- Density gradients are formed using water with different concentrations of salt and sugar. The different concentrations have different densities, keeping them from mixing.
Recrystallization is a process that takes an impure substance and purifies it, removing the unwanted substances from the wanted solid. This method involves heating the solid to dissolve it in a solvent , cooling it to reform the solid, then vacuuming and drying the newly formed pure crystals. First, you have to choose a proper solvent, something that the desired solid is insoluble in at room temperature, but soluble in when heated. If the solvent is too strong, the solid won’t fully reform, resulting in a poor yield of final product. If it is too weak, the start material will not fully dissolve and the final product will still contain impurities. You then dissolve the solid in the solvent using heat, when the solid is dissolved, the impurities are also dissolved and escape into the solvent. Then the solution is removed from heat and cooled to room temperature, allowing the crystals to reform, leaving the impurities in the solvent. The recrystallization process can be helped by placing the solution in an ice bath to cool it down to 0o C. Once the crystals have formed you filter out the solvent and impurities and dry the purified solid. Once you have obtained pure, dry crystals, you can take the mass of them to discover your mass yield, and test the melting point of the solid to determine the purity of the solid. For this experiment we had crystals that contained Lead bromide (PbBr2), Lead iodide (PbI2), and Lead chloride (PbCl2). We chose water as the solvent for all of our solids. We heated all of the solutions on hot plates while they were being stirred. Once all of the solid had dissolved, we cooled the solution at room temperature, continuing to stir them. Crystals as the solutions cooled, crystals began to form, see below for pictures. Since these recrystallizations were done for the purpose of show, we did not filter and dry the solids.