precipitate

Meaning: A precipitate is an insoluble solid that forms from solution. Typically two soluble salts are mixed and one pair of ions combines into a poorly soluble compound, as in Ag⁺(aq) + Cl⁻(aq) → AgCl(s). Solid forms when the ion product Q exceeds Ksp, and it keeps forming until the ion concentrations fall to Q = Ksp.

Equation: Q = [Ag⁺][Cl⁻]; a precipitate forms if Q > Ksp. Brackets are concentrations in mol/L; Q and Ksp are usually treated as unitless. For a salt AmBn, Q = [A]ᵐ[B]ⁿ.

On the MCAT: Passages mix two solutions and ask whether a solid forms. Find the concentrations after mixing (volumes add, so each ion is diluted), compute Q, and compare it with Ksp. Q < Ksp: no solid. Q = Ksp: saturated. Q > Ksp: precipitate. In selective precipitation, the salt that reaches its Ksp first comes out first. A common ion lowers solubility.

Common confusion: Students use concentrations from before mixing and skip the dilution step. Also, comparing Ksp values to rank solubility works directly only for salts with the same ion ratio; otherwise compare molar solubilities.