ChemWhiz ReferenceSolutions
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.
01Use this when
Predicting whether an ionic product precipitates in a double-replacement or net-ionic reaction.
02Conditions
In water at ~25 °C; “soluble” means roughly 0.1 M or more.
03Watch out
Check the exceptions column before deciding (AgCl, BaSO4, and PbI2 are classic insoluble exceptions).
Solubility Rules in Water
Read the general rule for each ion, then check the listed exceptions before predicting a precipitate.
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| 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" commonly means the compound forms at least about a 0.1 M solution at 25 °C (an operational convention, not a sharp cutoff).
- "Slightly soluble" means roughly 0.01 M to 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.
Verification references: OpenStax Chemistry 2e, Section 4.2 (Table 4.1, solubility rules for common ionic compounds in water).