Question

A student was recrystallizing a compound. Seeing some floating specks, the student continued to add hot solvent until she had filled the flask. At that point, she decided that the specks were never going to dissolve, and stopped worrying about them. She cooled the flask to room temperature, no crystals appeared so the student scratched the sides of the flask and cooled the solution in an ice bath. Finally, with relief, the student isolated small amount of crystals, a much lower yield than expected. Explain what happened.

Answer 1

Yeah. basically this floating specks is nothing but a symnol of crystallistation. basically crystallistion always prefers in less amount of solvent. here after addition of excess amount of hot solvent could make this specks dissolve because that specks was crystalline in nature. In general, it's very hard to make crystal soluble. Now come to its actual point. When she cooled this solution at room temperature no crystal is found, because at room temp rate of evapouration of the solvent is slow. But when she cooled the solution suddenly she has been observed some small amount of crystals. Once temperature suddnly falls then rate of evapouration and rate of deposition is suddenly falls and thats why crystal come. She could able to more crystal if she has used less amount of solvents.