C3bL7 Testing for Non-Metal Ions
Key Words
Ammonia gas - NH3, a pungent and toxic gas
Ammonium ions - NH4+, ions formed when ammonia dissolves in water.
Halide - negative ions formed by Group VII elements
Insoluble - does not dissolve.
Litmus paper - paper impregnated with an indicator to show acid or alkali.
Nitrate - the NO3- ion.
Non-metal ions - ions made up from non-metallic elements (really?)
Precipitate - a solid that forms when two solutions react.
Grade E
You will need to know the tests for non-metal ions:
- For sulphate, add some dilute hydrochloric acid, followed by dilute barium chloride. A white precipitate of insoluble barium sulphate is seen,
- For halide ions, dissolve in water with some dilute nitric acid. The colour of the precipitate indicates the ion:
|
Halide |
Precipitate |
| Chloride | White |
| Bromide | Cream |
| Iodide | Yellow |
- For nitrate ions, we add sodium hydroxide and a little aluminium powder, heating the mixture gently. Ammonia gas is given off and turns damp red litmus paper blue.
- For ammonium ions, add sodium hydroxide. Ammonia gas is given off, turning red litmus paper blue.
- Carbonate ions react with dilute hydrochloric acid to give off carbon dioxide, which turns limewater cloudy.
Grade C
These are the reactions for the tests above:
|
Ion |
Reaction |
| Sulphate | Ba2+(aq) + SO42-(aq) ® BaSO4(s) |
| Halide |
Ag+(aq) + Cl-(aq)
® AgCl(s) Ag+(aq) + Br-(aq) ® AgBr(s) Ag+(aq) + I-(aq) ® AgI(s) |
| Nitrate |
 |
| Ammonium | NH4+(aq) + OH-(aq) ® NH3(g) + H2O(l) |
Grade A
With silver halide tests, it can be difficult to tell whether the precipitate is white, cream, or yellow. So a further step can be needed:
|
Halide |
Further Test |
| Chloride | Dissolves readily in dilute ammonia solution |
| Bromide | Does not dissolve in dilute ammonia solution, but does dissolve in concentrated ammonia solution |
| Iodide | Does not dissolve in dilute or concentrated ammonia solution. |