Chemistry 30
Separating Ions Using Solubility
Certain combinations of ions are soluble in aqueous solution as described in a chart of
qualitative solubility. Chemists can use this fact to separate ions (either positive or negative) by
what is termed ______________ _______________. This means that in order to separate two
positive ions which are dissolved in a solution a chemist would add a negative ion that would
cause one positive ion to precipitate (form a solid with the ion) and other ion not to.
Selective Precipitation
From the chart of qualitative solubility , if sulfate SO4 2- is added to Barium, Strontium or
Lead (II) ions the compound formed will be insoluble and will ______________. All other
positive ions form soluble ionic compounds with sulfate.
Sulfate SO4 2-
Ba 2+, Sr 2+, Pb 2+ --------->
Insoluble
All Other positive ions --------->
Soluble
If sulfate was added to a solution containing sodium (Na+) ions and lead (II) ions (Pb2+), it would
cause the lead (II) ions to form an insoluble compound of lead (II) sulfate. The sodium ions,
however, would ________ __________ with the sulfate. The Lead (II) sulfate could then be
filtered out and this would effectively separate these ions.
The same technique could be used for combinations of three or four positive ions, as long as
______ ______ ______ in the group is insoluble with the negative ion that is added. It is
difficult in many cases to separate two ions that have precipitated together.
______ _________ can be used to describe the separation of the ions by selective precipitation.
Chemistry 30
Example: Separating positive ions by selective precipitation
Describe how to separate the following combination of positive ions present in a solution
with nitrate ions. (NO3-)
Ca2+(aq) ; Ag +(aq) ; Zn2+(aq)
a) If we add Chlorine ions, Bromine ions or Iodide ions, Silver ions will precipitate (insoluble)
b) If we add S2- ions, the Zn2+ ions will precipitate (insoluble) and the Ca2+ ions will be soluble
Practice
Create a flow chart that illustrates how to separate the following three positive ions by
selective precipitation
Pb2+(aq) ; Ba2+(aq) ; Cu 2+(aq)
Chemistry 30
Assignment: Separating Ions by Selective Precipitation
Using the solubility table create a flow chart that describes how to separate the following ions,
if they are dissolved together in a solution. The last ion may remain in solution.
a) Ag+(aq) ; Ba2+(aq) ; Mg2+(aq)
b) Ba2+(aq) ; K+(aq) ; Zn2+(aq)
c) Pb2+(aq) ; Al3+(aq) ; Sr2+(aq)
d) Sr2+(aq) ; Mg2+(aq) ; Fe3+(aq)