Chem 123
REVIEW OF SOLUBILITY RULES
page 1
Know these solubility rules WELL, including the exceptions listed below. You will NOT be provided
with this table at your exams.
There is a few other exceptions not mentioned here but you are not held responsible for the exceptions
not mentioned here.
FIRST Note: All ionic compounds are solids when not dissolved in water.
Soluble Compounds
1. All compounds of the alkali metals (Group IA) are soluble. Most common is Na+.
2. All salts containing NH4+, NO3, ClO4, ClO3, and C2H3O2 are soluble.
3. All chlorides, bromides and iodides (salts containing Cl, Bror I) are soluble
except when combined with Ag+, Pb2+, and Hg22+ (mercury(I) ion)
4. All sulfates (salts containing SO42) are soluble
except those of Ca2+, Sr2+, Ba2+, Pb2+ and Hg22+
Insoluble Compounds
5. All hydroxides and oxides* are insoluble
except those of NH4+ **, Group IA and Ca2+, Sr2+ and Ba2+
6. Assume all other ionic compounds are insoluble unless specified otherwise.
Special reactions you should remember:
I. Reactions of Oxides with Water:
*Metal oxides react with water to form hydroxides:
e.g. Na2O (s) + H OH (l)  2 NaOH (aq)
e.g. MgO (s) + H2O (l)  Mg(OH)2 (s)
**NH4OH (aq) is soluble but most of it converts to NH3 (g) + H2O (l)
Nonmetal oxides react with water to form acids:
e.g. SO3 (g) + H2O (l)  H2SO4 (aq)
e.g. NO2 (g) + H2O (l)  HNO3 (aq)
II. Gas-producing reactions:
These are gas-producing reactions you should know well:
NH4OH (aq)
NH3 (g) + H2O (l)
H2CO3 (g)
CO2 (g) + H2O (l)
H2SO3 (aq)
H2O (l) + SO2 (g)
This means that if your product in your equation is one of the above…
(NH4OH,
or H2CO3
or H2SO3)
it should be replaced with the formulas…
(NH3 + H2O
H2O + CO2
H2O + SO2)
Example of how you should be able to apply these solubility rules:
Which of the following is soluble in water?
a) CoCl2 b) Ba(C2H3O2)2
c) Cr(OH)3
d) (NH4)2CrO4 e) HgCl2
ANS. (a), (b), (d), (e) only
f) PbSO4
(See page 2 for practice on dissociation of ionic compounds)
Chem 121
Which Formulas Do We Dissociate Into Ionic form?
page 2
These are at Dr. Yau’s HomePage:
 Solubility Rules you should know well
 Practice Questions Involving Solubility Rules of Ionic Compounds
 How to Write Ionic Equations (A tutorial with practice questions and answers)
The rule to decide which formulas to dissociate into ionic form is very simple. You separate a formula
into cations and anions IF AND ONLY IF:
1) Substance has the physical state (aq)
and
2) it is either an ionic compound or a strong acid.
There are only 7 common strong acids. Memorize this list:
HNO3 H2SO4 HClO4 HClO3 HCl HBr HI
You can assume all other acids are “weak acids.”
For each of the following substances, if it fits the rule, write the formulas of the ions and specify how
many of each ion into which it dissociates. The first one has been done for you as an example:
1. Na2SO4 (aq)
2Na+ (aq) + SO42− (aq)
2. Mg(NO3)2 (aq) _______________________________
3. 3K3PO4 (aq)
_______________________________
4. 2HI (aq)
_______________________________
5. 2NaHCO3 (aq) _______________________________
6. CH3OH (aq)
_______________________________
7. MgCl2 (s)
_______________________________
8. H2O (l)
_______________________________
9. 2(NH4)2SO4 (aq) _______________________________
10. CO2 (g)
_______________________________
11. 3Al2(SO4)3 (aq) _______________________________
12. N2H4 (aq)
_______________________________
13. HClO4 (aq)
_______________________________
14. HC2H3O2 (aq)
_______________________________
Ans. (2) Mg2+(aq) + 2NO3 (3) 9K+(aq) +3PO43 (aq); (4)2H+(aq)+2I(aq) ;
(5) 2Na+(aq) +2HCO3(aq); (6) CH3OH (aq) ; (7) MgCl2 (s); (8) H2O (l); (9) 4NH4+(aq) +2SO42 (aq)
(10) CO2 (g); (11) 6Al3+(aq) + 9SO42 (aq) ; (12) N2H4 (aq)
(13) H+ (aq) + ClO4 (aq); (14) HC2H3O2 (aq)