Precipitation reactions occur when two soluble ionic compounds in aqueous solution react to form an insoluble solid product called a precipitate. For example, when solutions of silver nitrate and sodium chloride are mixed, silver chloride precipitates as a white solid. The chemical equation for this reaction is silver nitrate plus sodium chloride yields silver chloride precipitate plus sodium nitrate in solution.
Let's look at a molecular view of precipitation reactions. In solution, ionic compounds dissociate into their component ions. Silver nitrate dissolves to form silver ions and nitrate ions, while sodium chloride dissolves to form sodium ions and chloride ions. When these solutions mix, the silver ions and chloride ions are attracted to each other and form silver chloride, which is insoluble in water and precipitates out as a solid. The sodium ions and nitrate ions remain in solution as they form sodium nitrate, which is soluble in water.
To predict precipitation reactions, we need to know the solubility rules. Most compounds containing sodium, potassium, or ammonium ions are soluble in water. Most chlorides are soluble, with important exceptions like silver chloride, lead chloride, and mercury(I) chloride. Most sulfates are soluble except for barium sulfate, lead sulfate, and calcium sulfate. Most hydroxides are insoluble, with exceptions for Group 1 metals and some Group 2 metals. Using these rules, we can predict whether a precipitation reaction will occur when two solutions are mixed. For example, when silver nitrate solution is mixed with sodium chloride solution, silver chloride precipitates because it's insoluble according to our rules.
When writing precipitation reactions, we follow a three-step process. First, we write the molecular equation showing the complete formulas of all reactants and products. For example, when silver nitrate reacts with sodium chloride, we write: silver nitrate plus sodium chloride yields silver chloride solid plus sodium nitrate. Second, we write the complete ionic equation, showing all soluble compounds as dissociated ions. This gives us: silver ion plus nitrate ion plus sodium ion plus chloride ion yields solid silver chloride plus sodium ion plus nitrate ion. Finally, we write the net ionic equation by canceling out spectator ions—those that appear unchanged on both sides of the equation. The spectator ions here are sodium and nitrate. After canceling them out, our net ionic equation is simply: silver ion plus chloride ion yields solid silver chloride.
To summarize what we've learned about precipitation reactions: First, precipitation reactions occur when two soluble ionic compounds react to form an insoluble solid product called a precipitate. Second, solubility rules help us predict which combinations of ions will form precipitates. Third, in solution, ionic compounds dissociate into ions that can recombine to form new compounds. Fourth, net ionic equations show only the species that actually participate in the reaction, excluding spectator ions. Finally, precipitation reactions are important in water treatment, qualitative analysis, and many industrial processes. Understanding these reactions helps us predict chemical behavior and design useful applications.