Solubility rules help chemists predict whether an ionic compound will dissolve in water or remain as a solid. These rules are especially useful when two aqueous ionic solutions are mixed, because an insoluble product can form as a precipitate. Being able to predict precipitates lets you identify reactions, separate ions, and write correct chemical equations.
The rules are patterns based on how strongly ions interact with water compared with each other.
When an ionic compound dissolves, its positive and negative ions separate and become surrounded by water molecules. If the attraction between the ions is too strong, the compound stays together as a solid or forms a solid after solutions are mixed. In double-replacement reactions, ions exchange partners, and the solubility rules tell which possible products are aqueous and which are solid.
A precipitate is evidence that new ion combinations have produced an insoluble compound.
Understanding Chemistry: Solubility Rules
Solubility is a competition between energy changes. An ionic crystal has a repeating structure called a lattice. Pulling its ions apart requires energy because opposite charges attract.
Water can give back energy when its partly negative oxygen end faces positive ions and its partly positive hydrogen ends face negative ions. A salt tends to dissolve when these water ion attractions can compete successfully with the lattice.
Small ions with high charges often build especially strong lattices. This helps explain why compounds with similar looking formulas can behave very differently in water.
When solving a mixing problem, first write the ions present in each starting solution. Keep the charge on every ion clear, since charge determines the correct ratio in each possible product. Next exchange the positive ions between the negative ions.
Use the rules to label each product as dissolved or solid. If one product is insoluble, it is the precipitate.
For example, mixing a solution containing barium ions with one containing sulfate ions produces barium sulfate solid. The other ions may remain dissolved without taking part in the visible change.
The full ionic equation shows all strong dissolved ionic compounds as separate ions. Then remove spectator ions, which appear unchanged on both sides. What remains is the net ionic equation.
This shorter equation identifies the actual chemical event. It is useful because the same precipitate can form from many different soluble starting salts. A silver chloride precipitate, for instance, always depends on silver ions meeting chloride ions, regardless of which soluble compounds supplied those ions.
State symbols matter in this work. They show whether a substance is dissolved, solid, liquid, or gas.
The rules are reliable classroom shortcuts, not complete laws for every situation. Temperature can change how much of a substance dissolves. Concentration matters too.
A small amount of an insoluble substance may remain dissolved before enough ions are present to start crystal formation. Acids can change the result because they react with carbonate, sulfide, phosphate, or hydroxide ions. Complex ions can keep some metal ions dissolved when a simple rule suggests otherwise.
In school problems, pay close attention to the stated conditions and apply the listed exceptions before deciding that a solid forms. Careful ion charges, correct formulas, and state labels prevent most mistakes.
Key Facts
- All Group 1 metal ion compounds and NH4+ compounds are soluble.
- All NO3-, C2H3O2-, ClO3-, and ClO4- salts are soluble.
- Most Cl-, Br-, and I- salts are soluble except with Ag+, Pb2+, and Hg2^2+.
- Most SO4^2- salts are soluble except with Ba2+, Sr2+, Pb2+, and often Ca2+.
- Most CO3^2-, PO4^3-, S^2-, and OH- salts are insoluble except with Group 1 ions and NH4+.
- A precipitation reaction occurs when aqueous ions form an insoluble product, such as Ag+(aq) + Cl-(aq) = AgCl(s).
Vocabulary
- Solubility
- Solubility is the ability of a substance to dissolve in a solvent such as water.
- Precipitate
- A precipitate is an insoluble solid that forms from ions in solution during a reaction.
- Aqueous
- Aqueous means dissolved in water and is shown by the state symbol aq.
- Spectator ion
- A spectator ion is an ion that remains dissolved and unchanged during a reaction.
- Net ionic equation
- A net ionic equation shows only the ions and compounds that directly participate in a chemical change.
Common Mistakes to Avoid
- Treating every ionic compound as soluble is wrong because many common ions form insoluble salts, such as carbonates and phosphates.
- Forgetting exceptions is wrong because rules like chloride salts are soluble have important exceptions such as AgCl, PbCl2, and Hg2Cl2.
- Writing all products as solids is wrong because only insoluble products form precipitates, while soluble products remain aqueous ions.
- Including spectator ions in the net ionic equation is wrong because spectator ions do not change and should be canceled from the complete ionic equation.
Practice Questions
- 1 Predict whether a precipitate forms when 25.0 mL of 0.20 M AgNO3(aq) is mixed with 25.0 mL of 0.20 M NaCl(aq). Write the balanced molecular equation and net ionic equation.
- 2 A solution contains 0.010 mol BaCl2(aq) and is mixed with excess Na2SO4(aq). Use the reaction Ba2+(aq) + SO4^2-(aq) = BaSO4(s) to find the moles of BaSO4 precipitate formed.
- 3 Two clear solutions, K2CO3(aq) and Ca(NO3)2(aq), are mixed and a cloudy solid appears. Explain which ions form the precipitate and why the other ions remain in solution.