Unit 8 · Topic 8.11 Beta

pH and Solubility

The solubility of a salt depends on pH when its anion is a base.

Practice 4: Model AnalysisPractice 6: Argumentation

Question set for this topic

Part 1 · Hook

Why this matters

Limestone caves form because rainwater is slightly acidic: dissolved carbon dioxide makes it so, and calcium carbonate, almost insoluble in pure water, slowly dissolves in it. The same chemistry erodes tooth enamel after a soda. In each case the acid attacks the anion of a salt, and the solid dissolves to replace it.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. For CaF₂(s) ⇌ Ca²⁺ + 2 F⁻, what is the expression for Ksp?

  1. [Ca²⁺][F⁻]²
  2. [Ca²⁺][F⁻]
  3. [Ca²⁺][2F⁻]
  4. [CaF₂]/([Ca²⁺][F⁻]²)
Show the answer

Products raised to their coefficients; the solid is left out.

  • Correct: [Ca²⁺][F⁻]²:
  • [Ca²⁺][F⁻]:
  • [Ca²⁺][2F⁻]:
  • [CaF₂]/([Ca²⁺][F⁻]²):

2. If Q < Ksp for a salt with solid present, what happens?

  1. More solid dissolves
  2. More solid forms
  3. Nothing: the system is at equilibrium
  4. Ksp increases
Show the answer

Q < K means the net reaction goes forward: dissolving.

  • Correct: More solid dissolves:
  • More solid forms:
  • Nothing: the system is at equilibrium:
  • Ksp increases:

3. Which anion is the conjugate base of a weak acid?

  1. F⁻
  2. Cl⁻
  3. NO₃⁻
  4. I⁻
Show the answer

HF is weak; HCl, HNO₃ and HI are strong.

  • Correct: F⁻:
  • Cl⁻:
  • NO₃⁻:
  • I⁻:

Part 4 · See it

See it first

CaF₂(s) ⇌ Ca²⁺ + 2 F⁻. Adding hydronium turns fluoride into HF, so Q falls below Ksp and more CaF₂ dissolves. For AgCl, chloride comes from the strong acid HCl and does not react with hydronium, so pH has no effect on its solubility.
Hydronium removes fluoride, so more calcium fluoride dissolves. Chloride does not react, so silver chloride is unaffected by pH. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. A salt's anion is the conjugate base of a weak acidH₃O⁺ reacts with it: F⁻ + H₃O⁺ → HF + H₂O
  2. That reaction lowers the anion's concentrationQ = [Ca²⁺][F⁻]² falls below Ksp
  3. Q < Ksp with solid presentmore solid dissolves until Q = Ksp again, so solubility rises as pH falls
  4. An anion from a strong acid (Cl⁻, NO₃⁻) does not take protonsits salts' solubility does not depend on pH

Part 6 · Key ideas

Key ideas

  • Lowering the pH dissolves more of a salt whose anion is a base: F⁻, CO₃²⁻, PO₄³⁻, S²⁻, OH⁻.
  • Explain with Q and Ksp: H₃O⁺ removes the anion, Q < Ksp, more solid dissolves. Ksp itself does not change.
  • Anions from strong acids (Cl⁻, Br⁻, I⁻, NO₃⁻) are unaffected, so those salts' solubility does not depend on pH.
  • The weaker the conjugate acid, the stronger the base, and the more the solubility depends on pH.

Part 7 · Misconception

A common mistake

The wrong idea: Acid makes every salt more soluble.

What actually happens: Only salts whose anions are bases. AgCl is as insoluble in nitric acid as in water, because Cl⁻ does not react with H₃O⁺.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Data table

Five slightly soluble salts

Each salt dissolves only slightly in pure water at 25 °C. Ka values are given for the conjugate acid of each anion; "strong" means the conjugate acid is a strong acid.

