Solubility Rules for Ionic Compounds
Quick-reference solubility rules for predicting whether ionic compounds dissolve in water. Essential for precipitation reactions and net ionic equations.
| Ion | Solubility | Exceptions |
|---|---|---|
| Na+, K+, Li+, NH4+ | Always soluble | No exceptions |
| NO3- (nitrate) | Always soluble | No exceptions |
| CH3COO- (acetate) | Always soluble | Ag+ acetate is slightly soluble |
| MnO4- (permanganate) | Always soluble | No exceptions |
| ClO4- (perchlorate) | Always soluble | No exceptions |
| ClO3- (chlorate) | Always soluble | No exceptions |
| ClO2- (chlorite) | Soluble | No common exceptions |
| ClO- (hypochlorite) | Soluble | No common exceptions |
| Cr2O72- (dichromate) | Soluble | No common exceptions |
| NO2- (nitrite) | Soluble | Ag+ nitrite is slightly soluble |
| F- (fluoride) | Soluble | Insoluble with Mg2+, Ca2+, Sr2+, Ba2+, Pb2+ |
| Cl- (chloride) | Soluble | Insoluble with Ag+, Pb2+, Hg22+ |
| Br- (bromide) | Soluble | Insoluble with Ag+, Pb2+, Hg22+ |
| I- (iodide) | Soluble | Insoluble with Ag+, Pb2+, Hg22+ |
| SCN- (thiocyanate) | Soluble | Insoluble with Ag+, Cu+ |
| SO42- (sulfate) | Soluble | Insoluble with Ba2+, Pb2+, Ca2+ (slightly), Sr2+ (slightly) |
| S2O32- (thiosulfate) | Soluble | No common exceptions |
| HSO4- (hydrogen sulfate) | Soluble | No common exceptions |
| H2PO4- (dihydrogen phosphate) | Soluble | No common exceptions |
| HCO3- (hydrogen carbonate) | Soluble | No common exceptions |
| OH- (hydroxide) | Insoluble | Soluble with Na+, K+, NH4+, Ba2+, Ca2+ (slightly), Sr2+ (slightly) |
| SO32- (sulfite) | Insoluble | Soluble with Na+, K+, NH4+ |
| CO32- (carbonate) | Insoluble | Soluble with Na+, K+, NH4+ |
| PO43- (phosphate) | Insoluble | Soluble with Na+, K+, NH4+ |
| CrO42- (chromate) | Insoluble | Soluble with Na+, K+, NH4+ |
| C2O42- (oxalate) | Insoluble | Soluble with Na+, K+, NH4+ |
| S2- (sulfide) | Insoluble | Soluble with Na+, K+, NH4+, and Group 2 metals |
| CN- (cyanide) | Insoluble | Soluble with Na+, K+, NH4+ |
Important Notes
- "Soluble" means the compound dissolves to form at least a 0.1 M solution at 25°C.
- "Slightly soluble" means the compound dissolves to between 0.01 M and 0.1 M — too soluble to call insoluble, but not freely soluble. Examples: CaSO₄, AgCH₃COO, PbCl₂ in hot water.
- "Insoluble" means the compound forms less than a 0.01 M solution. Very small amounts may still dissolve (Ksp).
- When two soluble ionic compounds are mixed, a precipitate forms only if the product ion combination is insoluble.
- Acidic (hydrogen) salts are generally more soluble than their fully deprotonated counterparts: HSO₄⁻, HCO₃⁻, and H₂PO₄⁻ salts are all soluble, while SO₃²⁻, CO₃²⁻, and PO₄³⁻ are mostly insoluble. Caution: HPO₄²⁻ (hydrogen phosphate) does NOT follow this trend — CaHPO₄ and BaHPO₄ are insoluble.
- Lead(II) chloride (PbCl₂) is insoluble in cold water but slightly soluble in hot water.
- Silver acetate is slightly soluble (borderline case).
- Fluoride salts of Group 2 metals are notable exceptions to the general halide solubility trend.
- The chlorine oxyanions (ClO₄⁻, ClO₃⁻, ClO₂⁻, ClO⁻) are all soluble, derived from perchloric, chloric, chlorous, and hypochlorous acid respectively.
- SCN⁻ (thiocyanate) forms a characteristic blood-red complex with Fe³⁺, used as a qualitative test for iron(III) ions.