A solid substance may carry two kinds of impurity: those soluble, and those insoluble, in the same solvent as the main compound. Insoluble impurities are removed by dissolving the compound in a suitable solvent and separating the undissolved impurity by filtration — pouring the solution through a moistened filter-paper cone fitted in a funnel, with the trapped solid called the residue and the liquid that passes through called the filtrate. When speed matters, filtration under suction is used instead: a vacuum/suction pump draws the liquid through a porous Buchner funnel fixed onto a side-armed conical flask (via a rubber cork), with an intermediate safety bottle protecting the pump from the filtrate.
Soluble impurities are removed by crystallization, a four-step process: (i) boil the crude solid in a small amount of solvent to make an almost-saturated solution (not saturated with respect to the minor soluble impurities); (ii) filter this solution while hot (using suction, for speed) to remove insoluble impurities as residue; (iii) let the hot filtrate cool — as solubility drops with temperature, the main solute becomes supersaturated and crystallizes out, while the soluble impurities, being too dilute to supersaturate, stay dissolved; (iv) filter off these pure crystals, leaving the soluble impurities behind in the mother liquor. A good crystallization solvent should be only sparingly soluble for the compound cold but highly soluble hot, chemically unreactive with the compound, and volatile enough to remove easily (commonly water, ethyl/methyl alcohol, acetone, ether, or combinations, chosen by trial and error). Impure copper sulphate (CuSO4) and benzoic acid are both purified this way using water.
When a mixture contains two or more soluble substances with widely different solubilities in the same solvent, fractional crystallization separates them: the mixture is dissolved hot, filtered, and cooled so the less-soluble solute crystallizes out first and is filtered off; the mother liquor is then concentrated by evaporation so the more-soluble second solute crystallizes out in turn.