# 3.4: Crystallization Theory

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• 3.4A: Purification
Crystallization is an excellent purification technique for solids because a crystal slowly forming from a saturated solution tends to selectively incorporate particles of the same type into its crystal structure. A pure crystal is often slightly lower in energy than an impure crystal (or has a higher lattice energy), as packing identical particles into a lattice allows for maximized intermolecular forces.
• 3.4B: Cooling Slowly
The difference in crystal lattice energy between pure and impure solids is marginal, so a solution must be cooled SLOWLY to allow for differentiation. If a hot solution is plunged immediately into an ice bath, the system will favor the formation of a solid (any solid!) so strongly that there may be little preference for purity. Impurities can become engulfed in the developing solid and trapped as solutes are deposited unselectively onto the growing solid.
• 3.4C: Using the Minimum Amount of Hot Solvent
• 3.4D: The Unavoidable Loss of Recovery
• 3.4E: Quantitating Crystallization
• 3.4F: Second Crop Crystallization
As previously discussed, a portion of the compound of interest always remains dissolved in the mother liquor and is filtered away. This is not to say that this portion is lost, as it is possible to recover additional compound from the mother liquor. The solvent can be concentrated in the original vessel or on a rotary evaporator, and a second crystallization can be attempted. A second crystallization from the mother liquor of the first crystallization is called a "second crop crystallization".

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