Testing for Negative Ions (Anions)
| Ion | Test | Positive result |
|---|---|---|
| Carbonate, CO₃²⁻ | Add dilute hydrochloric or nitric acid | Effervescence; the gas turns limewater milky. CO₃²⁻(s) + 2H⁺(aq) → H₂O(l) + CO₂(g); CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l) |
| Sulphate, SO₄²⁻ | Add aqueous barium chloride, then dilute hydrochloric (or nitric) acid | White precipitate that does not dissolve in the excess acid. Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) |
| Nitrate, NO₃⁻ | Brown ring test — add freshly prepared iron(II) sulphate solution, then carefully run concentrated sulphuric acid down the side of the tube | A brown ring forms where the two layers meet |
| Sulphide, S²⁻ | Add dilute hydrochloric acid | Rotten-egg smell (hydrogen sulphide). S²⁻(s) + 2H⁺(aq) → H₂S(g) |
| Sulphite, SO₃²⁻ | Add dilute hydrochloric acid and warm | Colourless, choking gas that turns orange acidified potassium dichromate solution green. SO₃²⁻(s) + 2H⁺(aq) → SO₂(g) + H₂O(l) |
| Chloride, Cl⁻ | Acidify with dilute nitric acid, then add silver nitrate solution | White precipitate (AgCl), dissolves in aqueous ammonia |
| Bromide, Br⁻ | Same as above | Cream precipitate (AgBr), dissolves partially in aqueous ammonia |
| Iodide, I⁻ | Same as above | Yellow precipitate (AgI), insoluble in aqueous ammonia |
Testing for Positive Ions (Cations)
Add sodium hydroxide solution drop by drop, then in excess; separately, try the same with aqueous ammonia. The precipitate colour, and whether it redissolves in excess reagent, identifies the cation.
| Cation | With NaOH(aq) | With NH₃(aq) |
|---|---|---|
| Ca²⁺ | White precipitate, insoluble in excess | No precipitate |
| Mg²⁺ | White precipitate, insoluble in excess | White precipitate, insoluble in excess |
| Al³⁺ | White precipitate, soluble in excess | White precipitate, insoluble in excess |
| Zn²⁺ | White precipitate, soluble in excess | White precipitate, soluble in excess |
| Fe²⁺ | Dirty green precipitate, insoluble in excess | Dirty green precipitate, insoluble in excess |
| Fe³⁺ | Reddish-brown precipitate, insoluble in excess | Reddish-brown precipitate, insoluble in excess |
| Cu²⁺ | Blue precipitate, insoluble in excess | Blue precipitate, soluble in excess (deep blue solution) |
Aluminium and zinc look identical with NaOH alone (both redissolve), but ammonia tells them apart: zinc hydroxide redissolves in excess ammonia, aluminium hydroxide doesn’t.
The Flame Test
Holding a sample in a gas flame can reveal which metal ion it contains, from the flame’s colour.
| Cation | Flame colour |
|---|---|
| Lithium | Carmine (crimson) red |
| Sodium | Golden yellow |
| Potassium | Lilac (purple) |
| Calcium | Brick red |
| Strontium | Deep red |
| Barium | Apple (pale) green |
| Copper | Blue-green |
Identifying Common Gases
| Gas | Colour | Smell | Test |
|---|---|---|---|
| Ammonia, NH₃ | Colourless | Pungent (urine-like) | Turns moist red litmus blue |
| Carbon dioxide, CO₂ | Colourless | Odourless | Turns limewater milky |
| Chlorine, Cl₂ | Greenish-yellow | Choking (bleach-like) | Bleaches moist litmus paper |
| Hydrogen, H₂ | Colourless | Odourless | “Pop” sound with a lit splint |
| Oxygen, O₂ | Colourless | Odourless | Rekindles a glowing splint |
| Dinitrogen oxide, N₂O | Colourless | Faint, sickly-sweet | Rekindles a glowing splint |
| Nitrogen dioxide, NO₂ | Brown | Pungent | Turns moist blue litmus red |
| Sulphur dioxide, SO₂ | Colourless | Choking | Bleaches by reduction, depositing sulphur (unlike chlorine’s oxidative bleaching) |
| Hydrogen sulphide, H₂S | Colourless | Rotten eggs | Deposits sulphur with oxidising agents |
| Hydrogen chloride, HCl | White fumes | Pungent, choking | Turns moist blue litmus red |
