Analysis of an Unknown Mixture by Fractional Crystallization

PURPOSE OP EXPERIMENT Separate three solids (sand, K2Cr207, and NaCl) from each other by fractional crystallization, and determine the percent composition by mass of an unknown mixture. 

The process of fractional crystallization is a technique by which chemists separate and purify many substances. This process takes advantage of differing variations with temperature of the solubilities of several components in a given solvent. If one of the components is insoluble at all temperatures whereas other components are soluble at some temperature, the insoluble component can be separated easily by filtration. If the solubility of one of the soluble components in the mixture increases rapidly with temperature whereas the solubility of the other soluble component increases only slightly with temperature, an appropriate temperature can be chosen such that one of the components will be only slightly soluble whereas the other will be almost entirely in solution. With one component present primarily in the solid phase and the other in solution, a separation can again be achieved easily by filtration. The solvent used may dissociate components that are salts, but must not react further chemically with the components. The solvent must also allow the components to separate as well formed crystals and must be easily removed from the components, for example, by evaporation. Water serves as an appropriate solvent in this experiment. In many industrial operations mixed solvent systems must be used. 

In this experiment you will start with a solid mixture of sand (Si02), potassium dichromate (K2Cr207), and sodium chloride (NaCl) and will attempt to separate and obtain each of these components in a pure form by fractional crystallization from water. 

and Purcell, K. F., Chemistry and Chemical Reactivity. Saunders College Publishing, Philadelphia, 1987, sections 1.3, 2.12, and 12.2. 

TABLE 5.1. Solubilities (grams of anhydrous solute per 100. mL of water). 

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