Solubility
Lesson 6
Separating Ions
Positive ions react with negative ions to give a precipitate if they have low
solubility.
A precipitate can be separated from a solution by filtration. The precipitate will be
on the filter paper while the soluble ions will go through.
To separate ions one ion must be low solubility and all others high.
Separating Positive Ions
1.
Separate
Ba2+
&
Start at the top of page 4 and try Cl- first
Pb2+
Ba2+
1st add Cl-, it reacts with Pb2+ (low) and not Ba2+ (high)
2nd add SO42-, it reacts with Ba2+ (low)
NaCl
Cl-
Na+
Pb2+
Ba2+
PbCl2
Filter
NO3-
Separating Positive Ions
Ba2+
&
Pb2+
1.
Separate
i.
Add
NaCl
Pb2+
+
2Cl-
ii.
Add
Na2SO4 Filter out BaSO4(s)
Ba2+
+
SO42-
Filter out PbCl2(s)


PbCl2(s)
BaSO4(s)
Cl- does not exist
on its own
Add NaCl because
it is soluble- always
use Na+ for negative
ions
Separating Positive Ions
2.
Separate Cu2+,
Mg2+
&
Sr2+
Cu2+
Cu2+
Cu2+
Mg2+
Mg2+
Mg2+
Sr2+
Separating Positive Ions
2.
Separate Cu2+,
i.
Add
Na2SO4 Filter out SrSO4(s)
Sr2+
+
Add
Na2S
Cu2+
+
Add
NaOH
Mg2+
+
ii.
iii.
Mg2+
SO42-
&

Sr2+
SrSO4(s)
Filter out CuS(s)
S2-

CuS(s)
Filter out Mg(OH)2(s)
2OH-

Mg(OH)2(s)
SO42- does not exist
on its own
Add Na2SO4 because
it is soluble- always
use Na+ for negative
ions
Separating Negative Ions
Remove the lowest negative ion on the chart first by adding a positive cation.
3.
Separate
Cl-
&
OH-
Look for a cation that is low with OH- (lowest) and high with Cl-.
Ba2+
Look for a cation that is low with
with Cl-
Look for a cation that is low with
OH- (lowest) and high with Cl-.
Ba2+
Remove the lowest negative ion
first by adding a positive cation.
Separating Negative Ions
Cl-
Separate
i.
Ba2+ works so add Ba(NO3)2 and filter out Ba(OH)2(s)
Ba2+
ii.
2OH-
+
&
OH-
3.

Ba(OH)2(s)
Add Ag+ as AgNO3 and filter out AgCl(s)
Ag+
+
Cl-

AgCl(s)
Ba2+ does not exist
on its own
Add Ba(NO3)2 because
it is soluble- always
use NO3- to pair with
positive ions
Separating Negative Ions
4.
Separate
Cl-
S2-
CO32-
Look for a cation that is low with
with Cl-
Look for a cation that is low with
S2- and high with ClZn2+
Look for a cation that is low with
CO2- and high with Cl- and S2Ba2+
Separating Negative Ions
Cl-
S2-
CO32-
4.
Separate
i.
Ba2+ works so add Ba(NO3)2 and filter out BaCO3(s)
Ba2+
ii.
CO32-

BaCO3(s)
Add Zn2+ as Zn(NO3)2 and filter out ZnS(s)
Zn2+
iii.
+
+
S2-

ZnS(s)
Add Ag+ as AgNO3 and filter out AgCl(s)
Ag+
+
Cl-

AgCl(s)
Ba2+ does not exist
on its own
Add Ba(NO3)2 because
it is soluble- always
use NO3- to pair with
positive ions