Salts and the acid each anion comes from
SaltAnionConjugate acid of the anionKa of that acid
CaF₂F⁻HF6.8 × 10⁻⁴
AgClCl⁻HClstrong
BaCO₃CO₃²⁻HCO₃⁻4.7 × 10⁻¹¹
PbI₂I⁻HIstrong
Mg(OH)₂OH⁻H₂O(OH⁻ reacts with H₃O⁺ to make water)

1. Which salts become more soluble when the solution is made more acidic? Select all that apply.

  1. CaF₂
  2. AgCl
  3. BaCO₃
  4. PbI₂
  5. Mg(OH)₂
Show the answer

Each of these anions is a base: H₃O⁺ converts F⁻ to HF, CO₃²⁻ to HCO₃⁻ (and on to H₂CO₃ and CO₂), and OH⁻ to water. Removing the anion makes Q < Ksp, so more solid dissolves.

  • Correct: CaF₂: Right: F⁻ is the conjugate base of the weak acid HF.
  • AgCl: Cl⁻ comes from a strong acid and does not react with H₃O⁺.
  • Correct: BaCO₃: Right: carbonate is a fairly strong base.
  • PbI₂: I⁻ comes from a strong acid and does not react with H₃O⁺.
  • Correct: Mg(OH)₂: Right: H₃O⁺ + OH⁻ → 2 H₂O removes hydroxide.

2. Which explanation correctly says why adding HNO₃ dissolves more CaF₂(s)?

  1. H₃O⁺ turns F⁻ into HF, so Q < Ksp
  2. HNO₃ increases the Ksp of the CaF₂ solid
  3. NO₃⁻ ions react with Ca²⁺ to form a soluble compound
  4. The H₃O⁺ ions attack the solid and break its covalent bonds
Show the answer

The answer names the reaction (F⁻ + H₃O⁺ → HF + H₂O), its effect on Q, and the shift: the reasoning readers look for.

  • Correct: H₃O⁺ turns F⁻ into HF, so Q < Ksp: Right: Q vs Ksp with the specific reaction.
  • HNO₃ increases the Ksp of the CaF₂ solid: Ksp depends only on temperature; it is Q that changes.
  • NO₃⁻ ions react with Ca²⁺ to form a soluble compound: Calcium nitrate is soluble, but NO₃⁻ was not needed to remove Ca²⁺; the effect comes from removing F⁻.
  • The H₃O⁺ ions attack the solid and break its covalent bonds: CaF₂ is an ionic solid; dissolving separates ions rather than breaking covalent bonds.

3. How does adding HNO₃ change the solubility of AgCl?

  1. Very little: Cl⁻ does not react with H₃O⁺
  2. It increases, because H₃O⁺ reacts with Cl⁻ to form HCl
  3. It decreases, because NO₃⁻ is a common ion
  4. It increases, because salts dissolve better in acid
Show the answer

HCl is a strong acid, so Cl⁻ has essentially no tendency to take a proton. [Cl⁻] stays the same, Q is unchanged, and the solubility is unchanged.

  • Correct: Very little: Cl⁻ does not react with H₃O⁺: Right: no reaction with the anion, no shift.
  • It increases, because H₃O⁺ reacts with Cl⁻ to form HCl: HCl would immediately ionize again; Cl⁻ is not removed.
  • It decreases, because NO₃⁻ is a common ion: NO₃⁻ is not in AgCl, so it is not a common ion.
  • It increases, because salts dissolve better in acid: Only salts whose anions are bases dissolve better in acid.

Particle view

Calcium fluoride in water and in acid

Box 1 (water)Ca²⁺F⁻F⁻Box 2Ca²⁺Ca²⁺Ca²⁺Ca²⁺F⁻HFHFHFHFHFHFHFBox 3Ca²⁺Ca²⁺Ca²⁺Ca²⁺F⁻F⁻F⁻F⁻F⁻F⁻F⁻F⁻Box 4Ca²⁺HFHFH₃O⁺H₃O⁺H₃O⁺H₃O⁺

Key: Ca²⁺ calcium ion; F⁻ fluoride; HF hydrogen fluoride molecule; H₃O⁺ hydronium. Each box is the solution above undissolved CaF₂(s), not drawn, at the same temperature. Water and the anions of the added acid are not drawn.

