If the material that you are re-crystallizing fails to precipitate out of the cold solvent, propose what would you do to recover the material from the solution?

Answer 1

There are a few different things you could do depending on the chemical composition of the solution and the desired precipitate. Given that the solvent in the example is cold, heating it up could trigger the precipitation reaction if the dissolution reaction is exothermic. That type of reaction is possible, but rare, so if the precipitation reaction is instead endothermic, you could try cooling the solution further, stirring or jostling the solution, or adding a substance with nucleation sites. Nucleation sites can come from an inert material with a rough surface, or a small amount of the desired precipitate. Adding either of these to the solution would speed up crystallization by providing a starting surface on which the precipitate can form, rather than relying on it to fall out from the solution directly.

This is why, for example, adding a wooden stick or a small crystal of sugar to a solution of water with dissolved sugar will encourage precipitation of the sugar on the added material, which occurs more rapidly than the sugar from the solution falling out as small precipitate crystals throughout the solution when it is supersaturated.