A key step in the recrystallization process is: a) dissolve the solid in a large quantity of cold solvent b) remove all of the soluble impurities by vacuum filtration c) allow the hot solution to cool slowly and precipitate the solid d) take a melting point of the wet crystals e) all of the above The consensus is that it’s C. However, a substantial amount of people are saying it’s E, and a few say it’s B. Why is there such a variation?
The correct answer and explanation is:
Correct Answer: C) allow the hot solution to cool slowly and precipitate the solid
✅ Explanation (300 words):
Recrystallization is a purification technique used to isolate pure compounds from a mixture. The key step—central to the process—is allowing the hot solution to cool slowly so that pure crystals of the desired compound can form, leaving impurities in solution. This slow cooling allows the molecules of the desired compound to arrange themselves into a well-ordered crystal lattice, which excludes impurities.
Let’s examine why there’s confusion among the options:
- Option A (“dissolve in a large quantity of cold solvent”) is incorrect because hot solvent is typically used to fully dissolve the compound. Cold solvent won’t dissolve enough and can prematurely precipitate impurities.
- Option B (removing soluble impurities by vacuum filtration) is part of the process, but it typically refers to removing insoluble impurities after dissolving the compound in hot solvent. Soluble impurities remain dissolved and do not get removed by filtration—they are left behind when crystals form.
- Option C is correct because slow cooling is what actually drives the purification. This is the defining moment where the pure solid separates from the impurities.
- Option D (taking a melting point of wet crystals) is incorrect. Wet crystals can give inaccurate melting point readings. Crystals must be dry before analysis.
- Option E (“all of the above”) is incorrect because not all steps listed are correct or equally critical. Only C is the essential, defining step. The others either misstate the procedure or describe supporting but non-defining steps.
⚠️ Why the Variation?
The variation in answers often stems from:
- Misunderstanding the role of each step.
- Thinking of recrystallization as a sequence rather than identifying the critical moment.
- Test-taking habits—assuming “all of the above” is often correct.
In reality, C is the only answer that represents the key step in recrystallization.