[Jaccobtw]

Let's say you have some NaCl(s) dissolved in some water

NaCl(s)  Na⁺ + Cl^-

If you add some strong acid, which direction will the solubility increase or decrease?

Answer: I think it will increase, because the strong acid will consume the chloride ion and the NaCl will have to dissociate more to compensate. Is this right?

[AWK]

http://www.csun.edu/~ml727939/coursework/695/common%20ion%20effect/recrystallization%20of%20NaCl%20solution%20with%20HCl.htm

[Jaccobtw]

But what if you're adding H₃O⁺, not HCl. Won't the reaction shift to the right? Adding H₃O⁺ will reduce chloride ions, so the reaction has to shift to the right

[Borek]

HCl is so strong this effect is negligible (no stronger acids in water solution, where you are actually limited by the strength of hydronium).

For weaker acids the effect can be important.

Trick is, in the really concentrated solutions effects are actually very difficult to predict, as ions compete for water molecules they require for solvation.