Final answer:
For insoluble ionic compounds with anions from weak acids, lower pH increases solubility due to more of the solid salt dissolving to restore equilibrium when hydronium ions react with the anions, reducing their concentration in solution.
Step-by-step explanation:
For insoluble ionic compounds that contain anions of weak acids, the lower the pH, the higher the solubility. This is explained by the reaction A-(aq) + H2O(l) ⇒ OH-(aq) + HA(aq), which shows that the anion (A-) from the sparingly soluble salt can react with water to produce hydroxide ions (OH-) and the corresponding weak acid (HA). When the pH of the solution decreases, which means the concentration of hydronium ions (H+) increases, these hydronium ions will react with the anion to form the weak acid, thus reducing the concentration of A- in the solution.
According to Le Chatelier's principle, this shift in concentration causes more of the sparingly soluble salt (MA) to dissolve until the ion product Q equals the solubility product (Ksp). Therefore, in acidic conditions (low pH), the solubility of salts with anions that are the conjugate bases of weak acids is increased. This effect is significant for salts like magnesium hydroxide (Mg(OH)2) and does not apply to salts with anions derived from stronger acids.