[curiousgeorge1234]

Why does recrystallizing a compound purify it? I kind of get the theory behind it, but not all of it.

I understand if you boil the compound in a solvent and filter it, any impurities that didn't dissolve will be filtered out.  Then, I know you usually reboil the solution and then let it cool to crystallize.  Why would the crystallization be purified?  How can you be sure there aren't impurities in the crystals?  Is it that if there are impurities they remain liquid.

[Yggdrasil]

Recrystallization can separate compounds on the basis of solubility.  Compounds with completely different solubilities than your compound will not dissolve in your hot solvent and be filtered out as you mentioned.  Upon cooling, impurities that are significantly more soluble in the recrystallization solvent or that are present in small amounts will stay in the aqueous phase (since they're not present at a high enough concentration to precipitate out).  Theoretically, only your compound should crystallize.  Only impurities with the same solubility as your compound (soluble in the recrystallization solvent at high temperature and insoluble at low temperature) and that are present in significant amount would recrystallize with your product.