Filtration isolates a solid from a liquid. In the chemistry lab, one pours a liquid–solid mixture onto a
paper filter that allows only the solvent to pass through, much like a coffee filter. At the end of
filtration, one is left with the solid, called the residue, and the flask full of liquid that passed through
the filter, known as the filtrate.
Filtration can be modified depending on whether the substance of interest is the solid or is dissolved
in the filtrate. Gravity filtration, in which the solvent’s own weight pulls it through the filter, is
more commonly used when the product of interest is in the filtrate. Hot solvent is generally used to
keep the product dissolved in liquid. Vacuum filtration, in which the solvent is forced through the
filter by a vacuum connected to the flask, is more often used when the solid is the desired product.
Recrystallization
Recrystallization is a method for further purifying crystals in solution. In this process, we dissolve
our product in a minimum amount of hot solvent and let it recrystallize as it cools. The solvent
chosen for this process should be one in which the product is soluble only at high temperatures.
Thus, when the solution cools, only the desired product will recrystallize out of solution, excluding
the impurities.
MCAT Concept Check 12.1:
Before you move on, assess your understanding of the material with these questions.What must be true about the two solvents used for an extraction to work?When doing an extraction, would it be better to do three extractions with 10 mL of solvent,
or one extraction with 30 mL?