Chemical tests identify which ions or gases are present. Learn the test and the positive result for each.
Flame tests (metal ions)Dip a clean nichrome wire loop in the sample and hold it in a blue Bunsen flame.
| Ion | Flame colour |
|---|---|
| Lithium | Crimson |
| Sodium | Yellow |
| Potassium | Lilac |
| Calcium | Orange-red |
| Copper | Blue-green |
Sodium hydroxide test (metal ions)Add a few drops of sodium hydroxide solution and look at the precipitate.
| Ion | Precipitate |
|---|---|
| Copper(II) | Blue |
| Iron(II) | Green |
| Iron(III) | Brown |
| Aluminium | White — dissolves in excess sodium hydroxide |
| Calcium and magnesium | White — does not dissolve in excess |
Negative ions
| Ion | Test | Positive result |
|---|---|---|
| Carbonate | Add dilute acid; bubble the gas through limewater | Fizzing; limewater turns milky |
| Sulfate | Add dilute hydrochloric acid, then barium chloride | White precipitate |
| Chloride, bromide, iodide | Add dilute nitric acid, then silver nitrate | Chloride white, bromide cream, iodide yellow precipitate |
Gases
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen | Lit splint | Squeaky pop |
| Oxygen | Glowing splint | Relights |
| Carbon dioxide | Bubble through limewater | Turns milky |
| Chlorine | Damp blue litmus paper | Turns red, then bleached white |
| Ammonia | Damp red litmus paper | Turns blue |
Flame photometryAn instrument that identifies metal ions from their line spectrum and measures their concentration. Unlike a flame test, it works on mixtures of ions.