Unit 7.1 Predicting Precipitates Using Solubility Rules
Will it rain? Predicting the weather is tricky business. A thorough examination
of a large amount of data is needed to make the daily forecast. Wind patterns,
historical data, barometric pressure – these and many other data are fed into
computers that then use a set of rules to predict what will happen based on
past history.
Predicting Precipitates Using Solubility Rules
Some combinations of aqueous reactants result in the formation of a solid precipitate as a product. A
precipitate is a solid formed when two solutions are mixed with an insoluble compound produced. Any ions
that do not react to form a solid precipitate are called spectator ions. They are like the spectators at a
ball game. They watch, but do not participate. It is useful to be able to predict when a precipitate will
occur in a reaction. To do so, you can use a set of guidelines called the solubility rules of common
compounds in water.
Rule
1
Negative Ions (anions)
Positive Ions (cations)
Solubility
Alkali ions (Li+1, Na+1, K+1, Rb+1, Cs+1)
Essentially all
+1
Soluble
2
Essentially all
Hydrogen ions (H )
Soluble
3
Essentially all
Ammonium ion (NH4+1)
Soluble
Essentially all
Soluble
Essentially all
Soluble
Ag+1, Pb=2, Hg2+2, Cu+1
Low Solubility
All others
Soluble
Ca+2, Sr+2, Ba+2, Pb+2, Ra+2. Ag+1
Low Solubility
4
_1
Nitrate (NO3 )
-1
-1
5
Acetate (C2H3O2
6
Chloride Cl-1, Bromide Br-1, Iodide I-1
7
or CH3COO )
Sulfate SO4-2
All others
8
Sulfide S
-2
Soluble
+1
+1
+2
+2
Alkali ions, H , NH4 , Be , Mg ,
Soluble
Ca+2, Sr+2, Ba+2, Ra+2
9
10
Hydroxide OH-1
-3
-2
Phosphate PO4 , Carbonate CO3 ,
Sulfite SO3-2
11
Chromate CrO4-2, Silicate SiO3-2
All others
Low Solubility
Alkali ions, H+1, NH4+1, Sr+2, Ba+2, Ra+2
Soluble
All others
Low Solubility
+1
+1
Alkali ions, H , NH4
Soluble
All others
Low Solubility
Alkali ions, H+1, NH4+1
Soluble
All others
Low Solubility
Soluble is a term meaning that the compound formed would dissolve in water and be found as ions. Low
Solubility (or insoluble) means that a precipitates (solid) would form. A few molecules may dissolve, but
very few. Most of the ions would ionically bond and settle out of the solution as a solid.
However, some combinations will not produce such a product. If solutions of sodium nitrate and
ammonium chloride are mixed, no reaction occurs. One could write a molecular equation showing a doublereplacement reaction, but both products, sodium chloride and ammonium nitrate, are soluble and would
remain in the solution as ions. Every ion is a spectator ion and there is not net ionic equation at all.
As an example on how to use the solubility rules, predict if a precipitate will form when solutions of
cesium bromide (CsBr) and lead (II) nitrate (Pb(NO3)2) are mixed.
CsBr (aq)
+
-1
+2
Pb(NO3)2 (aq)

?
The reactants actually
exist as ions in solution.
+1
Cs
(aq)
+ Br
(aq)
+ Pb
(aq)
-1
+ 2 NO3
(aq)
 ?
The potential precipitates from a double-replacement reaction are cesium nitrate and lead (II) bromide.
According to the solubility rules table, cesium nitrate is soluble because all compounds containing the
nitrate ion (rule 4), as well as all compounds containing the alkali metal ions (rule 1), are soluble. Most
compounds containing the bromide ion are soluble, but lead (II) is an exception (top half of rule 6).
Therefore, the cesium and nitrate ions are spectator ions and the lead (II) bromide is a precipitate. The
balanced net ionic equation is:
Pb+2 (aq)
+
2 Br-1(aq)

PbBr2 (s)
Spectator ions are not shown in net ionic equation.
Practice Problem: Predict the Precipitates Formed
Four solutions are available to mix. What precipitates can be formed by mixing 2 of the solutions at a
time? Once precipitates have been identified, write the net ionic equations representing each possible
precipitate. The solutions available are:
Sodium chloride
NaCl
Silver nitrate
AgNO3
Potassium sulfate
Barium acetate
K2SO4
Ba(C2H3O2)2
Step 1: Plan the Problem
Determine the ions that are found in each solution
Check each possible combination of ions with the Solubility of Commons Compounds Chart
Step 2: Solve
Ions found in the solutions are:
NaCl
Na+1
AgNO3
Cl-1
Ag+1
K2SO4
NO3-1
K+1
SO4-2
Ba(C2H3O2)2
Ba+2
C2H3O2-1
Check all possible combinations with the Solubility of Commons Compounds Chart and write net ionic
equations to show the formation of the precipitates.
Na+1
Cl-1
Ag+1
NO3-1
K+1
SO4-2
Ba+2
C2H3O2-1
Both Na+1 and K+1 are alkali ions which according to rule 1 are essentially all soluble.
NO3-1 is nitrate which according to rule 4 is essentially all soluble.
C2H3O2-1 is acetate which according to rule 5 is essentially all soluble.
The following includes all possibilities using other rules, and all with low solubility.
Na+1
+
SO4-2
Ag+1
+
Cl-1
2 Ag+1
+
K+1
Cl-1
Ba+2
+
+
SO4-2
Cl-1

soluble (rule 1 and bottom half of 7)

AgCl (s)
low solubility

Ag2SO4 (s)
low solubility

soluble (rule 1 and 6)

soluble (bottom half of rule 6)
Two precipitates could be formed using the solutions.
Summary

Solubility rules allow prediction of what products will be insoluble in water.

Soluble means the combination of ions will remain dissolved in water.

Low solubility or insoluble means the combination will ionically bond and settle out of the solution
as a precipitate.
Review
1.
Are all alkali metal salts soluble?
2.
What chloride salts are not soluble?
3.
Would you expect sodium silicate to dissolve?
4.
Is lead (II) sulfate soluble?
5.
Which of the following combinations would form a precipitate?
a.
Pb+2 and I-1
b.
Al+3 and SO4-2
c.
Rb+1 and OH-1
d.
Ca+2 and PO4-3
e.
NH4+1 and CrO4-2
Answers
1.
Yes, all alkali metal compounds are soluble.
2.
Chloride salts of Ag+1, Hg2+2, and Pb+2 are low solubility.
3.
Sodium silicate has an alkali metal, Na+1 in it. It will dissolve.
4.
Lead (II) sulfate is not soluble.
5.
Which of the following combinations would form a precipitate?
a. Precipitate
Ag+ and Br-1
b.
Soluble
Al+3 and SO4-2
c.
Soluble
Rb+1 and OH-1
d.
Precipitate
Ca+2 and PO4-3
e.
Soluble
NH4+1 and CrO4-2