Flexi Says: A precipitate forms in a double-replacement reaction when the cations from one of the reactants combine with the anions from the other reactant to form an insoluble ionic compound. A precipitate appears suddenly as its formation depends on the relative concentrations of anions and cations that form the precipitate. If the value of the ion product is less than the value of the Ksp (solubility product constant), then the solution will remain unsaturated. No precipitate will form because the concentrations are not high enough to begin the precipitation process. If the value of the ion product is greater than the value of the Ksp, then a precipitate will form. The formation of the precipitate lowers the concentration of each of the ions until the ion product is exactly equal to the Ksp, at which point precipitation ceases.