Solubility is the extend to which a substance dissolves in a solvent to make a solution. Most of the time we are talking about substances dissolving in water to form an aqueous solution. Obviously, not everything dissolves in water, and some reactions even result in a precipitate, which is an insoluble product formed during the reaction. In order to know what dissolves and what does not dissolve, there are rules which we can follow.
Here is a list of solubility rules for ionic compounds in water:
1. Most nitrate (NO3–) and acetate (C2H3O2–) salts are soluble.
2. Most salts of alkali metals (Li, Na, K, Cs, Rb) and ammonium (NH4+) cations are soluble.
3. Most chloride, bromide, and iodide salts are soluble.
Exceptions: Salts containing Ag+, Pb2+, and Hg22+ ions are insoluble.
4. Most sulfate (SO42-) salts are soluble.
Exceptions: Sulfates containing Ag+, Ca2+, Ba2+, Pb2+ and Hg22+ ions are insoluble.
5. Most hydroxide (OH–) salts are insoluble.
Exceptions: Hydroxides containing alkali metals are soluble. Hydroxides containing Ba2+, Sr2+ and Ca2+ ions are marginally soluble.
6. Most sulfide, carbonate (CO32-), chromate (CrO42-), and phosphate (PO43-) salts are insoluble.
Exceptions: Salts of alkali metals and ammonium cations are soluble.
Note: Click below if you would like to print the solubility rules above as a reference sheet. Solubility Rules Reference Sheet
By following the rules above, it is easy to determine if an ionic compound will dissolve in water or not. Here are a few examples:
Magnesium sulfate will dissolve in water. According to rule 4, most compounds containing sulfates are soluble, and since magnesium is not listed as an exception to this rule we can assume that it will dissolve.
Sodium phosphate will dissolve in water. According to rule 2, most salts containing alkali metals are soluble, so since sodium is an alkali metal this compound will dissolve. Rule 6 tells us that most phosphates are insoluble, but because in this case phosphate is combined with the alkali metal sodium this makes the compound soluble.
Lead (II) chloride will not dissolve in water. According to rule 3, most chloride containing compounds are soluble. However, if we look at the exceptions we see that lead (II) is listed as an insoluble exception.
Click here to go to a practice worksheet about solubility rules