THE DEFINITIVE GUIDE TO ‘CRACKBACK’

‘Crackback’ is basically a ghetto version of testing the melting point range. A scientist might describe it as ‘qualitative observation of crystallization as a means of approximating melting point range’. Basically, a pure substance will solidify in a noticeably different way than a mixture will.

Now, when watching a molten pool of whatever you happen to have, you will typically see one of two things:

In some cases, small number of crystals will form at the edges, and then the whole pool will slowly solidify in a wave outward from the initial crystals. This means that the substance is most likely pure. It doesn’t necessarily tell you what it is, only that it’s reasonably pure. So it’s good to check using at least one other method to identify what it is.

In other cases, it’ll sort of solidify around the edges at first, then gradually get slushy-looking, usually with a molten spot in the middle as the last part to finally solidify. This means it’s NOT pure, or it’s racemate which means it contains both left and right handed isomers.

It’s sort of like having a bin full of plastic cups and trying to stack them inside each other. If they’re all the same, then it goes a lot faster, because any two will fit inside each other. But if there are multiple sizes and shapes, then it takes longer to match them up with each other, and sometimes they’ll match up wrong, and the whole process will generally be a lot less orderly.

It’s common for people to post pictures online and ask about the patterns, but that’s not the part that’s going to be the most helpful. You have to watch how it transitions from liquid to solid. Sometimes the lines in the crystals can hint at how the crystals formed, but that can often be misleading.

Bottom line: Watching how it solidifies will tell you a lot more than what it looks like after it’s already solid.