Figuring out which cations and anions we might have in an unknown aqueous solution is a form of qualitative analysis. The observations we might make whilst carrying out these chemical tests include effervescence / bubbles, changes in the colour of a solution (including colourless), formation of precipitates, colour changes to litmus paper etc.
Cations
You can find out how to test for transition metal ions in this post.
1. Flame tests
We can identify some common metal cations using a flame test (which is a practical demonstration of atomic emission spectroscopy).
Essentially we take a nichrome or platinum wire, dip it into concentrated hydrochloric acid and then place into a blue Bunsen flame until it burns clean. We dip the hot wire back into the acid and into a solid sample of a compound containing the metal cation, for example lithium chloride or copper chloride, and then place back into the flame.
Electrons are promoted to higher energy levels by absorbing heat energy. When the electrons drop back to lower energy levels they emit a specific frequency of radiation that corresponds to the difference in energy between the two levels (ΔE = h𝛎). If that specific frequency falls within the visible range of the electromagnetic spectrum we see a coloured flame.
| Element | Flame colour |
|---|---|
| Li | red |
| Na | yellow-orange |
| K | lilac |
| Rb | red-violet |
| Ca | orange-red |
| Sr | red |
| Ba | pale green |
| Cu | blue-green |
2. Testing for ammonium ions, NH4+
If we add aqueous sodium hydroxide to aqueous ammonium ions, ammonia gas is formed which invariably dissolves in the solution. We can warm the test tube to release the gas which will turn damp red litmus paper blue because ammonia is strongly alkaline.
NH4+(aq) + OH–(aq) ⇾ NH3(g) + H2O(l)
Anions
1. Testing for carbonate ions, CO32-
Carbonate ions react with acids giving off bubbles of carbon dioxide.
K2CO3(aq) + 2HNO3(aq) ⇾ 2KNO3(aq) + CO2(g) + H2O(l)
Examiners are very keen on asking you for ionic equations …
CO32- (aq) + 2H+(aq) ⇾ CO2(g) + H2O(l)
The observation here is that we will see bubbles. To prove that the gas is carbon dioxide we would need to bubble it through limewater (saturated aqueous calcium hydroxide) which reacts with the CO2 to form a fine cloudy white precipitate (calcium carbonate).
CO2(g) + Ca(OH)2(aq) ⇾ CaCO3(s) + H2O(l)
We always use nitric acid for this test because if we use hydrochloric acid or sulphuric acid there will be chloride ions or sulphate ions left in solution which would interfere with further tests for either of these two ions.
2. Testing for sulfate ions, SO42-
Sulphate ions react with aqueous barium ions, Ba2+ in the form of either BaCl2(aq) or Ba(NO3)2(aq), to give a white precipitate of barium sulphate. We always acidify the solution first with nitric acid to remove any carbonate ions that might be present as barium carbonate is also a white precipitate.
Ba2+(aq) + SO42-(aq) ⇾ BaSO4(s)
3. Testig for halide ions, Cl–, Br–, I–
Halide ions react with aqueous silver ions, Ag+ in the form of AgNO3(aq), to give precipitates. Sliver chloride is a fine white precipitate, silver bromide is a cream precipitate and silver iodide is a yellow precipitate.
Ag+(aq) + Br–(aq) ⇾ AgBr(s)
In reality though, the precipitates are often very faint and it’s difficult to tell them apart so we add ammonia dropwise to the precipitates.
Silver chloride is soluble in aqueous ammonia dissolving to form a colourless solution.
Silver bromide is soluble in concentrated ammonia dissolving to form a colourless solution.
Silver iodide is insoluble in both aqueous and concentrated ammonia.
Once again, we acidify the solution first with nitric acid to remove carbonate ions as silver carbonate is a white insoluble precipitate and will mask any silver halide precipitates formed.
Silver sulphate is also an insoluble white precipitate so sulphate ions must be removed by the addition of Ba2+(aq). Any BaSO4 formed can be filtered off before we test for the presence of halide ions.
Practice questions
- A student carries out a series of test tube reactions on solutions of iron(II) sulfate and chromium(III) chloride.
(a) Identify compounds A – E and state the expected observation for each test.
(b) Write an ionic equation for each reaction.
(c) Explain why HNO3(aq) is added to FeSO4(aq) before the reagent Ba(NO3)2(aq)
- Suggest how the following pairs of compounds could be distinguished from one another:
(a) lithium bromide and potassium bromide
(b) sodium bromide and sodium iodide
- A test tube is thought to contain a weak solution of ammonium iodide. Describe the qualitative tests you would carry out to show that this was the case.
Answers
- (a) A is a green precipitate of Fe(OH)2 / Fe(H2O)4(OH)2 ; B is a white precipitate of BaSO4 ; C is a grey-green precipitate of Cr(OH)3 / Cr(H2O)3(OH)3 ; D is a violet solution of [Cr(NH3)6]3+ ; E is a white precipitate of AgCl
(b) A: Fe2+(aq) + 2OH–(aq) ⇾ Fe(OH)2(s) / Fe2+(aq) + 2OH–(aq) + 4H2O(l) ⇾ Fe(H2O)4(OH)2(s)
B: Ba2+(aq) + SO42-(aq) ⇾ BaSO4(s)
C: Cr3+(aq) + 3OH–(aq) ⇾ Cr(OH)3(s) / Cr3+(aq) + 3OH–(aq) + 3H2O(l) ⇾ Cr(H2O)3(OH)3(s) /
[Cr(H2O)6]3+(aq) + 3NH3(aq) ⇾ [Cr(H2O)3(OH)3](s) + 3NH4+(aq)
D: Cr(OH)3(s) + 6NH3(aq) ⇾ [Cr(NH3)6]3+(aq) + 3OH–(aq) /
[Cr(H2O)3(OH)3](s) + 6NH3(aq) ⇾ [Cr(NH3)6]3+(aq) + 3OH–(aq) + 3H2O(l)
E: Ag+(aq) + Cl–(aq) ⇾ AgCl(s)
(c) HNO3 will react with any carbonate ions that might be present, releasing CO2. Barium carbonate is also a white precipitate and will prevent us from seeing the white precipitate of a positive result for sulfate ions.
2. (a) Dip nichrome wire into concentrated hydrochloric acid and then place into a blue Bunsen flame until it burns clean. Dip the hot wire back into the acid and into a solid sample of lithium bromide. The flame will burn red. Repeat with potassium bromide and the flame will burn lilac.
(b) Add aqueous silver nitrate drop wise to each an aqueous solution of each substance. A cream precipitate that dissolves when concentrated ammonia solution is added indicates bromide ions and a yellow precipitate that does not dissolve on addition of concentrated ammonia solution indicates iodide ions.
3. To test for ammonium ions add aqueous sodium hydroxide, warm the test tube and place damp red litmus paper into the mouth of the tube. It will turn blue indicating ammonia is present.
To test for iodide ions we need to first remove carbonate ions by addition of dilute nitric acid, and remove sulfate ions by adding aqueous barium nitrate and filtering off any precipitate that forms. We can then add aqueous silver chloride to the solution and look for the formation of a yellow precipitate which will not dissolve if we add a few drops of concentrated ammonia solution.