Methods of crystallisation from solution: Which set of strategies can induce crystallisation of a solute from its solution?
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ARemove pure solvent by evaporation only
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BChange temperature to create supersaturation only
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CAdd another solute to alter solubility (salting-out) only
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DAll of the above (a), (b) and (c)
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ENone of the above
Answer
Correct Answer: All of the above (a), (b) and (c)
Explanation
Introduction / Context:Crystallisation is a cornerstone separation and purification technique in chemical and pharmaceutical processing. Engineers often control supersaturation by manipulating solvent removal, temperature, or solution composition to nucleate and grow crystals selectively.
Given Data / Assumptions:
- Solute has finite solubility in the solvent.
- No chemical reaction; only physical separation by phase change.
Concept / Approach:Crystallisation requires supersaturation. Three common routes are: (1) evaporating solvent to increase solute concentration; (2) cooling (or heating, for retrograde cases) to reduce solubility; and (3) adding a second solute/antisolvent to reduce solubility of the target (salting-out or antisolvent crystallisation).
Step-by-Step Solution:
Route (a): Evaporation increases concentration at fixed T → supersaturation.Route (b): Cooling most solutions lowers solubility → supersaturation forms.Route (c): Adding a soluble salt or antisolvent changes activity coefficients/solubility → supersaturation.Hence all listed methods are valid approaches.Verification / Alternative check:Industrial crystallisers (evaporative and cooling types) and antisolvent crystallisation in pharma validate each route. Process selection depends on thermodynamics, kinetics, and impurity profile.
Why Other Options Are Wrong:
- (a), (b), (c) alone are incomplete—each is valid, but the best answer is that all apply.
- (e) is false because multiple standard methods exist.
Common Pitfalls:Overcooling causing excessive nucleation and fines; evaporating too fast causing occluded impurities; using an inappropriate antisolvent leading to oiling out instead of crystallisation.
Final Answer:All of the above (a), (b) and (c)