What Are Solubility Rules Used For?

Solubility rules dictate whether a compound will dissolve in water. Therefore, solubility rules can help you determine what state the products of a chemical reaction will have. When you write out a chemical equation, you can use solubility rules to label the predicted states of the compounds involved. This is especially useful when monitoring a solution for precipitates.

All nitrates are soluble in water. Therefore, any compound containing the nitrate ion, NO3-, will dissolve in water. Most chlorides, or compounds containing Cl- ions, are soluble in water. Most sulfates, or compounds containing SO4 2- ions, are also soluble in water. Finally, all ionic compounds containing group IA metals or ammonium ions are soluble. For example, NH4OH will dissolve into NH4+ and OH- ions in solution.

Chlorides containing copper(I), mercury(II) or silver ions will not dissolve. Lead(II) chloride is slightly soluble, but will form as a precipitate in many reactions. Sulfates containing strontium, barium and lead(II) are insoluble. Sulfates containing calcium and silver are slightly soluble. Like lead(II) chloride, calcium sulfate and silver sulfate will precipitate in many reactions.

All hydroxides are insoluble, with the exceptions of hydroxides involving the group IA metals or ammonium ions. Furthermore, calcium hydroxide, strontium hydroxide and barium hydroxide are slightly soluble. All sulfides are insoluble, with the exceptions of sulfides involving the group IA metals or ammonium ions. Sulfides contain the S2- ion, and should not be confused with sulfates. Finally, all carbonates and phosphates are insoluble, except those involving group IA metals or ammonium ions.

Consider the reaction of silver nitrate with calcium chloride, in solution. Silver nitrate is soluble in solution per the solubility rules for nitrates. Calcium chloride is soluble in solution per the solubility rules for chlorides. If all of these ions are floating in a solution, you must then think of their other possible combinations. Silver chloride is an exception to the chloride rule. Therefore, you can expect to see silver chloride precipitate out of the solution. The remaining calcium nitrate ions will stay in solution due to the solubility rules for nitrates.