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Ions
Tests for ions
The presence of certain ions in an aqueous solution can be detected by making chemical tests.
1) Tests for ions with a precipitation reaction
The presence of certain ions in an aqueous solution can be detected by making chemical tests.
A test for ions is performed in several steps:
- A small part of the tested solution is taken and placed in a test tube.
- A few drops of a chemical called "reactive" are added in this test tube. The reactive is supposed to react with tested ions.
- The result is observed: if the searched ion is present in the solution then a chemical transformation takes place and produces a solid compound which looks a bit like gelatin and that is called "precipitate". Its color must be checked and compared to the expected color.
Example:test for copper ions.
Step
1: some
solution is poured in a test tube
Step
2: A few drops of an
aqueous solution of sodium hydroxide (which
acts as a reactive) are added
Step
3: A
blue precipitate forms which confirms the presence of copper ions
To realize a test, we must therefore know the reagent to be used and the color of the precipitate that should form.
2) Some tests to know
| Ion | Iron (II) ion Fe2+ | Iron (III) ion Fe3+ | Copper ion Cu2+ | Chloride ion Cl- |
| Reactive | Sodium hydroxide | Sodium hydroxide | Sodium hydroxide | Silver nitrate |
| Color of the precipitate | Green | Red | Blue | White (Blackens in the light) |
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