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PCAT Chemical Processes – Recrystallization

A student is having trouble successfully carrying out a recrystallization procedure. The student can improve the odds of success by doing each of the following EXCEPT:

 

A) Adding a seed crystal of the purified substance that he is attempting isolate.

B) Cooling the solution very slowly.

C) Submerging the beaker in a cool water bath.

D) Scratching the side of the beaker.

Explanation

In a recrystallization procedure, a substance with impurities in dissolved in a small amount of hot solvent. The solvent is then very slowly cooled and the pure substance will crystallize out as its solubility decreases. To improve the outcome, the crytal needs a nucleation site to begin precipitating out. That can be provided either by scratching the wall of the beaker or by adding a seed crystal. Thus, (A) and (D) are incorrect.

 

If the solution is cooled too quickly, then both the pure substance and the contaminants may preciptate out all at once. Thus (B) is a way to improve the procedure, but (C), which would result in faster cooling would not solve the student’s problem. Thus (C) is the correct answer.

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