Solubility is difficult to predict with confidence. The most reliable way to obtain a substance’s solubility is to look it up on a table of physical properties in a reference book. When that is not possible, you can use the following guidelines for predicting whether some substances are soluble or insoluble in water. They are summarized in the table below .
- Ionic compounds with group 1 (or 1A) metallic cations or ammonium cations, NH4+, form soluble compounds no matter what the anion is.
- Ionic compounds with acetate, C2H3O2-, or nitrate, NO3-, ion form soluble compounds no matter what the cation is.
- Compounds containing the chloride, Cl-, bromide, Br-, or iodide, I-, ion are water‑soluble except with silver ions, Ag+ and lead(II) ions, Pb2+.
- Compounds containing the sulfate ion, SO42-, are water‑soluble except with barium ions, Ba2+, and lead(II) ions, Pb2+.
- Compounds containing carbonate, CO32-, phosphate, PO43-, or hydroxide, OH-, ions are insoluble in water except with group 1 metallic ions and ammonium ions.
| Category | Ions | Except with these ions | Examples |
| Soluble cations | Group 1 metallic ions and ammonium, NH4+ |
No exceptions | Na2CO3, LiOH, and (NH4)2S are soluble. |
| Soluble anions | NO3- and C2H3O2- | No exceptions | Bi(NO3)3, and Co(C2H3O2)2 are soluble. |
| Usually soluble anions |
Cl-, Br-, and I-
|
Soluble with some exceptions, including with Ag+ and Pb2+ | CuCl2 is water soluble, but AgCl is insoluble. |
| SO42- | Soluble with some exceptions, including with Ba2+ and Pb2+ | FeSO4 is water soluble, but BaSO4 is insoluble. |
|
| Usually insoluble anions | CO32-, PO43-, and OH- | Insoluble with some exceptions, including with group 1 elements and NH4+ | CaCO3, Ca3(PO4)2, and Mn(OH)2 are insoluble in water, but (NH4)2CO3, Li3PO4, and CsOH are soluble. |