Solubility Rules for Ionic Compounds in Water
| Ion or Compound Type | Solubility | Notable Exceptions |
|---|---|---|
| Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ (Group 1 cations) | Soluble | No common exceptions |
| NH₄⁺ (ammonium) | Soluble | No common exceptions |
| NO₃⁻ (nitrate) | Soluble | No common exceptions |
| C₂H₃O₂⁻ (acetate) | Soluble | AgC₂H₃O₂ is slightly soluble |
| Cl⁻ (chloride) | Soluble | AgCl, PbCl₂, Hg₂Cl₂ are insoluble |
| Br⁻ (bromide) | Soluble | AgBr, PbBr₂, Hg₂Br₂ are insoluble |
| I⁻ (iodide) | Soluble | AgI, PbI₂, Hg₂I₂ are insoluble |
| SO₄²⁻ (sulfate) | Soluble | BaSO₄, PbSO₄, SrSO₄, CaSO₄ are insoluble or slightly soluble; Ag₂SO₄ is slightly soluble |
| OH⁻ (hydroxide) | Insoluble | Group 1 hydroxides and Ba(OH)₂ are soluble; Ca(OH)₂ and Sr(OH)₂ are slightly soluble |
| S²⁻ (sulfide) | Insoluble | Group 1 sulfides, (NH₄)₂S, CaS, BaS, and SrS are soluble |
| CO₃²⁻ (carbonate) | Insoluble | Group 1 carbonates and (NH₄)₂CO₃ are soluble |
| PO₄³⁻ (phosphate) | Insoluble | Group 1 phosphates and (NH₄)₃PO₄ are soluble |
| CrO₄²⁻ (chromate) | Insoluble | Group 1 chromates, (NH₄)₂CrO₄, and MgCrO₄ are soluble |
| C₂O₄²⁻ (oxalate) | Insoluble | Group 1 oxalates and (NH₄)₂C₂O₄ are soluble |
Rules apply at roughly 25 °C and 1 atm. The thresholds are approximate: 'soluble' means more than 0.1 mol/L dissolves, 'slightly soluble' lands between 0.01 and 0.1 mol/L, and 'insoluble' means less than 0.01 mol/L. Even 'insoluble' compounds dissolve to a tiny extent set by their solubility product (Ksp) — the rules are a qualitative screen, not a substitute for Ksp data. Rules are listed in priority order: when two conflict, the earlier rule wins (e.g., NaOH is soluble because Group 1 overrides the hydroxide rule). For quantitative work, use Ksp values from the CRC Handbook.
Frequently Asked Questions
How do you use solubility rules to predict precipitates?
Swap the cations between the two reactants to write the possible products, then check each against the rules. If a product matches an insoluble rule and isn't covered by an exception, it precipitates. For NaCl(aq) + AgNO₃(aq), the swap gives AgCl and NaNO₃. Nitrate is always soluble, so NaNO₃ stays in solution. Chloride is soluble except with Ag⁺, Pb²⁺, and Hg₂²⁺ — AgCl falls under that exception and drops out as a white precipitate.
Why are all nitrates soluble in water?
The NO₃⁻ ion is large and singly charged, which keeps lattice energies in nitrate crystals relatively low. At the same time, the polyatomic geometry lets water molecules hydrate the ion effectively. The hydration enthalpy comfortably exceeds the lattice energy for every common nitrate salt, so dissolution is thermodynamically favorable across the board. This is why nitrates show up so often in qualitative analysis as the 'spectator' counterion — they never precipitate out.
What is the difference between insoluble and slightly soluble?
The convention in introductory chemistry: 'insoluble' means less than 0.01 mol/L dissolves at 25 °C, 'slightly soluble' covers 0.01–0.1 mol/L, and 'soluble' is anything above. CaSO₄ and Ca(OH)₂ sit in the slightly-soluble band — they cloud a solution but don't drop a sharp precipitate. Nothing is truly insoluble; the dissolved fraction is set by Ksp. For AgCl (Ksp ≈ 1.8 × 10⁻¹⁰), that fraction is about 1.3 × 10⁻⁵ M — small enough to call insoluble for stoichiometric work.