Problem with Salt Crystals...?

Question

So I'm trying to make crystals with salt in a supersaturated solution. It seems to work, I DON'T boil the water, but for some reason, all the solution does is sinks down to the bottom like a salt dust. What am I doing wrong? Why wont my crystals form?

btw, when I say I don't Boil it, that doesn't mean the water's not heated. It's just not to the point where it bubbles.....it's still hot though

Answer 1

To start with, is the "salt" you're using sodium chloride? This salt doesn't change solubility with change in temperature of the water very much; only going from 39.12g/100ml @ 100 C to 35.7g/ 100ml @ 0 C; so NaCl is not a good salt for making experiments in super saturation. Try something like potassium nitrate: the solubility goes from about 247g/100ml @ 100 C to 13.3 g/100ml @ 0 C.

If you want to grow large crystals, you want a saturated solution (not super saturated) and you want to cool or let solvent evaporate SLOWLY this is how to get big crystals. For some materials, it helps to use some pre-made "seed crystals" to act as a starting place for the big crystals to grow.

Answer 2

I assume by salt you mean sodium chloride. Part of the difficulty with sodium chloride is that it has a pretty flat solubility curve in just water. It could be that if you are trying to grow a large crystal quickly another salt may be a better choice. I know tetrasodium pyrophosphate makes a decahydrate salt that can get quite large very quickly.

Anyway, if you are set on using sodium chloride, heat the solution to near boiling and dissolve as much sodium chloride you can until a few crystals do not dissolve. Then allow the solution to cool very slowly. The slow drop in temperature will tend to dissolve nucleating crystals and allow for the growth of larger crystals. Once the temperature has reached ambient conditions, remove the largest crystal, heat the solution back to close to boiling and add as much sodium chloride as you can until a few crystals will not dissolve. Then put your large crystal back into the container and repeat the process.