[impulse29]

This is a question from my organic chemistry course, but due to the nature of it, I have chosen to post in the high school forum.

We have extracted o-acteylsalicylic acid from aspirin and crystallized it. A question is what is the difference between crystallization and precipitation. I know that both require a supersaturated solution, but do not know what differentiates one from the other. I know that a precipitate is the result of a chemical reaction, so would that mean that crystallization is only a physical change? Do precipitates form in solution and crystals form through vacuum filtration?

Any help is appreciated!
-Impulse.

[Yggdrasil]

Basically the difference between crystallization and precipitation is the end product.  Both are phase changes in which a solid is formed; however, crystalization produces crystalline solids while precipitation produces amorphous solids.  Crystalline solids are more highly ordered than amorphous solids, and so it is generally harder to produce crystals than precipitates (this is especially true for biologists like me who try to crystalize proteins).  In general, if you let the phase change occur more slowly (i.e. supersaturate a solution then let it cool) you will form crystals wheras if you let the phase change occur rapidly (i.e. mix two solutions and a solid forms immediately), you will form a precipitate.