8.4 Identification of ions and gases
Aqueous cations:
| Test | Ion | Result |
| Add a few drops of dilute sodium hydroxide
solution. A precipitate will form. |
Cu2+ | Pale blue precipitate |
| Fe2+ | Green precipitate | |
| Fe3+ | Red-Brown precipitate | |
| Al3+, Zn2+, Ca2+ | White precipitate | |
| Divide the solutions into two equal volumes.
To one, add double the volume of sodium hydroxide solution. To the other, add double the volume of aqueous ammonia. |
Al3+ | The precipitate dissolves again in sodium
hydroxide solution giving a colourless solution but not in excess ammonia solution |
| Zn2+ | Precipitate dissolves in both solutions,
giving a colourless solution. |
|
| Ca2+ | Dissolves in neither. | |
| Take a small amount of the solid or solution.
Add a little dilute sodium hydroxide solution and heat gently. |
NH +
4 |
Ammonia gas given off (it has a strong
sharp smell and turns red litmus blue) |
Anions:
| Test | Ion | Result |
| Take a small amount of the
solid/solution. Add a little dilute hydrochloric acid. |
Carbonate | Carbon dioxide gas is produced so
there are bubbles and limewater goes milky |
| Take a small amount of the solution.
Add an equal volume of dilute nitric acid. Then add silver nitrate solution. Silver halides are insoluble, so a precipitate forms. |
Chloride | White precipitate |
| Iodide | Yellow | |
| Take a small amount of the solution.
Add a little sodium hydroxide solution. Add aluminium foil and heat gently. |
Nitrate | Ammonia gas given off |
| Take a small amount of the solution.
Add an equal volume of dilute hydrochloric acid. Then add barium nitrate solution. Barium sulphate is insoluble so… |
Sulphate | White precipitate (barium sulphate) |
Gases:
| Test | Gas | Result |
| Smell gas, use indicator paper. | Ammonia | Recognizable odour, indicator paper
turns blue |
| It is a weak acid, so reacts with
calcium hydroxide (lime water) to form insoluble calcium carbonate |
Carbon Dioxide | white precipitate, solution goes milky |
| Green poisonous gas which
bleaches dyes. Hold damp indicator paper in the gas (in a fume cupboard). |
Chlorine | Indicator paper turns white. |
| Collect in a tube then put a lighted
splint to it. |
Hydrogen | Squeaky pop |
| Collect in a tube then put a glowing
splint to it. |
Oxygen | Splint immediately bursts into a flame |