Final answer:
To solve recrystallization problems, one can cool the solution slowly or use seed crystals to prevent oiling out, concentrate the solution for better crystal formation, use activated charcoal to remove colored impurities and maintain warmth to prevent premature crystallization during filtration.
Step-by-step explanation:
The student has inquired about resolving several recrystallization problems in a laboratory setup. To address these issues:
- Oiling out: This occurs when a substance solidifies as an oil rather than crystallizing. To solve this, one can slowly cool the solution to promote proper crystallization or add seed crystals to initiate crystal formation.
- Lack of crystal formation: This could be due to an insufficient degree of supersaturation. One can evaporate some of the solvent to increase concentration or scratch the sides of the flask with a glass rod to create nucleation sites.
- Presence of colored impurities: If the impurities are more soluble in the solvent, they may remain in the solution after crystallization. Using activated charcoal can help adsorb these impurities, followed by hot filtration to remove the charcoal and any adsorbed impurities before crystallization.
- Premature recrystallization in the funnel stem during gravity filtration: This can be avoided by keeping the funnel and the receiving flask warm, ensuring the crystals remain dissolved in the filtrate until the solvent has fully passed through the funnel.
Successful recrystallization relies on the compound of interest being more soluble in a solvent at higher temperatures and the impurities being more soluble in the solvent at all temperatures. This principle ensures that cooling the saturated solution leads to the desired product crystallizing out while leaving impurities in the solution.