Vacuum filtration is a chemistry technique that can allow for a greater rate of filtration in the lab. In normal filtration, the gravity can provide the force drawing the liquid through the filter paper, but a pressure gradient performs this function in vacuum filtration, offering a variable rate due to the different strength of the pump extracting the air from the Büchner flask.
The process of vacuum filtration has advantages in comparison to gravity filtration. First, vacuum filtration is often faster than the gravity one. It takes less than one minute if you set it up with good seals and a good vacuum source. And vacuum filtration is much more efficient at removing residual liquid, in the result of a purer solid. You will find it very important in crystallization, because the liquid may contain soluble impurities, and when the solvent evaporates, the impurities might adsorb back onto the solid surface.
On the other hand, compare with gravity filtration, vacuum filtration also has some disadvantages. The force of vacuum may draw fine crystals through the filter paper pores, and a quantity of material cannot be recovered from the filter paper. So we suggest that you use vacuum filtration when dealing with large crystals.
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