a). Treat the benzoic acid-sand mixture with 1 M NaOH solution. Only the benzoic acid will dissolve. Filter the mixture to remove the sand. Then, add concentrated acid (like 6 M HCl) to the solution - this will cause benzoic acid to precipitate. Filter this mixture to obtain the pure benzoic acid. b). Solvent pair recrystallizations make use of the fact the the compound of interest is relatively soluble in one solvent, and relatively insoluble in the other. Thus, a saturated solution can be made simply by varying the ratio of the two solvents, rather than slowly adding small portions of a single solvent to a solid. When the ethanol - water mixtures become cloudy, it indicates the the compound is no longer soluble, and too much of the \"bad\" solvent (the one in which the compound is less soluble) has been added. Addition of more of the \"good\" solvent will remove the cloudiness. Solution a). Treat the benzoic acid-sand mixture with 1 M NaOH solution. Only the benzoic acid will dissolve. Filter the mixture to remove the sand. Then, add concentrated acid (like 6 M HCl) to the solution - this will cause benzoic acid to precipitate. Filter this mixture to obtain the pure benzoic acid. b). Solvent pair recrystallizations make use of the fact the the compound of interest is relatively soluble in one solvent, and relatively insoluble in the other. Thus, a saturated solution can be made simply by varying the ratio of the two solvents, rather than slowly adding small portions of a single solvent to a solid. When the ethanol - water mixtures become cloudy, it indicates the the compound is no longer soluble, and too much of the \"bad\" solvent (the one in which the compound is less soluble) has been added. Addition of more of the \"good\" solvent will remove the cloudiness..