solubility
Summary
Solubility is the maximum amount of solute that dissolves in a given amount of solvent at a set temperature, giving a saturated solution at equilibrium. Most testable: for a salt, solubility is linked to Ksp, and a common ion lowers it.
Detail
Meaning: Solubility is how much solute can dissolve in a given amount of solvent at a given temperature before the solution is saturated, meaning dissolved solute is in equilibrium with undissolved solute. "Like dissolves like": ionic and polar solutes dissolve well in polar solvents such as water, and nonpolar solutes dissolve well in nonpolar solvents. Temperature raises the solubility of most solids but lowers that of gases. Gas solubility rises with the pressure of that gas above the liquid (Henry's law).
Equation: For a 1:1 salt AB ⇌ A⁺ + B⁻, Ksp = [A⁺][B⁻] = s², where Ksp is the solubility product (treated as unitless) and s is the molar solubility in mol/L (SI: mol/m³). For AxBy, Ksp = (xs)ˣ(ys)ʸ.
On the MCAT: Expect a Ksp given and a request for s, or a comparison of how much salt dissolves in pure water versus in a solution containing a common ion (it dissolves less). Passages also ask you to compare the ion product Q with Ksp: if Q > Ksp, a precipitate forms. Solubility curves versus temperature may need reading.
Common confusion: Ksp is not itself the solubility. A smaller Ksp does not guarantee a smaller molar solubility unless the salts have the same ion ratio, because s is found from Ksp using the formula's stoichiometry.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- Principles of Modern Chemistry
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.