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Introduction to Solubility Equilibria

A slightly soluble salt reaches a dissolution equilibrium: the undissolved solid is in dynamic equilibrium with its dissolved ions, described by the solubility product, Ksp. The Ksp expression is the product of the ion concentrations, each raised to its coefficient (the solid is excluded).

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
A slightly soluble salt sits in dynamic equilibrium with its dissolved ions, governed by the solubility product Ksp. Once saturated, adding more solid does not increase the dissolved ion concentration, and solubility is derived from Ksp, not read off directly.
Ksp Dashboard · Open the sandbox →

The traps here are that adding more solid dissolves more (a saturated solution holds a fixed ion concentration; extra solid just sits there), reading solubility straight off Ksp (it must be derived through the equilibrium), and treating dissolution as a hard stop (it is a dynamic equilibrium).

The work

3 ways in · any order
Lesson
Solubility Equilibria

A slightly soluble salt is in dynamic equilibrium with its ions via Ksp; solubility is derived, not read off. The lesson sets up Ksp and derives solubility, then closes with a ten-scenario check.

Skill check · 10 scenarios
Diagnostic
10-item topic check

Ten items spanning the Topic 7.11 misconceptions: more solid thought to give more dissolved ions, solubility read straight from Ksp, and dissolution taken as a hard stop rather than a dynamic equilibrium.

Not started · 10 items · ~15 min
Targeted Practice
Drill a single misconception

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions