insoluble

Meaning: An insoluble compound dissolves to such a small extent that almost none enters solution, so a solid stays visible. Dissolving needs ion–solvent attractions (hydration) to make up for the lattice energy, and in insoluble salts the lattice energy wins out. Typical solubility rules: salts of group 1 cations, ammonium and nitrate are soluble. Most chlorides, bromides and iodides are soluble, except those of Ag⁺, Pb²⁺ and Hg₂²⁺. Most hydroxides, carbonates, phosphates and sulfides are insoluble, except those of group 1 and ammonium.

Equation: Ksp = [Mⁿ⁺]ᵐ[Xᵐ⁻]ⁿ for MₘXₙ(s) ⇌ mMⁿ⁺ + nXᵐ⁻, where the brackets are equilibrium concentrations in mol/L and Ksp is the solubility product. A precipitate forms when Q > Ksp, where Q uses the same expression with the current concentrations.

On the MCAT: Passages give solubility rules or Ksp values. You predict whether a precipitate forms when two solutions mix, write a net ionic equation that leaves out spectator ions, or rank salts by molar solubility. Adding a common ion lowers solubility. Adding acid tends to raise the solubility of salts with basic anions, such as hydroxides and carbonates.

Common confusion: Insoluble does not mean zero solubility. Even an insoluble salt has a Ksp and a small ion concentration in solution. Also, a smaller Ksp means lower solubility only when salts have the same ion ratio. Otherwise, compare molar solubilities.