it looks like. You need to take the solution off the heat as soon as you see this skin forming. And you can see the whitish powdery stuff on the side of the pot (upper right corner of this picture, in the shadow) which is sodium acetate crystals that have already formed.

I'll leave off the explanation of the chemistry involved in this reaction---I'd probably mangle it (find it in this video or this less-detailed one)---but suffice it to say, when you drop a crystal of the sodium acetate into the supersaturated sodium acetate solution, it initiates immediate nucleation and you can actually SEE the crystals form. It is really quite amazingly cool. And, probably thanks to our meticulous adherence to the directions (and our low expectations)---we actually got it to work! And we repeated the process several different times. Once you have the sodium acetate made, you can use it again and again (you just heat it to dissolve the crystals, then start the reaction over).
It was interesting, because sometimes our crystals looked more sharp and needle-like, like this^^

And other times they spread out in fan shapes, like this ^^

So, in spite of my fears that this would be one of those really frustrating and time-consuming demonstrations that never actually works---it wasn't! And it turned out to be one of the coolest things we've ever done. Watching the crystals spread through the solution, so orderly and yet so organic-seeming, and then reaching your hand in feeling the instant heat created by the chemical reaction, is incredible. We LOVED it. We wanted to see the reaction over and over again, and to show it to everyone we knew! And we will definitely be making this again (since after many uses, we have now thrown our sodium acetate away). 

Here are a couple videos---if you're only going to watch one, the first one is most impressive.
Hot ice crystallization---best version

Hot ice crystallization---slower, more needlelike crystals

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