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What is crystillization? - Scientific Explorer My First Chemistry Kit

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Hi, Sajidalidud,

Crystallization is a simple, yet powerful method for purifying solid substances, whether or not they are molecular or ionic. The main idea of the method is to dissolve the solid to make a solution of it, then causing it to fall back out of solution without its impurities. You'd just have to filter the solution away from the recovered, but purified solid. Before drying the purified solid you would "wash" it, while it is still on top of the filter paper, with 2 or 3 small portions of cold solvent (usually water). Finally, you would dry it by either letting it dry in the air, or by oven-drying it (that's quicker, but only advisable if the pure solid is stable to heat, or if the heat is set low enough to prevent melting).

The trick to successful crystallization is using a satisfactory solvent, one in which the solid is not excessively soluble (you'd never get it to fall back out as a solid if it is too soluble). The solvent has to be "good" enough, or course, to dissolve the solid. Depending on your chemical intuition or knowledge, you could narrow down the long list of possible solvents to a reasonable first choice. If the solid is polar (for example, like one of the thousands of salts), you would know to use only polar solvents, like water. Most ionic solids would not dissolve in anything except water, or some mixture of water and another polar solvent like ethanol or methanol - just how much alcohol would require some trial and error (experimentation).

If your solid were an organic compound, you would probably want to avoid using water; instead use something like alcohol, or alcohol-ether, etc. If your organic compound were a very polar organic compound, you might be able to use a small amount of water mixed with alcohol.

How does the desired solid crystalize free of the impurities?
Separation of desired solid from impurities is not merely a matter of differential solubility.
What allows the solid to crystallize pure is its particles' (ions or molecules) greatly selective attraction for other particles in solution like themselves. A solid's natural pure state is a crystalline matrix, which is very stable, and naturally "preferred" - something that can be explained best by thermodynamics theory - a topic best discussed in advanced chemistry courses.

Impurities that are present would not satisfy the most stable state, and would only interfere with it. The growing crystals of the pure solid can grow only by its natural attraction for other ions of the desired substance. The different ions of an ionic compound stack in well-defined ways, creating the most stable angles, patterns, and layers (the geometry) that are characteristic for the solid substance. This process would also probably be occurring much faster than the impurities could interfere.

The impurities, even if they could bind to the growing crystal's ions, would not be attracted as strongly; so as they momentarily drifted away from the crystal, the ions in higher concentration surrounding the growing sites on the crystal would stick and further block the interference of the impurities. So after a few hours of growing, the set of pure crystals of the solid would have formed on the sufaces of the container, and the impurities would still be dissolved in the solution.

Some practical considerations
If you have a low yield of crystals of the solid, how do you recover the rest of the solid in pure form?
You can recover much of it by processing the "mother liquor," a term going back to the days of alchemy. That's what you call the solution above the crystals, or the filtrate that goes through the filter paper. You could concentrate it to a smaller volume and allow crystallization to go further. So, it's best not to discard this solution after the first crop of crystals. I have usually obtained 2 or 3 crops of crystals, but generally, the second and third crops are not as pure. To increase their purity you could redissove the crystals in minimum solvent and repeat the usual process of crystallization ("recrystallization").

How to test the purity of your crystals
Take a melting point. If the melting point range is narrow, that is a good indicator of solid's purity. For example, suppose you had a solid that melted at 103.1 - 103.3 0C, that would indicate high purity. On the otherhand, if the range were, say, 103 - 106 0C, that would indicate low to moderate purity. A minimum acceptable purity depends on what application you have in mind for the solid. Often a 2-degree range is acceptable.

If you are lucky, you will have a sample of the pure crystalline solid as a reference. This will probably be the case, if your purpose is only to purify a sample of the solid that is a known substance, and not a new substance from your research. To prove the purity of your crystalline solid, take a melting point of it side by side with the reference sample. Their melting points should be very close to each other. If you don't have a sample of the commercial pure solid, you could just refer to the mp in a reference manual.

I hope my description is useful to you. Good luck in using crystallization!

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Posted on Jan 11, 2011

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