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Predicting Solubility of Ionic Compounds in Water

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.

 

Copyright 2010 Mark Bishop markbishop@preparatorychemistry.com