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Recrystallization methods that I have seen online and in manuals involve adding a minimum amount hot solvent to the solid mixture. Is there a reason I should do this instead of adding an equivalent amount of solvent at room temperature and then heating?

I've tried dissolving table salt and sugar both ways, but I didn't notice a substantial difference other than adding hot solvent directly requires less time to dissolve because it took time to heat the cooler solvent.

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  • $\begingroup$ You want to dissolve the compound in the minimal amount of hot water possible. If you add lots of cold water to dissolve all of it, then heat it up, then cool it down, it won't precipitate out. $\endgroup$ – orthocresol♦ Oct 5 '16 at 16:54
  • $\begingroup$ That being said, table salt and sugar are both terrible candidates for recrystallization, albeit for different reasons. $\endgroup$ – Ivan Neretin Oct 5 '16 at 17:00
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    $\begingroup$ Yes you may add just the right amount of solvent and then heat it together with the solid substance (which would gradually dissolve in the meantime). That should make no difference. $\endgroup$ – Ivan Neretin Oct 5 '16 at 17:10
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    $\begingroup$ In some cases the material you want to recrystallize is sensitive, and extra heating may result in decomposition, etc $\endgroup$ – Greg Oct 5 '16 at 18:30
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    $\begingroup$ The issue is that, without knowing beforehand the exact solubility of your compound, you do not know what is the "right amount" of solvent. Yeah, if you know the solubility perfectly, fair game, apart from decomposition/side reactions. However, if you're trying to recrystallise something new that you made in the lab (probably a much more common scenario), you don't want to start by guessing how much cold water/ethanol/whatever to add. $\endgroup$ – orthocresol♦ Oct 5 '16 at 20:38

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