Brief Introductory Discussion
These rules are general and qualitative in nature. Each rule has exceptions and the important ones are noted.
The most common definition for solubility is this:
1) Soluble substances can form a 0.10-molar solution at 25 °C.
2) Insoluble substances cannot form a 0.10-molar solution at 25 °C.
This definition means there are only two categories: soluble and insoluble.
However, there are other definitions of solubility since a third term called "slightly soluble" is one that some in chemistry prefer to use. Indeed, I found one website with four terms.
This website has definitions for soluble, insoluble and slightly soluble plus a good set of rules for solubility at the end of the file. The most important slightly soluble substance is calcium hydroxide, Ca(OH)2.
The Solubility Table
|1. All nitrate (NO3¯), nitrite (NO2¯), chlorate (ClO3¯) and perchlorate (ClO4¯) salts are soluble.||Silver nitrite and potassium perchlorate are considered slightly soluble.|
|2. Essentially, all alkali metal (Li+, Na+, K+, Rb+, Cs+) and ammonium (NH4+) salts are soluble.||Some Li+ are insoluble, with Li3PO4 being the most common example.|
|3. Most halogen (Cl¯, Br¯, I¯) salts are soluble.||Ag+, Pb2+, Hg22+, Cu+, Tl+ (Pb2+ halogens are soluble in hot water.) HgBr2 is slightly soluble.|
|4. Most acetate (C2H3O2¯) salts are soluble.||Ag+, Hg22+|
|5. Most sulfate (SO42¯) salts are soluble.||Ca2+, Sr2+, Ba2+, Ra2+, Pb2+, Ag+, Hg2+ (Some sources consider calcium sulfate and silver sulfate to be slightly soluble.)|
|6. Many sulfides (S2¯) are insoluble.||All alkali metal and alkaline earth (Be2+, Mg2+, Ca2+, Sr2+, Ba2+, Ra2+) sulfides are soluble. Ammonium sulfide is soluble. (Some sources consider MgS, CaS and BaS to be slightly soluble.)|
|7. Most borates (BO32¯), carbonates (CO32¯), chromates (CrO42¯), phosphates (PO43¯), and sulfites (SO32¯) are slightly soluble.||MgCrO4 is soluble, MgSO3 is slightly soluble.|
|7. Most hydroxide (OH¯) salts are insoluble||Alkali metal hydroxides are soluble.|
Ba2+, Sr2+, Ca2+, Tl+ are considered slightly soluble.
Note concerning #7 just above: Some older sources might include ammonium hydroxide (NH4OH) as a soluble hydroxide. This substance does not exist; it cannot be isolated as a pure substance. When ammonia gas (NH3) is bubbled through water, it dissolves and a small number of ammonia molecules react with the water to produce NH4+ and OH¯ ions. However, most of the NH3 molecules remain as NH3. There is no such thing as NH4OH as a separate substance which can be isolated in pure form.
Other solubility tables can be seen via this solubility table search.
Keep in mind that there will be slight variations from table to table. In the final analysis, the table your teacher wants you to use is the most correct one for you to use.