We will be conducting a crystallization experiment this week. I was wondering what the most common materials used for this experiment are and what are their best solvents.

Question

Thanks from Sheryl

alum in water is really good, however, it takes some time for the water to evaporate.

I have done a fractional crystallization using KClO3 and CuSO4*5H2O. It is possible to obtain WHITE KClO3 (not blue from contaminating CuSO4).

Answer 1

1. Why is the final product from the crystallization process isolated by vacuum filtration and not by gravity filtration? 2. Environmentally speaking, what are the advantages and disadvantages of recrystallization versus column chromatography? Recrystallization –Report Form

3. Explain, in your own words, why dissolving and then reforming crystals improves the purity of those crystals. 4. Explain why a solvent that can dissolve the solute, even when it is cold, is useless for recrystallizations.

Answer 2

The vacuum produces a lower pressure which cools the solution and promotes crystallization. Also, it dries the crystal well. When you are using a vaccum, you decrease the pressure from the lower side of the filter, keeping a big pressure difference which makes the solvent pass through the filter quickly, regardless of the amount of solution above the filter.
This way you minimize the residual amount of solvent in your solid product, but also avoid losses due to temperature increase. Vacuum filtration is used instead of gravity filtration in order to achieve a dryer product in less time. The faster process of vacuum filtration allows the slurry to remain cold and in cases such as with benzoic acid more crytals will remain formed at colder temperatures.