Question

mention any one use of crystallization method

Answer 1

Crystallization is a common technique used to purify solids. Two common methods of
crystallization are “gradual cooling” and “diffusion”.
Gradual Cooling
Gradual cooling involves dissolving the impure solid in a minimum amount of a hot solvent and
allowing the resulting solution to cool slowly to room temperature. During the cooling process,
pure (or almost pure) crystals form and are then collected by vacuum filtration.
The first step in the crystallization process is to find an "ideal” solvent by testing a small amount
of the impure solid. The desired compound should be very soluble in the solvent when it is hot
(near the boiling), but only slightly soluble at room temperature. Sometimes a solvent mixture
(two or more solvents in a specific ratio) is required to achieve this temperature dependent
solubility. At the same time, the dissolved impurities (byproducts and unreacted starting
material) should be highly soluble in the solvent at BOTH high and low solvent temperatures.
The compound must precipitate out of solution at cooler temperatures to allow its separation
from the solvent and the dissolved impurities by filtration.
The next step is to dissolve the solid to be purified in a minimum amount of hot solvent. This
step should be performed quickly, with stirring if necessary. (Note: The solid and solvent
should NOT be combined and then heated – this may cause decomposition of the desired
compound. The solvent should always be heated first, and then added to the solid.)
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