Video Transcript
The given diagram shows the setup for a crystallization process. When should the heat be removed from the evaporating dish?
To answer this question, let’s begin by briefly discussing the purpose and application of the crystallization process. Crystallization, similar to evaporation, is a separation technique used to isolate or recover a dissolved solid, which is also called the solute, from a liquid, which is also called the solvent. One main difference in their application is that evaporation is used when there is only one solute in the solution with no known impurities, while crystallization is used when the solution is known to have two or more solutes with different solubilities and/or known impurities.
As far as crystallization goes, it is the differing solubilities of the solutes that allows for the solute with the lower solubility to be removed from the solution, while the other more soluble solute or solutes remain in solution. As a result, when performing crystallization, the goal is to supersaturate the solution and then allow it to cool slowly to form large regular crystals. With the purpose of heating the solution to be to drive off some of the solvent or liquid, we know that a supersaturated solution has been achieved when small crystals just begin to form on the surface of the liquid or on the edges of the dish. This happens because the added heat has caused enough of the solvent to evaporate that there aren’t enough solvent particles available to keep the solute dissolved in the solution.
When it comes to crystallization, we should stop heating when small crystals start to form in the dish, in contrast to evaporation where the solution can be heated until most of the solvent is gone to generate solute crystals. In order for the crystallization process to be successful and for only one solute to crystallize while the other solute or solutes remain in solution, it is imperative that once the solution has reached the supersaturated point indicated when small crystals start to form that the heat be removed so the solution is allowed to cool slowly. Otherwise, if a solution with multiple solutes is left to heat after the initial crystals form, it is possible that more than one solute will crystallize and separation will not be achieved.
So when should the heat be removed from the evaporating dish in the crystallization process? The answer is the heat should be removed when small crystals start to form in the dish.
