pH and Solubility
Connect a buffered pH environment to free-ion removal and conditional total solubility.
Before this connection: Chemical equilibrium: dissolution equations, Q versus K, and molar solubility.
AP Chemistry Topic 8.11 · CED printed p. 159 / PDF p. 166. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Ksp and common ions
- Conjugate acid–base reactions
Solubility can depend on pH when a released ion reacts with hydronium or hydroxide. Consider M(OH)2(s) ⇌ M2+ + 2OH−. Added acid consumes OH−, lowering the free-ion product and allowing more solid to dissolve. Added hydroxide instead supplies a common ion and suppresses dissolution. For a carbonate or another weakly basic anion, acid protonates the anion; removing that free dissolution product can also increase total dissolved salt. The same coupled-equilibrium logic applies when a constituent ion has relevant weak-acid behavior. Identify the actual proton-transfer reaction rather than assuming all anions respond alike.
An acidic constituent ion gives the complementary case. Use a hypothetical sparingly soluble 1:1 salt BHX(s) ⇌ BH+(aq) + X−(aq). BH+ is a weak acid: BH+ + H2O ⇌ B(aq) + H3O+. X− has negligible acid–base behavior. Raising pH favors loss of the proton from BH+; equivalently, BH+ + OH− → B + H2O. Removing free BH+ lowers Qsp = [BH+][X−] below Ksp, so more solid dissolves. Total dissolved B-containing material includes both BH+ and B. Lowering pH reverses this deprotonation tendency and can lower the saturated total solubility. This stipulated model requires B to stay dissolved, the original solid to remain available, negligible dilution, and no competing precipitation, complexation or additional acid–base reactions over the pH range. It is not a claim that every metal salt or amphoteric solid has this behavior.
Ksp is still the constant for the specified solid’s dissolution at fixed temperature. A pH-induced change in total solubility changes the concentrations and the distribution of an ion among protonation states; it does not redefine Ksp. A salt whose ions do not appreciably undergo acid–base reactions may show little pH effect from this mechanism. Real metals may also complex or precipitate another phase, so the simplified reasoning needs its stated assumptions. AP assessment here is qualitative: write the dissolution and proton-transfer links, identify which free ion is removed or supplied, and predict the response. Solubility as a numerical function of pH is excluded.
Let's work through it
- For MC2O4(s) ⇌ M2+ + C2O42−, suppose the anion is a weak base and no other reactions dominate.
- Write C2O42− + H3O+ ⇌ HC2O4− + H2O. Lowering pH favors protonation and lowers the free dianion concentration.
- The dissolution quotient falls below Ksp, so more solid dissolves if present. Dissolved anion is now shared between protonation states.
- At the new saturated equilibrium, [M2+][C2O42−] matches the same Ksp even though total dissolved material can be higher.
A saturated hypothetical BHX solution contains some BHX(s). BH+ is a weak acid, X− is acid–base inactive, and neutral B remains soluble. A buffer raises pH without appreciable dilution or a common-ion addition. Only dissolution and BH+/B proton transfer are allowed in the stated pH window.
- Write BHX(s) ⇌ BH+ + X− and BH+ + H2O ⇌ B + H3O+.
- Lower hydronium favors deprotonation of BH+ to B. Free BH+ falls immediately relative to the saturated free-ion product.
- Qsp falls below the same fixed-temperature Ksp, so net dissolution supplies additional BH+ and X−. Some BH+ becomes B at the imposed pH.
- At the new saturated equilibrium, [BH+][X−] again equals Ksp, but total dissolved BH+ plus B is larger. A new solid or insoluble B phase would require a different model; it is excluded here.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Why can acid dissolve more carbonate salt?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Why can acid dissolve more carbonate salt?
Worked answer and reason
It protonates free carbonate, removing a dissolution product.
Linked equilibria replace the removed free-ion concentration.
Retrieval 2
Does a higher total dissolved concentration require a higher Ksp?
Worked answer and reason
No. Some dissolved ion can be converted to another protonation state while the free-ion product stays fixed.
Ksp uses the species in the specified dissolution equation.
Catch the tempting mistake
- pH affects every salt identically.
- It depends on whether its ions have relevant acid–base reactions and on any other allowed chemistry.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 8 problem 15 · ap application · symbolic
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For the teacher
- Ask students to distinguish a free ion in Ksp from total material distributed among protonation states.
- Use the acidic-ion teaching paragraph and separate JHY task to check the complementary higher-pH case. Keep the original solid, soluble neutral conjugate base and no-competing-phase assumptions explicit; do not extend this model to amphoteric metals.
- Assess the coupled dissolution/proton-transfer argument qualitatively, without a numerical pH-solubility curve.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 8.11.A
- Identify the qualitative effect of changes in pH on the solubility of a salt.
- 8.11.A.1
- The solubility of a salt is pH sensitive when one of the constituent ions is a weak acid, a weak base, or the hydroxide ion. These effects can be understood qualitatively using Le Châtelier’s principle.
- Computations of solubility as a function of pH will not be assessed on the AP Exam.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 — Required topic-numbered learning objectives and essential knowledge, all continuations and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §15.1, Precipitation and Dissolution — Effects of protonation on ion availability in a dissolution equilibrium.
Glossary and deeper reading · 10 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Solubility product
A solubility product is an equilibrium constant for a specified dissolution reaction. Its expression includes dissolved-species activities and omits the pure solid. For example, dissolving one formula unit that releases two anions gives an anion activity squared in the expression.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
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Conjugate acid–base pair
A conjugate acid–base pair consists of species differing by one proton. Losing a proton forms an acid's conjugate base, while gaining one forms a base's conjugate acid. For example, ammonium and ammonia form a conjugate acid–base pair.
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pH
pH is the negative base-ten logarithm of hydronium activity. In suitable dilute models, activity is approximated by molar concentration relative to the standard concentration. For example, lowering hydronium concentration by a factor of ten raises pH by one unit.
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Solubility
Solubility describes how much of a substance can dissolve in a specified solvent under stated conditions. It depends on the solute, solvent, temperature, and other relevant conditions and is distinct from how fast dissolution occurs. For example, two salts can have different equilibrium solubilities in water at the same temperature.
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Hydronium
Hydronium is the H₃O⁺ ion used in the introductory model of proton behavior in water. Aqueous H⁺ notation is shorthand for a solvated proton rather than a bare proton moving independently in the liquid. For example, a Brønsted acid can transfer a proton to water and form hydronium.
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Anion
An anion is a negatively charged ion. It has more electrons than needed to balance its positive nuclear charge. For example, a chlorine atom gaining one electron forms a chloride anion.
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Weak acid or base
A weak acid or base reacts only partially with water and establishes an equilibrium. Its equilibrium constant describes strength, while analytical concentration describes how much was prepared per volume. For example, acetic acid leaves both acid molecules and its conjugate base in water.
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Precipitate
A precipitate is a solid phase that forms from species in a solution. Whether it forms depends on the ion concentrations and the relevant equilibrium conditions. For example, silver ions and chloride ions can produce a silver chloride precipitate.
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Brønsted acid and base
A Brønsted acid donates a proton, and a Brønsted base accepts a proton. These roles describe a particular reaction, so some substances can act in either role. For example, water accepts a proton from hydrogen chloride but donates one to ammonia.