4. Which box best represents the solution after a strong acid is added to Box 1 and equilibrium is restored?

  1. Box 2
  2. Box 3
  3. Box 4
  4. Box 1, unchanged
Show the answer

More CaF₂ dissolved (4 Ca²⁺ instead of 1), and most of its fluoride became HF. F atoms are conserved (1 F⁻ + 7 HF = 8 = 2 × 4 Ca²⁺), and the free-ion product [Ca²⁺][F⁻]² is 4 × 1² = 4, the same as Box 1 (1 × 2² = 4), as Ksp requires.

  • Correct: Box 2: Right: more dissolved, fluoride mostly protonated, ion product back at Ksp.
  • Box 3: Box 3 shows more dissolved with no HF: its ion product (4 × 8² = 256) is far above Ksp, so it is not at equilibrium.
  • Box 4: Box 4 has no free F⁻, so Q = 0 < Ksp: more solid would still dissolve. It is not at equilibrium.
  • Box 1, unchanged: Adding acid removes F⁻, so the solution cannot stay the same.

5. In every box, the total number of F atoms (in F⁻ and HF) should be twice the number of Ca²⁺ ions. Why?

  1. Both come from CaF₂, with two F per Ca
  2. Ca²⁺ has a 2+ charge, so it attracts two water molecules
  3. HF molecules contain two atoms
  4. Ksp has [F⁻] squared
Show the answer

Dissolving CaF₂ releases one Ca²⁺ and two F⁻; protonating F⁻ to HF does not change the number of F atoms.

  • Correct: Both come from CaF₂, with two F per Ca: Right: the formula sets the ratio.
  • Ca²⁺ has a 2+ charge, so it attracts two water molecules: The ratio comes from the formula of the salt, not from hydration.
  • HF molecules contain two atoms: The number of atoms in HF does not set the Ca : F ratio.
  • Ksp has [F⁻] squared: The square in Ksp follows from the same formula; it is a consequence, not the cause.

Experimental setup

Dissolving a hydroxide

Three test tubes each contain 0.10 g of solid Mg(OH)₂ in 10 mL of liquid: Tube A, distilled water; Tube B, 1.0 M HCl; Tube C, 1.0 M NaOH. The tubes are shaken and left to settle.

Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2 OH⁻(aq)

6. In which tube does the most Mg(OH)₂ dissolve?

  1. Tube B
  2. Tube A
  3. Tube C
  4. The same amount in each
Show the answer

H₃O⁺ in HCl reacts with OH⁻ (H₃O⁺ + OH⁻ → 2 H₂O), driving [OH⁻] down, so the dissolving equilibrium shifts right until the solid dissolves.

  • Correct: Tube B: Right: acid removes OH⁻.
  • Tube A: Water dissolves only a trace of Mg(OH)₂.
  • Tube C: Added OH⁻ is a common ion; it pushes the equilibrium left.
  • The same amount in each: The liquids affect [OH⁻] very differently.

7. Why does even less Mg(OH)₂ dissolve in Tube C than in Tube A?

  1. OH⁻ is a common ion, so Q reaches Ksp sooner
  2. NaOH reacts with Mg(OH)₂ to form a new solid
  3. Na⁺ ions lower Ksp
  4. NaOH raises the temperature
Show the answer

The large [OH⁻] makes Q = [Mg²⁺][OH⁻]² exceed Ksp at a tiny [Mg²⁺], so the dissolving stops much earlier.

  • Correct: OH⁻ is a common ion, so Q reaches Ksp sooner: Right: the common-ion effect.
  • NaOH reacts with Mg(OH)₂ to form a new solid: No new solid is involved; the effect is on Q.
  • Na⁺ ions lower Ksp: Ksp does not change with added ions.
  • NaOH raises the temperature: Dissolving a little NaOH in 10 mL does warm it slightly, but that is not why the solubility falls.

Part 9 · Summary

Summary

The solubility of a salt depends on pH when its anion is a base. Added H₃O⁺ reacts with the anion, its concentration drops, Q falls below Ksp, and more solid dissolves; Ksp itself does not change. Salts of anions from strong acids are unaffected by pH. On the exam this is reasoned qualitatively.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections