Recrystallization: Cooling the Solution

Cool the solution SLOWLY.

The most fascinating part of the recrystallization technique is observing the crystals as they form. However, it is best not to rush this step. When the crystals too fast, the impurities you are trying to remove may become trapped in the forming crystalline structure.

It is best to allow the crystals to form as slowly as possible. This means letting the solution cool slowly in a safe location where it is unlikely to be disturbed. Don't use an ice bath initially as recrystallization will occur too quickly. Also, position a watch glass over the flask to prevent dust and debris from entering and contaminating the solution.

Crystals that have formed slowly are usually larger and prettier (they twinkle in the light) than crystals formed quickly. It is not uncommon for a product grade to be determined by the shape, quality and appearance of the final recrystallized product.

Crystals Forming

In the following photographs, a hot solution of benzoic acid dissolved in water is allowed to slowly cool. Notice the desirable long, spearlike crystals that form.