Solubility Rules for Ionic Compounds in Water
The standard qualitative rules predict whether a binary ionic compound dissolves in water at 25 °C. They work by looking at the ions independently: if one of the ions belongs to a soluble class with no relevant exception, the compound dissolves. If one of the ions is in an insoluble class without an exception that rescues it, the compound precipitates. The rules are operational shortcuts, not derivations from first principles. For a quantitative answer (saturation concentration, Qsp vs Ksp comparison), use the Ksp & Precipitation Checker.
Soluble ions (the compound dissolves)
These ions push a compound toward the soluble side. Read "soluble" here as "solubility above 1 g per 100 mL of water at 25 °C".
| Ion | Exceptions | Note |
|---|---|---|
| Alkali metal cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) | none | Essentially all compounds soluble; no documented exceptions for common salts. |
| Ammonium (NH₄⁺) | none | All common ammonium salts dissolve readily. |
| Nitrate (NO₃⁻) | none | All nitrates are soluble. No exceptions. |
| Acetate (CH₃COO⁻) | AgCH₃COO is slightly soluble | Most acetates are soluble. |
| Chlorate (ClO₃⁻), perchlorate (ClO₄⁻) | none | Generally soluble. KClO₄ is slightly soluble at low temperature. |
| Halides (Cl⁻, Br⁻, I⁻) | AgX, PbX₂, Hg₂X₂ (X = Cl, Br, I) are insoluble | |
| Sulfate (SO₄²⁻) | BaSO₄, PbSO₄, SrSO₄, CaSO₄ (slightly), Ag₂SO₄ (slightly) |
Insoluble ions (the compound precipitates)
These ions push the compound toward the insoluble side. The exception column lists the alkali metal and ammonium rescues that almost always apply.
| Ion | Exceptions |
|---|---|
| Hydroxide (OH⁻) | Alkali metal hydroxides and NH₄OH are soluble; Ca(OH)₂, Sr(OH)₂, Ba(OH)₂ are slightly soluble |
| Sulfide (S²⁻) | Alkali metal sulfides and (NH₄)₂S are soluble; alkaline earth sulfides hydrolyze in water |
| Carbonate (CO₃²⁻) | Alkali metal carbonates and (NH₄)₂CO₃ are soluble |
| Phosphate (PO₄³⁻) | Alkali metal phosphates and (NH₄)₃PO₄ are soluble |
| Chromate (CrO₄²⁻) | Alkali metal chromates and (NH₄)₂CrO₄ are soluble |
| Oxalate (C₂O₄²⁻), fluoride (F⁻ for most metals) | Alkali metal oxalates and fluorides are soluble |
How to apply the rules
Take the compound apart into its ions, then check each side. For AgCl, the silver is Ag⁺ (a halide-rescue exception) and the chloride is Cl⁻ (normally soluble, but Ag⁺ is in the exception list). Result: insoluble. The rule pair encodes which side wins, and silver wins for the halides.
Compound precipitation predictions use the same logic. Mix AgNO₃ with NaCl in solution: Ag⁺ and Cl⁻ are now both present. Could AgCl form? The soluble-halide rule has Ag⁺ as an exception, so yes, AgCl precipitates. Na⁺ and NO₃⁻ stay in solution (both unconditionally soluble). The net ionic equation is Ag⁺ + Cl⁻ → AgCl(s).
The rules are qualitative. They tell you precipitation will occur, not how much. For quantitative work you need Ksp values and the Ksp & Precipitation Checker. The rules also assume 25 °C. Hot water dissolves many of the "insoluble" carbonates and sulfates appreciably.
pH matters more than the rules let on. Metal hydroxides and carbonates are listed as insoluble, but they dissolve in acid because the conjugate base (OH⁻, CO₃²⁻) gets protonated away from equilibrium. CaCO₃ won't dissolve in pure water (Ksp ≈ 3.4 × 10⁻⁹) but dissolves cleanly in 1 M HCl. For pH-dependent precipitation, pair these rules with the pH Calculator.
Sources
- CRC Handbook of Chemistry and Physics, current edition (solubility data and Ksp values).
- OpenStax Chemistry 2e Chapter 11 (solubility and precipitation reactions).
- LibreTexts Chemistry for the qualitative rule formulations used in undergraduate teaching.