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General Chemistry I · Aqueous Reactions

Precipitation and Solubility Rules

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

A precipitation reaction occurs when two aqueous solutions are mixed and an insoluble solid — the precipitate — forms from ions that combine in solution. Most such reactions are double-displacement (metathesis) reactions: the cations and anions of two dissolved ionic compounds swap partners. Whether a precipitate forms is decided by the solubility rules, a set of empirical guidelines stating which ion combinations dissolve and which do not.

Why this matters

Precipitation is a workhorse of chemistry: it is how water-treatment plants remove dissolved metals, how ores are separated, how qualitative analysis identifies unknown ions, and how pigments and nanoparticles are synthesized. The solubility rules turn a table of ions into a prediction of what solid — if any — will fall out of a mixture.

The college version

Key Ideas

The double-displacement pattern

  • General form: AB + CD → AD + CB, where A and C are cations and B and D are anions.
  • If one of the new combinations (AD or CB) is insoluble, it precipitates out.

Core solubility rules (memorize these)

  • Always soluble: compounds of Na⁺, K⁺, NH₄⁺, and NO₃⁻.
  • Usually soluble: most chlorides, bromides, and iodides — except those of Ag⁺, Pb²⁺, and Hg₂²⁺.
  • Usually soluble: most sulfates — except Ba²⁺, Sr²⁺, Pb²⁺ (and Ca²⁺, Hg₂²⁺ only slightly).
  • Usually insoluble: carbonates (CO₃²⁻), phosphates (PO₄³⁻), and hydroxides (OH⁻) — except with the always-soluble cations (Na⁺, K⁺, NH₄⁺) and, for OH⁻, the soluble group 1 and some group 2 hydroxides.

The anion-exchange logic

  • In a double displacement, each cation in the reactant mixture is "tested" against each anion present.
  • A precipitate forms when any cation–anion pairing is insoluble.

Equations and Variables

  • General double displacement: AB(aq) + CD(aq) → AD(?) + CB(?)
  • Classic example: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
  • Solubility is reported as (aq) = soluble, (s) = insoluble precipitate.

How It Works (Problem-Solving Method)

  1. Write the formulas of both reactants as aqueous ionic compounds.
  2. Swap partners to write the two possible products.
  3. Apply the solubility rules to each product — label soluble ones (aq) and insoluble ones (s).
  4. Identify the precipitate as the insoluble product (s).
  5. Balance the equation using coefficients.

Worked Example

Predict the products and the precipitate when aqueous AgNO₃ and NaCl are mixed.

  1. Reactants: AgNO₃(aq) + NaCl(aq).
  2. Swap partners: Ag⁺ pairs with Cl⁻ → AgCl; Na⁺ pairs with NO₃⁻ → NaNO₃.
  3. Solubility: AgCl is an insoluble chloride (Ag⁺ exception) → (s). NaNO₃ contains NO₃⁻ and Na⁺ → always soluble → (aq).
  4. Precipitate = AgCl.
  5. Balanced: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).

No-reaction case: Mixing NaCl and KNO₃ gives possible products KCl and NaNO₃ — both always soluble, so no precipitate forms (all ions stay in solution).

Common Confusions

  • "'Insoluble' means absolutely nothing dissolves." — It means very slightly soluble; a tiny amount always dissolves, but so little that a solid appears.
  • "Two clear solutions always react visibly." — Only if an insoluble pairing exists; NaCl + KNO₃ stays clear with no reaction.
  • "AgNO₃ is insoluble because it contains Ag⁺." — Ag⁺ makes chlorides (and bromides/iodides) insoluble, but AgNO₃ contains NO₃⁻ and is therefore soluble.
  • "NaCl and NaNO₃ are both solids." — They are aqueous (soluble); only AgCl is the solid.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine two fish tanks: one holds pairs of fish where a red fish swims with a blue fish, the other holds green fish paired with yellow fish. Pour the tanks together, and every fish gets to re-pick a partner. If a red fish and a yellow fish form a pair that "can't swim" (they sink to the bottom as a solid), you've made a precipitate. The solubility rules are the list of which fish-pairs sink. The catch: if every possible new pairing can swim fine, nothing sinks and no visible reaction happens — the ions just stay mixed in the water.

Key takeaways

  • Precipitation = formation of an insoluble solid from aqueous ions.
  • Double displacement swaps cation/anion partners.
  • Na⁺, K⁺, NH₄⁺, and NO₃⁻ salts are always soluble.
  • Ag⁺, Pb²⁺, and Hg₂²⁺ chlorides/bromides/iodides are insoluble.
  • Most carbonates, phosphates, and hydroxides are insoluble (exceptions: Na⁺, K⁺, NH₄⁺).
  • A reaction occurs only if at least one new combination is insoluble.
  • Double displacement: swap partners; look for an insoluble product.
  • Always soluble: Na⁺, K⁺, NH₄⁺, NO₃⁻.
  • Ag⁺, Pb²⁺, Hg₂²⁺ are exceptions for Cl⁻/Br⁻/I⁻.
  • Carbonates, phosphates, hydroxides mostly insoluble.
  • Precipitate = the (s) product.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Define precipitate and precipitation (double-displacement) reaction.
  • State and apply the common solubility rules to predict precipitates.
  • Predict the products of a double-displacement reaction, including physical states.
  • Identify the spectator ions in a precipitation reaction.

Sources & references

  1. Chemistry LibreTexts, "4.2: Precipitation Reactions."
  2. Purdue University, "Solubility."
  3. Chemistry LibreTexts, "7.5: Aqueous Solutions and Solubility — Compounds Dissolved in Water."

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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