The document discusses the common ion effect, which is when adding a soluble compound containing one of the ions of a precipitate decreases the solubility of the precipitate. It provides examples like adding dilute sulfuric acid (which contains sulfate ions) to a saturated calcium sulfate solution, causing more calcium sulfate to precipitate out. It also discusses how adding sodium chloride to a solution of hydrochloric acid and water, or during soap making, provides more chloride or sodium ions that reduce the solubility of salts through competition.