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I've been searching around the web and the consensus seems to be sodium salicylate dissolves poorly in HCl because it would form salicylic acid. However, I don't understand why forming salicylic acid product (which is poorly soluble in water) would make sodium salicylate (which is the reactant) dissolve poorly in HCl. If HCl doesn't contribute to making it more soluble, wouldn't the polarity of the water molecule dissolve it anyway?

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HCl is in almost all cases already aqueous for common chemistry, mostly dissociated into $\ce{H^+} + \ce{Cl^-}$ because it is a strong acid, which means water is the primary solvent, not HCl. Pure HCl under ambient conditions is actually a gas (thus certainly not a pure liquid solvent).

That leaves the compound in question insoluble in the water after being protonated by $\ce{H^+}$ to form the insoluble acid molecule.

The process I mentioned in the comment is illustrated here: enter image description here

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  • $\begingroup$ The HCl is aqueous. But if that's the case, why would sodium salicylate be poorly soluble in HCl just because it forms an acid when there are so many water molecules around? $\endgroup$ – L to the V Feb 1 '16 at 3:48
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    $\begingroup$ The thing rendered insoluble is not the sodium salt itself; rather the post-reacted salicylic acid. It is an induced solubility, as it were $\endgroup$ – khaverim Feb 1 '16 at 3:51
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    $\begingroup$ That's right -- the new compound doesn't revert to the sodium salt after accepting the proton. That means you can actually reduce the concentration of insoluble salicylic acid by adding base, e.g. NaOH, and tipping equilibrium $\endgroup$ – khaverim Feb 1 '16 at 3:53
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    $\begingroup$ The principle here is that minimization of molecular energy wins every time. Resonance is a stabilizing, energy minimizing effect, but it does not render every molecule with resonance unreactive. Accepting a proton in this case actually renders a molecule with lower energy than the negatively charged ion. Many such molecules react with acids and other compounds, and that's a big chunk of organic chemistry. $\endgroup$ – khaverim Feb 1 '16 at 3:58
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    $\begingroup$ Yes, effectively. You're just making a compound that's insoluble in water (salicylic acid), from one that isn't (sodium salicylate), using something else in the water (HCl), through ionic combination. It's a precipitation reaction much like what you've probably learned before, just now in the class of organic salts, not metallic salts. $\endgroup$ – khaverim Feb 1 '16 at 4:05

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