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fractional crystallization[¦frak·shən·əl ‚krist·əl·ə′zā·shən]
a means of separating and purifying substances by converting them into a solid state during crystallization from a solution or melt. Crystallization can occur as a result of cooling, introduction of impurities that lower the solubility, or isothermal evaporation of the solvent. The effectiveness of the separation depends on the ratio of the quantities of the separable components, on their solubility, and on the conditions of the fractional crystallization. If substances being separated are present in comparable quantities, each component solidifies independently. If one of the components is present in trace quantities, it may coprecipitate rather than forming an independent solid phase with the macrocomponents. Fractional crystallization is a multiphase process that is used for the separation of substances with nearly identical chemical properties—for example, compounds of niobium and tantalum and of radium and barium.