A precipitation reaction happens when two aqueous ionic solutions are mixed and an insoluble solid forms. The solid is called a precipitate, and it often appears as cloudiness, crystals, or particles settling to the bottom. These reactions matter because they are used to identify ions, remove pollutants from water, make useful compounds, and understand double replacement reactions.
Predicting whether a precipitate forms depends on knowing which ion combinations are soluble or insoluble in water.
At the particle level, dissolved ionic compounds separate into mobile cations and anions surrounded by water molecules. When two ions meet that form a low-solubility compound, they leave the solution and arrange into a solid crystal lattice. The ions that stay dissolved are spectator ions, and they are removed when writing the net ionic equation.
For example, mixing AgNO3(aq) and NaCl(aq) forms AgCl(s), so the net ionic equation is Ag+(aq) + Cl-(aq) = AgCl(s).
Key Facts
- A precipitation reaction forms an insoluble solid from ions in aqueous solution.
- General form: AB(aq) + CD(aq) = AD(s) + CB(aq), if AD is insoluble.
- Complete ionic equations show strong aqueous electrolytes as separate ions.
- Spectator ions appear unchanged on both sides of the equation and are canceled.
- Net ionic example: Ag+(aq) + Cl-(aq) = AgCl(s).
- Solubility rule: most nitrates, NO3-, and alkali metal salts are soluble in water.
Vocabulary
- Precipitate
- An insoluble solid that forms when ions in solution combine during a chemical reaction.
- Aqueous solution
- A mixture in which a substance is dissolved in water, shown by the symbol aq.
- Spectator ion
- An ion that remains dissolved and unchanged during a reaction and does not appear in the net ionic equation.
- Net ionic equation
- A chemical equation that shows only the particles directly involved in forming products.
- Solubility rules
- Guidelines used to predict whether ionic compounds dissolve in water or form insoluble solids.
Common Mistakes to Avoid
- Writing all double replacement reactions as precipitations is wrong because a precipitate forms only if one product is insoluble by solubility rules.
- Forgetting to split strong aqueous electrolytes into ions is wrong because complete ionic equations must show dissolved salts, strong acids, and strong bases as separate particles.
- Canceling ions that are not identical on both sides is wrong because spectator ions must have the same formula, charge, and physical state before they can be removed.
- Labeling the precipitate as aq is wrong because the solid product must be written with s to show that it has left the solution.
Practice Questions
- 1 Predict the precipitate, if any, when 25.0 mL of 0.100 M AgNO3 is mixed with excess NaCl. Write the net ionic equation and calculate the moles of precipitate formed.
- 2 When BaCl2(aq) is mixed with Na2SO4(aq), BaSO4(s) forms. If 0.0200 mol BaCl2 reacts with 0.0150 mol Na2SO4, identify the limiting reactant and calculate the moles of BaSO4 produced.
- 3 A student mixes KNO3(aq) and NaCl(aq) and expects a precipitate because two ionic solutions were combined. Use solubility rules and particle-level reasoning to explain whether the student is correct.