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Solubility equilibria

A pinch of 'insoluble' salt in water is not truly inert — a tiny amount dissolves and sits in dynamic equilibrium with the solid, governed by Ksp. Pile on more solid, though, and nothing extra dissolves.

§1

A solid in equilibrium with its ions.

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A slightly soluble salt reaches a dissolution equilibrium: the undissolved solid is in dynamic equilibrium with its dissolved ions. It is described by the solubility product, Ksp.

The Ksp expression is the product of the dissolved ion concentrations, each raised to its coefficient. The solid is excluded (it is a pure solid). For AgCl ⇌ Ag⁺ + Cl⁻, Ksp = [Ag⁺][Cl⁻].

Once the solution is saturated, adding more solid does not increase the dissolved ion concentration — the extra solid just sits at the bottom. Solubility (how much dissolves) is derived from Ksp through the equilibrium, not read off directly.

UNIT 7 TOPIC 7.11 • INTRODUCTION TO SOLUBILITY EQUILIBRIA KSP DASHBOARD DISSOLUTION EQUILIBRIUM AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq) Ag⁺ Cl⁻ Ag⁺ Cl⁻ Some solid dissolves and some ions recombine. At saturation the two rates are equal: Qsp = Ksp KSP EXPRESSION Ksp = [Ag⁺][Cl⁻] Solid AgCl is omitted because pure solids do not appear in equilibrium expressions. For a 1:1 salt: Ksp = s² QSP VERDICTS (compare Q to Ksp) Qsp < Ksp unsaturated — more solid dissolves Qsp = Ksp saturated — system at equilibrium Qsp > Ksp supersaturated — precipitate forms MOLAR SOLUBILITY LINK Ksp = 1.8 × 10⁻¹⁰ s = √Ksp = 1.3 × 10⁻⁵ M A larger Ksp means more ions in solution at equilibrium, so the salt is more soluble. THE TAKEAWAY Ksp is the equilibrium constant for a dissolving ionic solid — it links ion concentrations, precipitation, and molar solubility. AP Chemistry · Unit 7 · Equilibrium
Fig. 7.11.1 A slightly soluble salt sits in dynamic equilibrium with its dissolved ions, governed by Ksp. Once saturated, adding more solid does not increase the dissolved ion concentration, and solubility is derived from Ksp, not read off directly.
§2

Working with Ksp.

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Write Ksp, then derive solubility through the equilibrium.

  1. Write the dissolution equilibrium. Solid ⇌ its dissolved ions, with the solid excluded from Ksp.
  2. Write the Ksp expression. The product of ion concentrations, each raised to its coefficient.
  3. Relate solubility to the ions. Let s be the molar solubility; express each ion concentration in terms of s.
  4. Solve Ksp for s. Derive the solubility from Ksp; do not read it off directly.
§3

The pieces you'll meet.

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A dynamic solid-ion equilibrium.

dissolution eq
Dissolution equilibrium
Solid in dynamic equilibrium with its ions.
Ksp
Solubility product
Product of ion concentrations, coefficients as exponents.
solid excluded
Solid excluded
The pure solid is not in Ksp.
saturated
Saturated
Adding more solid does not dissolve more.
solubility
Molar solubility
How much dissolves; derived from Ksp.
dynamic
Dynamic
Dissolving and precipitating continue at equal rates.
§4

Worked example: set up Ksp.

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Equilibrium. AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq).

Ksp expression. Ksp = [Ag⁺][Cl⁻] — the solid AgCl is excluded.

Solubility. If molar solubility is s, then [Ag⁺] = s and [Cl⁻] = s, so Ksp = s², giving s = √Ksp. The solubility is derived from Ksp, not equal to it.

Saturation. Once saturated, tossing in more AgCl solid changes nothing about [Ag⁺] and [Cl⁻] — the extra just remains undissolved.

§5

Mistakes that cost real points.

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Pitfall · 01

"Adding more solid dissolves more ions."

In a saturated solution, the dissolved ion concentrations are fixed by Ksp; adding more solid does not raise them. The extra solid simply remains undissolved at the bottom. The amount dissolved is capped once saturation is reached.

Fix. Recognize that a saturated solution holds a fixed ion concentration; extra solid does not dissolve further.

Pitfall · 02

"The solubility equals Ksp."

Solubility must be derived from Ksp through the equilibrium, not read off directly. For AgCl, s = √Ksp, not Ksp. Different salt stoichiometries give different relationships between s and Ksp.

Fix. Set molar solubility s, express the ions in terms of s, and solve Ksp for s — do not equate s with Ksp.

Pitfall · 03

"Dissolution stops once the solid appears to stop dissolving."

A saturated solution is a dynamic equilibrium: solid keeps dissolving and ions keep precipitating at equal rates. It is not a hard stop — the concentrations are constant because the two processes balance.

Fix. Picture saturation as a dynamic equilibrium of dissolving and precipitating, not as dissolution halting.

§6

Skill Check.

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