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I received a comment on one of my other chemistry questions stating that

table salt and sugar are both terrible candidates for recrystallization, albeit for different reasons.

Why are salt and sugar terrible candidates for recrystallization?

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    $\begingroup$ What do you know about recrystallization, to begin with? $\endgroup$ – Ivan Neretin Oct 5 '16 at 18:16
  • $\begingroup$ Meanwhile, if somebody else attempts to answer this, please keep in mind that my comment implied the conditions of a small-scale home experiment. Industrial conditions can be pretty different. $\endgroup$ – Ivan Neretin Oct 5 '16 at 19:42
  • $\begingroup$ I understand the physical principle of separation by recrystallization. I don't know what properties of sucrose or NaCl are by themselves making those compounds unlikely to be purified successfully by recrystallization. $\endgroup$ – Galen Oct 5 '16 at 20:02
  • $\begingroup$ What are the principles involved, then? See, I need to know what you already know, otherwise how am I to know where to start? Why would a substance dissolve and then crystallize, to begin with? $\endgroup$ – Ivan Neretin Oct 5 '16 at 20:07
  • $\begingroup$ Solubility is a function of temperature. I want to recrystallize A from a mixture. A has a higher solubility than the other components of the solid. Hot solvent at BP and volume at saturation of A is added to mixture. A good solvent will have certain properties, especially dissolving the whole mixture at its BP. Slow cooling to form seed crystals of A. Continued cooling favors more A being added to existing crystals of A. $\endgroup$ – Galen Oct 5 '16 at 20:18
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The solubility of $\ce{NaCl}$ has very poor temperature dependency. If you dissolve it at $100^\circ\rm C$ and precipitate at $20^\circ\rm C$, you'll essentially lose about 35 g to purify 3 g. Even using a fridge, you'll still lose more than half of the product, which is quite a bad business. Industrial process relies on evaporation, I think.

Sugar is another story. It does have a nice temperature dependence, but it dissolves too well, even when cold. Just imagine dealing with that awfully viscous syrup, like molasses. It would take forever to filter, and quite a while to crystallize. Industry can handle that, but for home chemistry I'd suggest something more pleasant.

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  • $\begingroup$ Haha, reminds me of an experiment a colleague told me about, where students in Bochum tried to create a saturated solution of sugar in water (in a coke bottle). Every now and again, they would drop another two lumps of sugar in but it would just keep dissolving. $\endgroup$ – Jan Oct 6 '16 at 19:26

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