Crystallization

Crystallization is a process in which a substance (ionic or covalent and some elements) is forced out of solution forming a crystalline structure. This is done by reducing the amount of the substance which can be dissolved in a solvent and is achieved in two ways:

• evaporation of the solvent
• heating the solvent to increase its solubility so that a amount greater than normal of the substance can be dissolved and then letting it cool down

Re-crystallization

Re-crystallization is a process used in chemistry to purify polluted reagents. For re-crystallization to happen the wanted chemical must be soluble in the solvent of choice, whilst the impurities can be both soluble and insoluble.

If the impurities are insoluble then the process is as follows:

1. dissolve the polluted reagent in the solvent of choice
2. since the impurities can’t dissolved they are filtered
3. the now filtered solution of your purified reagent is dried so as to crystallize out your wanted chemical

Example: If you want to separate the mixture of NH₄NO₃ and CaCO₃ (a fertilizer) you can use this method since NH₄NO₃ is soluble in water, but CaCO₃ is not.

If the impurities are soluble, but in amounts smaller than that of your wanted chemical or are more soluble than your chemical, the process is as follows:

1. dissolve the polluted reagent in the solvent of choice
2. then the solution is brought into a super-saturated state either by evaporating the solvent or heating the solvent so that even more reagent can be dissolved and the it is left to gradually cool down
3. after some time the substance that is going to crystallize out is the one which was present in greater amounts or had a lower solubility, now just separate the two by filtration and take whichever one is that you wanted

Example: You can use this method to separate out the Iodine out of table salt, because it is present in very small amounts.

Another possible way of re-crystallization is through sublimation, but that is only possible for certain chemicals like iodine.

References

Relevant Sciencemadness threads

• Crystal Growing
• Crystallization - Cooling vs. Evaporation