Mr. Chapman
Chemistry 30
Norquay School
Qualitative Solubility
The solubility of a compound is the maximum amount of that substance that will dissolve in a
given amount of solute. The concentration can be
expressed in any of the measures previously used in
calculating concentrations. When solutions reach this
point they are said to be saturated. Although we have
not covered the topic in this class yet, saturated
solutions are at equilibrium. This means that the
forward reaction (the dissolving solid) and the reverse reaction (the dissolved ions forming a
solid) are happening at the same rate. More on this will follow later in the course.
Solubility Rules
The solubility of a substance is often given a relative reference. We may say a substance is
soluble or it is insoluble. No substance in fact is either perfectly soluble or insoluble. Chemists
generally agree that insoluble means that the solubility of the solid is less than 1 gram per liter.
Some combinations of ions (ionic solids) have differing degrees of attraction for each other and
therefore are not all equal in their abilities to dissolve.
The following are examples of ionic solids:

NaCl

PbSO4

BaNO3
Notice that ionic solids are simply ionic
compounds, and that ionic solid is
simply an interchangeable term.
Mr. Chapman
Chemistry 30
Norquay School
Some combinations of ions (ionic solids) have differing degrees of attraction for each other and
therefore are not all equal in their abilities to dissolve. Chemists unfortunately have no reliable
way of predicting theoretically what combination of ions (ionic compound) will have greater or
lesser attractions and therefore be soluble or insoluble. The more attracted ions are to each
other, the less soluble they will be. Chemists therefore through experimentation, have
determined certain patterns in the solubility of various ionic substances.
Key Idea: Some combinations of ions are more soluble than
others in water.
What happens to the compound when it dissolves in water?
When an ionic compound (remember that ionic compounds are compounds formed by a metal
and a non-metal) dissolves in water, it is broken down into its constituent ions (the ions that it
is formed by).
Examples:
BaSO4 (s)  Ba2+(aq) + SO42-(aq)
This is known as a dissociation equation.
Write dissociation equations for each of the following ionic compounds:
i)
NaCl
ii)
FeI2
iii)
AgNO3
iv)
Mg3(PO4)2
Mr. Chapman
Chemistry 30
Norquay School
Predicting Solubility
Since chemists cannot predict the solubility of a particular ionic compound by analyzing what it
is made out of, they have had to experimentally determine what compounds are soluble and
which ones are not soluble (or insoluble).
The rules (as to which compounds are soluble and which aren’t) are summarized into the form
of a table that can be used to predict the relative solubility of most common ionic compounds.
To use the Table of Qualitative Solubility (Solubility Rules):
Combine Negative ion in column A with positive ion in column B read across table to
determine solubility.
Example: CaSO4




Find SO4 2- : read across,
Ca 2+ ion is not part of first group,
therefore part of All other positive ions
therefore soluble
Chemists can use a table like the one above to predict the solubility of many different
combinations of ions. The assignment below gives you some practice in using the chart:
Using solubility rules
Write out the formula for each of the following ionic compounds ant then using the table of
relative solubility, predict the solubility of the following ionic compounds:
a) Silver chloride
b) Ammonium carbonate
c) Lead (II) sulfate
d) Copper (I) Iodide
e) Sodium phosphate
f) Calcium sulfide
g) Magnesium nitrate
h) Potassium sulfite
i) Barium Hydroxide
j) Iron (III) carbonate
Mr. Chapman
Chemistry 30
Norquay School
A Quick but Important Note: The Common Ion Effect
The solubility of an ionic compound is also affected by changes in the concentrations of the
ions in solution.
Hypothetical Situation:
Imagine that you have a saturated solution of NaCl in water. Before proceeding, write in your
own words below what that means:
______________________________________________________________________________
The ions present in the solution currently are Na+ and Cl-, because these are the constituent
ions of NaCl.
Now imagine that you add to the saturated solution of NaCl a little bit of hydrochloric acid, HCl.
In the space below, write down what you think would happen to the solution, given that it was
already saturated with NaCl:
______________________________________________________________________________
______________________________________________________________________________
Mr. Chapman
Chemistry 30
Norquay School
Common Ion Effect:
In a saturated solution the addition of one of the ions present in the equilibrium lowers the
solubility of the solid. In other word it causes some of the solid to precipitate.
In the demonstration above the addition of Cl- ions causes NaCl (s) to precipitate. Examine the
equation below
NaCl (s)
Na +(aq) + Cl -(aq)
In the future, we will learn about the concept of chemical equilibrium, and LeChatelier’s
Principle. We will find that the addition of ions present in the product side of the equation
favors the reactant side of the equation. In other words, the chemical equilibrium will shift
backwards.
For now, we can summarize the common ion effect as follows:
The Common Ion Effect:
The presence of ions in solution common to those that make up the solid dissolved in solution
will decrease the solubility of the solid. As a result, adding common ions to a saturated solution
will cause some of the solid to come out of solution.