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Why does the solubility of certain salts depend on the pH of the solution?

Topic 7.13 pH and Solubility: explain why the solubility of salts of weak acids or bases depends on pH, using Le Chatelier's principle applied to the dissolution and acid-base equilibria.

A focused answer to AP Chemistry Topic 7.13, covering how pH affects the solubility of salts containing basic anions (such as hydroxides, carbonates and fluorides), using Le Chatelier's principle on the coupled dissolution and acid-base equilibria, with full worked examples.

Generated by Claude Opus 4.810 min answer

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  1. What this topic is asking
  2. Why pH can affect solubility
  3. Salts that show the effect
  4. The direction of the shift
  5. Try this

What this topic is asking

The College Board (Topic 7.13) wants you to explain why the solubility of salts of weak acids or bases depends on pH, using Le Chatelier's principle applied to the coupled dissolution and acid-base equilibria. The key is that a basic anion can react with added acid, pulling the dissolution equilibrium forward.

Why pH can affect solubility

The mechanism is a coupling of two equilibria: the dissolution salt(s)⇌cation+anion\text{salt}(s) \rightleftharpoons \text{cation} + \text{anion} and the acid-base reaction of the anion with H+\text{H}^+. When acid removes the anion, Le Chatelier drives the dissolution forward to replace it, so more solid dissolves.

Salts that show the effect

So Mg(OH)2\text{Mg(OH)}_2, CaCO3\text{CaCO}_3 and CaF2\text{CaF}_2 all dissolve more in acid, because their anions are basic. By contrast, NaCl\text{NaCl} or AgCl\text{AgCl} (where chloride is the anion of strong hydrochloric acid) show no pH dependence, because chloride does not react with acid.

The direction of the shift

In acidic solution, H+\text{H}^+ consumes the basic anion, lowering its concentration; the dissolution equilibrium shifts right, so solubility increases. In basic solution, added OH−\text{OH}^- either acts as a common ion (for hydroxides) or suppresses the anion's reaction with acid, so solubility decreases or stays low. Throughout, KspK_\text{sp} is constant; only the position of equilibrium (the ion product) moves, exactly as in the common-ion effect.

Try this

Q1. State whether the solubility of AgCl\text{AgCl} depends on pH, and explain. [2 points]

  • Cue. No; chloride is the anion of the strong acid HCl and does not react with H+\text{H}^+, so the solubility is pH-independent.

Q2. Predict the effect of adding acid on the solubility of Zn(OH)2\text{Zn(OH)}_2. [2 points]

  • Cue. Acid removes OH−\text{OH}^-, shifting the dissolution right, so the solubility increases.

Exam-style practice questions

Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AP 2022 (style)4 marksSection II (long FRQ, part). Consider the slightly soluble salt Mg(OH)2\text{Mg(OH)}_2, Mg(OH)2(s)⇌Mg2+(aq)+2OH−(aq)\text{Mg(OH)}_2(s) \rightleftharpoons \text{Mg}^{2+}(aq) + 2\text{OH}^-(aq). (a) Predict and justify the effect on its solubility of adding acid. (b) Predict and justify the effect of adding base. (c) Explain why a salt such as NaCl\text{NaCl} does not show a pH-dependent solubility. (d) Identify another type of anion whose salts show pH-dependent solubility.
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A 4-point conceptual FRQ on pH and solubility.

(a) Adding acid (1 point): acid (H+\text{H}^+) reacts with the OH−\text{OH}^- produced, removing it; by Le Chatelier the dissolution equilibrium shifts right, so the solubility of Mg(OH)2\text{Mg(OH)}_2 increases.
(b) Adding base (1 point): added OH−\text{OH}^- is a common ion; it shifts the equilibrium left (toward the solid), so the solubility decreases.
(c) NaCl (1 point): Cl−\text{Cl}^- is the conjugate base of a strong acid and does not react with H+\text{H}^+, so the solubility of NaCl is independent of pH.
(d) Another anion (1 point): anions of weak acids such as carbonate (CO32−\text{CO}_3^{2-}) or fluoride (F−\text{F}^-) react with acid, so their salts also show pH-dependent solubility.

Markers reward the increased solubility in acid, the decreased solubility in base, the strong-acid-anion reasoning for NaCl, and a valid example of a basic anion.

AP 2021 (style)1 marksSection I (multiple choice). The solubility of CaCO3\text{CaCO}_3 increases when the solution is made more acidic because (A) Ca2+\text{Ca}^{2+} reacts with acid (B) CO32−\text{CO}_3^{2-} reacts with H+\text{H}^+, shifting the dissolution equilibrium right (C) KspK_\text{sp} increases (D) carbonate is a strong base that does not react. Justify your choice.
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A 1-point conceptual MCQ. The answer is (B).

Carbonate is the anion of a weak acid, so it reacts with added H+\text{H}^+ (forming HCO3−\text{HCO}_3^- and eventually CO2\text{CO}_2 and water), removing it from solution; by Le Chatelier the dissolution shifts right and solubility increases. KspK_\text{sp} is unchanged. The trap is (C): acid changes the solubility, not KspK_\text{sp}.

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