What is the difference between precipitation and crystallization?

1 Answer
Apr 5, 2017

Not much.............


As you know, crystallization is an important step in purification. As a solute slowly crystallizes from solution you get well-formed crystals of the desired product, perhaps with a few so-called waters of crystallization incorporated in the crystal structure.

Have a look at the crystals of salt in your salt cellar at home, or at the crystals of sugar in the sugar bowl. If you look closely (with a magnifying glass) I think you would be surprised at the regularity of the individual crystals: they have uniform size; they have uniform faces, and uniform shapes.

Usually, the slower a crystal is grown, the larger it becomes. And this is an important issue for inorganic chemists. If you can grow a decent crystal of a new compound, you can perform an X-ray crystallography experiment on the crystal, which would eventually give the non-equivocal structure of the new compound - but it is not straightforward to grow crystals of sufficient quality.

Back to your question (finally!), when a material precipitates from solution, it is conceived to have crystallized RAPIDLY, such that the crystals are very small and powdery. This material may be a bit difficult to handle (because the particles are so small and powdery), and it may have #"occluded"# (so-called) several solvents of crystallization. At least you have got the material solid, and isolated. And perhaps you can recrystallize the precipitate from fresh solvent to get better quality crystals.

I have written four paragraphs for a simple question! I am not going to win prizes for brevity.