Precipitation Reactions and Solubility Rules

Jul 22, 2025

Overview

This lecture covers precipitation reactions, how to predict precipitate formation, and the solubility rules used to determine product phases in chemical reactions.

Precipitation Reactions

  • A precipitation reaction occurs when two soluble salts in water react to form an insoluble solid (precipitate).
  • Example: Mixing lead(II) nitrate and potassium iodide forms lead(II) iodide (precipitate) and potassium nitrate (aqueous).
  • Dissolved ionic compounds split into their ions in solution before reaction.
  • Precipitates form when certain ions are more strongly attracted to each other than to water.

Types of Chemical Equations

  • Molecular equation: shows compounds as intact units.
  • Ionic equation: shows dissolved ionic compounds as separated ions.
  • Net ionic equation: includes only ions involved in the formation of the precipitate, omitting spectator ions.

Solubility Rules Summary

  • Most ionic compounds with a +1 cation or -1 anion are soluble.
  • Rule 1: All group 1A elements and ammonium compounds are soluble (no exceptions).
  • Rule 2: All acetates, nitrates, and perchlorates are soluble (no exceptions).
  • Rule 3: Most chlorides, bromides, and iodides are soluble except with silver, mercury(I), or lead(II).
  • Rule 4: Most sulfates are soluble except those of silver, mercury(I), lead(II), calcium, strontium, or barium.

Insolubility Rules

  • Rule 1: Most phosphates and carbonates are insoluble (except with group 1A or ammonium).
  • Rule 2: Most sulfides and hydroxides are insoluble (except with group 1A, ammonium, calcium, strontium, or barium).

Predicting Precipitation & Writing Equations

  • To predict precipitate:
    1. List the ions of the reactants.
    2. Pair the inside/outside ions to form new products.
    3. Use solubility rules to determine phases (solid = precipitate, aq = stays dissolved).
    4. Balance the overall chemical equation.
  • Practice problems reinforce identifying precipitates and balancing equations.

Laboratory & Exam Applications

  • Solubility rules help separate ions in mixtures and predict outcomes in the lab.
  • Understanding exceptions to solubility rules is crucial for exams and lab work.

Key Terms & Definitions

  • Precipitate — Solid formed in a solution during a chemical reaction.
  • Solubility — Ability of a substance to dissolve in water.
  • Spectator ion — Ion that does not participate in the chemical change of a reaction.
  • Net ionic equation — Equation showing only the ions involved in creating the precipitate.
  • Aqueous (aq) — Dissolved in water.
  • Solid (s) — Insoluble in water.

Action Items / Next Steps

  • Memorize the solubility and insolubility rules, including all exceptions.
  • Practice predicting solubility for given ionic compounds.
  • Complete assigned exercises on writing full, ionic, and net ionic equations.
  • Print and review provided solubility rules handout if available.

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