Identifing unknown ions in solution
This standard is primarily about being able to carry out practical procedures using the identification flow charts provided to identify unknown ions.
NB: The flow charts are provided during your internal, however, the ionic equations are not.
How do the flow charts work?
If you added 2 drops of NaOH to your unknown solution and a white precipitate formed, you would then add excess NaOH. If the precipitate remained in excess, you would get a new sample of your unknown solution and add dilute H2SO4. If the solution was colourless, you can conclude the unknown solution contained Mg2+ ions but if a white precipitate formed, you can conclude that it contained Ba2+ ions.
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Recording observations
To record our observations, we set up a simple table and make sure we rule off after each procedure to keep the information linked.
Example 1: we have a colourless solution and suspect magnesium ions are present.
NB: Please note, the charges of all ions should be written as superscripts. Due to the formatting on this page, they are not.
NB: Please note, the charges of all ions should be written as superscripts. Due to the formatting on this page, they are not.
Description of procedure |
Observations |
Name and formula of precipitate |
Complex ion formation |
Balanced ionic equation |
Add 2 drops of NaOH |
A white precipitate forms |
Magnesium hydroxide Mg(OH)2 |
- |
Mg2+(aq) + 2OH-(aq) -> Mg(OH)2(s) |
Add excess NaOH |
The white precipitate remains |
- |
- |
- |
To a new sample add dilute H2SO4 |
The solution stays colourless |
- |
- |
- |
Example 2: we have a colourless solution and suspect lead ions are present.
NB: Please note, the charges of all ions should be written as superscripts. Due to the formatting on this page, they are not.
NB: Please note, the charges of all ions should be written as superscripts. Due to the formatting on this page, they are not.
Description of procedure |
Observations |
Name and formula of precipitate |
Complex ion formation |
Balanced ionic equation |
Add 2 drops of NaOH |
A white precipitate forms |
Lead hydroxide Pb(OH)2 |
- |
Pb2+(aq) + 2OH-(aq) -> Pb(OH)2(s) |
Add excess NaOH |
The white precipitate disapears to form a colourless solution |
- |
[Pb(OH)4]2- |
Pb(OH)2(s) + 2OH-(aq) -> [Pb(OH)4]2-(aq) |
To a new sample add 2 drops of NH3 |
A white precipitate forms |
Lead hydroxide Pb(OH)2 |
- |
Pb2+(aq) + 2OH-(aq) -> Pb(OH)2(s) |
Add excess NH3 |
The precipitate remains |
- |
- |
- |
To a new sample add dilute H2SO4 |
A white precipitate forms |
PbSO4(s) |
- |
Pb2+(aq) + SO42-(aq) -> PbSO4(s) |
Example 3: For a solution containing carbonate ions.
NB: Please note, the charges of all ions should be written as superscripts. Due to the formatting on this page, they are not.
NB: Please note, the charges of all ions should be written as superscripts. Due to the formatting on this page, they are not.
Description of procedure |
Observations |
Name and formula of precipitate |
Complex ion formation |
Balanced ionic equation |
Add red litmus paper |
Litmus paper stays red |
- |
- |
- |
Add dilute HCl |
Bubbles are observed |
- |
- |
2H+(aq) + CO32-(aq) -> CO2(g) + H2O(l) |
Example 4: For a solution containing chloride ions
NB: Please note, the charges of all ions should be written as superscripts. Due to the formatting on this page, they are not.
NB: Please note, the charges of all ions should be written as superscripts. Due to the formatting on this page, they are not.
Description of procedure |
Observations |
Name and formula of precipitate |
Complex ion formation |
Balanced ionic equation |
Add red litmus paper |
Litmus paper stays red |
- |
- |
- |
To a new sample, add AgNO3 solution |
A white precipitate forms |
Silver chloride AgCl |
- |
Ag+(aq) + Cl-(aq) -> AgCl(s) |
Add dilute NH3 |
The precipitate dissolves and a colourless solution forms |
- |
[Ag(NH3)2]+ |
Ag+(aq) + 2NH3(aq) -> [Ag(NH3)2]+ (aq) |