Solubility of Ionic Compounds in Water
If a compound is comprised of a metal or ammonium (NH4+) cation and a nonmetal or
polyatomic ion cation, then it is an ionic compound.
All ionic compounds are strong electrolytes. This means that any molecule which
dissolves will dissociate into cations and anions. The aqueous solution will contain only
ions and no undissociated molecules. Although all ionic compounds are strong
electrolytes, not all soluble. In the case of an insoluble ionic compound, most of the
sample will be present on the bottom of the test tube or as suspended particles in the
solution. The filtered solution will contain a few ions or only water, but still no
undissociated salt.
Solubility determines how the formula of an ionic compound in water is written.
When writing a molecular equation involving an insoluble ionic compound, the
symbol (s) follows the formula, while soluble ionic compounds in solution are written
with the symbol (aq) following the formula. In a net ionic equation, the formula for an
insoluble ionic compound is still written as the undissociated molecule followed by the
symbol (s), while a soluble ionic compound is written as the dissociated ions, each
followed by the (aq) symbol.
(Note that if a strong electrolyte is not very soluble, there won’t be many ions in solution
because of the solubility, but there will me NO dissolved molecules. A solution of a
weak electrolyte will have BOTH dissolved molecules and some ions. A solution of
nonelectrolyte can have many dissolved molecules but will contain NO ions.
It is the solubility of an ionic compound (strong electrolyte) which determines whether
you write (s) or (aq) in the molecular equation, and whether you write it in undissociated
form or dissociate it into ions in the net ionic equation.
In addition solubility determines the presence of a precipitate, the yield of a reaction, the
feasibility of using filtration for separation and other important aspects of “bench
chemistry”.
Insoluble is defined as < 1 X 10 -2 mol/Liter going into solution.
Fill out the following chart by filling in the appropriate ions:
Not all ionic compounds dissolve in water. Many dissolve only to a small extent, and
still others are essentially insoluble. Fortunately, we can make some general statements
about which types of ionic compounds are water-soluble.
Guidelines to predict the solubility of ionic compounds: If a compound
contains one of the ions in the column to the left in the chart, the compound is predicted
to be at least moderately soluble in water. There are a few exceptions, and these are
noted. Most ionic compounds formed by anions listed at the bottom of the chart are
poorly soluble (with exceptions such as compounds with NH4+ and the alkali metal
cations.)
*(Note that solutions of weak electrolytes will contain both molecules and ions, while a
solution of a nonelectrolyte will contain only molecules).
SOLUBLE COMPOUNDS
EXCEPTIONS
ALL salts of group I metals
NO EXCEPTIONS - TRUMP
ALL salts of NH4+
NO EXCEPTIONS - TRUMP
ALL salts of:
NO EXCEPTIONS - TRUMP
nitrate; NO3chlorate; ClO3perchlorate; ClO4acetate; CH3CO2Salts of Cl-, Br-, and I-
Ag+, Hg22+, and Pb2+ cations
Salts of F-
Mg2+, Ca2+, Sr2+, Ba2+, Pb2+ cations
Salts of sulfate; SO42-
Sulfates of Ca2+, Sr2+, Ba2+, Pb2+,
Hg2 2+
INSOLUBLE COMPOUNDS
EXCEPTIONS
Salts of
NH4+ and alkali metal cations
carbonate; CO32phosphate; PO4-3
oxalate; C2O42chromate; CrO42sulfide; S2-
Hydroxides and oxides; OH-, O -2
NH4+, alkali metal cations and
heavy group II (Ca +2, Sr +2, Ba +2)
Name ________________________________________
Section ______________
Worksheet on Solubility
All ionic compounds are strong electrolytes. That means those molecules that dissolve
will dissociate into ions. However, not all ionic compounds dissolve, even though they
are strong electrolytes. It is the solubility that determines whether your solution is clear
or cloudy, whether you see a precipitate or not, the yield of your reaction, whether you
can filter it or not and other important aspects of “bench chemistry”.
Insoluble, does not dissolve, is defined as < 1 X 10 -2 mol/Liter go into solution.
Fill out the following chart by filling in the appropriate ions:
ANIONS
CATIONS
1. Always soluble TRUMP CARDS
1. Always soluble TRUMP CARDS
2. Usually soluble
2. Make usually soluble anions insoluble
3. Usually insoluble
3. Make hydroxides and oxides soluble
4. Make sulfates insoluble