Salts of weak acids dissolve more in acidic solutions. For CaCO3: CaCO3 <=> Ca2+ + $CO3^{2-}$. In acid, H+ reacts with $CO3^{2-}$ to form HCO3- and then H2CO3 (which decomposes to CO2 + H2O). This removes $CO3^{2-}$ from solution, shifting equilibrium right — more CaCO3 dissolves. Salts of strong acids (AgCl) are unaffected by pH changes because Cl- does not react with H+. Hydroxide salts dissolve in acids: Mg(OH)2 + 2H+ -> Mg2+ + 2H2O. Metal sulfides dissolve in acid: CuS + 2H+ -> Cu2+ + H2S (though some like HgS are too insoluble). This pH-dependent solubility is the basis for separation in qualitative inorganic analysis.