Identifying which ions we might have in a solution is known as qualitative analysis. The chemical tests we carry out to identify transition metal or aluminium ions are often precipitation reactions involving a base (hydroxide ions, ammonia or carbonate ions).
Precipitation reactions with ammonia
- you can find out about the reactions between ammonia and transition metal ions in detail here.
Precipitation reactions with hydroxide ions
- Fe2+, Fe3+, Cu2+ and Mn2+ ions all form an insoluble metal hydroxide with OH–(aq) that does NOT dissolve on addition of excess OH–.
- Al3+ ions form a white precipitate of [Al(H2O)3(OH)3] with OH–(aq) which dissolves to form a colourless solution of [Al(OH)4]– on addition of excess OH–.
- Cr3+ ions form a grey-green precipitate of Cr(OH)3 with OH– which also dissolves to give a dark green solution of [Cr(OH)6]3- on addition of excess OH–.
Precipitation reactions with carbonate ions
- Fe2+ and Cu2+ both form an insoluble metal carbonate precipitate with CO32- ions (the reagent is aqueous sodium carbonate)
- however, Fe3+ and Al3+ form an insoluble metal hydroxide when added to CO32- and bubbles of carbon dioxide are also observed. This is because solutions of [Fe(H2O)6]3+ and [Al(H2O)6]3+ are acidic (you can find a full explanation here).
You can find details of all the other cation and anion qualitative tests you need to know, with practice Q&A, here!
Practice questions
- [Fe(H2O)6]2+ ions in solution react with aqueous sodium carbonate to give a precipitate.
(a) Give the colour of the precipitate formed.
(b) Write an ionic equation for the reaction.
- [Al(H2O)6]3+ ions in solution react with aqueous sodium carbonate to give a white precipitate.
(a) Give the formula of the precipitate.
(b) What other observation would be seen?
(c) Write an ionic equation formation of the precipitate.
3. Describe what would be seen when aqueous sodium hydroxide is added drop by drop to a solution of chromium(III) nitrate until in excess.
4. The following tests were carried out on a solution containing [CuCl4]2- ions:
| Test | Observation | Formula of complex ion formed |
|---|---|---|
| (i) drop wise addition of excess water | ||
| (ii) drop wise addition of excess aqueous ammonia | ||
| (iii) addition of excess water followed by aqueous Na4EDTA | pale blue solution |
(a) Complete the table
(b) Write ionic equations for the reactions observed in (i) and (ii)
Answers
- (a) green
(b) [Fe(H2O)6]2+(aq) + CO32-(aq) ⇾ FeCO3(s) + 6H2O(l)
2. (a) [Al(H2O)3(OH)3]
(b) bubbles / effervescence
(c) 2 [Al(H2O)6]3+(aq) + 3 CO32-(aq) ⇾ 2 [Al(H2O)3(OH)3](s) + 3H2O(l) + 3CO2(g)
3. A grey-green precipitate would be seen that then dissolves to give dark green solution.
4. (a)
| Test | Observation | Formula of complex ion formed |
|---|---|---|
| (i) drop wise addition of excess water | yellow solution turns green and then pale blue * | [Cu(H2O)6]2+ |
| (ii) drop wise addition of excess aqueous ammonia | yellow solution turns deep blue * | [Cu(NH3)4(H2O)]2+ |
| (iii) addition of excess water followed by aqueous Na4EDTA | pale blue solution | [Cu(EDTA)]2+ |
(b) [CuCl4]2-(aq) + 6H2O(l) ⇾ [Cu(H2O)6]2+(aq) + 4Cl–(aq)
[CuCl4]2-(aq) + 2H2O(l) + 4NH3(aq) ⇾ [Cu(NH3)4(H2O)]2+(aq) + 4Cl–(aq)