• Crystallization is a simple,
effective and very important technique to separate and purify solids. It is
based on the fact that all organic compounds are more soluble in hot than in
cold solvents, so that solid gets dissolved on heating and is obtained back on
cooling.
• A crystalline organic substance is
made up of a three dimensional array of molecules held together primarily by
van der Waals forces.
These intermolecular attractions are fairly weak: most organic solids melt in
the range of room temperature to 250oC.
• Crystals can be grown from the
molten state just as water is frozen into ice, but it is not easy to remove
impurities from crystals made into this way. It involves dissolving the
material to be purified in the appropriate hot solvent. As the solvent cools,
the solution becomes saturated with respect to the substance, which then
crystallizes. Regular array of a crystal is formed, foreign molecules are
excluded and thus the crystal is one pure substance. Soluble impurities stay in the solution because they are not
concentrated enough to saturate the solution. The crystals are collected by
filtration, the surface of the crystal is washed with cold solvent to
remove the adhering impurities.
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