Chemistry · Ch 3 — Some Analytical Techniques
Crystallization
Crystallization
The Four Steps of Crystallization
When a crude solid consists mainly of one substance along with some soluble impurities, it is purified by crystallization, carried out in four steps.
- Preparation of a saturated solution. A saturated solution — one that cannot dissolve any additional quantity of solute — is prepared by boiling the crude solid in a small but sufficient quantity of a suitable solvent. The main solute forms a solution that is almost saturated, but because the soluble impurities are present in only small proportion, the solution does not become saturated with respect to them.
- Hot filtration. The hot solution is quickly filtered — filtration under suction is used here because it allows rapid filtration — which removes any undissolved impurities as the residue.
- Cooling of the filtrate. The hot filtrate is then allowed to cool. Since a substance's solubility decreases as temperature falls, the filtrate becomes supersaturated with respect to the main solute, and the excess solute separates out of solution as crystals. The small quantity of dissolved impurities does not reach supersaturation and so stays dissolved even on cooling — meaning the crystals that form are already free of soluble impurities.
- Filtration. The pure crystals are separated from the remaining solution, called the mother liquor, by a final filtration. The crystals obtained this way are free of both soluble and insoluble impurities.
Choosing the Solvent …
What this figure shows. A sequence showing the crystallization of common salt: a beaker of water into which salt is stirred with a glass rod until no more dissolves, the resulting solution then heated and filtered hot through a labelled 'Filter paper' set in a funnel on a stand, the hot filtrate collected in an evaporating dish and left to cool so that crystals of pure salt separate out, leaving the dissolved impurities behind in …
Worked out. Market common salt often carries siliceous matter and other impurities that can be removed to obtain larger, purer NaCl crystals. Apparatus: beaker, glass rod, funnel, filter paper and stand, evaporating dish. Materials: impure market salt, or a mixture of salt and fine sand. Procedure: dissolve salt in water in a beaker, stirring with the glass rod and adding more salt until no further salt dissolves (a saturated solution); heat this solution and filter it while hot, so the insoluble impurities remain behind on the filter paper as residue; collect the hot filtrate in an evaporating dish and allow it to cool, so crystals of pure NaCl separate out while the soluble impurities stay behind in the mother liquor; finally filter the c …