Solubility and Ksp

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Solubility describes how much of a substance can dissolve in a solvent before the solution becomes saturated. For ionic solids in water, dissolving means the crystal separates into positive and negative ions. The solubility product constant, Ksp, gives a precise way to describe the equilibrium between undissolved solid and dissolved ions. This matters in chemistry because precipitation, water hardness, mineral formation, and many lab separations depend on whether ions stay dissolved or form a solid.

Key Facts

For AgCl(s) ⇌ Ag+(aq) + Cl-(aq), Ksp = [Ag+][Cl-].
Pure solids are not included in Ksp expressions because their activity is treated as constant.
For CaF2(s) ⇌ Ca2+(aq) + 2F-(aq), Ksp = [Ca2+][F-]^2.
Molar solubility is the number of moles of solid that dissolve per liter of solution at equilibrium.
A precipitate forms when the ion product Qsp is greater than Ksp.
Adding a common ion usually lowers solubility because equilibrium shifts toward the solid.

Vocabulary

Solubility: Solubility is the maximum amount of a substance that can dissolve in a given amount of solvent at a specific temperature.
Saturated solution: A saturated solution contains the maximum dissolved solute possible while remaining in equilibrium with undissolved solute.
Ksp: Ksp is the equilibrium constant for the dissolving of a slightly soluble ionic compound.
Ion product: The ion product, Qsp, is the current product of ion concentrations before or during equilibrium comparison.
Common ion effect: The common ion effect is the decrease in solubility caused by adding an ion already present in the dissolving equilibrium.

Common Mistakes to Avoid

Including the solid in the Ksp expression is wrong because pure solids do not appear in equilibrium constant expressions.
Forgetting coefficients become exponents is wrong because a salt such as CaF2 gives Ksp = [Ca2+][F-]^2, not [Ca2+][F-].
Assuming larger Ksp always means larger solubility is wrong because different formulas produce different ion ratios, so Ksp values must be compared carefully.
Ignoring the common ion effect is wrong because added ions change equilibrium concentrations and can greatly reduce how much solid dissolves.

Practice Questions

1 The Ksp of AgCl is 1.8 x 10^-10 at 25°C. What is the molar solubility of AgCl in pure water?
2 For CaF2, Ksp = 3.9 x 10^-11. If the molar solubility is s, write the Ksp expression in terms of s and solve for s.
3 A solution contains dissolved Pb2+ and I- ions, and a small amount of solid PbI2 is visible at the bottom. Explain what must be true about the rates of dissolving and crystallizing at equilibrium.