Flame tests for metal ions, gas identification tests, cation tests with sodium hydroxide and ammonia solution, anion tests for carbonate, sulfate, and halides, and tests for the presence of water.
Qualitative analysis is the branch of chemistry concerned with identifying what substances are present in a sample. Unlike quantitative analysis, which measures how much, qualitative analysis gives a yes or no: does this ion exist here? Each test relies on a characteristic, reproducible observation — a colour change, a precipitate, a gas that reacts with a reagent — that is specific enough to distinguish one ion from another.
When a metal salt is held in a flame, the metal cation absorbs energy and its electrons jump to higher energy levels. When they fall back, they emit light at characteristic wavelengths visible as a coloured flame. The test is carried out using a clean nichrome or platinum wire loop dipped in the sample and held in the hottest part of a Bunsen burner flame.
| Ion | Flame colour |
|---|---|
| Sodium (Na⁺) | Persistent yellow-orange |
| Potassium (K⁺) | Lilac (pale violet) |
| Calcium (Ca²⁺) | Brick red |
| Copper (Cu²⁺) | Blue-green (verdigris) |
| Barium (Ba²⁺) | Apple green |
| Lithium (Li⁺) | Crimson red |
The wire must be cleaned between tests by dipping in concentrated hydrochloric acid and re-holding in the flame until no colour is imparted. A dirty wire carrying traces of sodium will give a yellow flame that masks every other colour — in particular it makes the pale lilac of potassium almost invisible.
A gas produced in a reaction can be identified by directing it at a reagent or applying a simple test. Each has a specific, named reagent and a specific, observable result.
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen (H₂) | Apply a burning splint | Burns with a squeaky pop |
| Oxygen (O₂) | Apply a glowing splint | Splint relights |
| Carbon dioxide (CO₂) | Bubble through limewater | Limewater turns milky (white precipitate of CaCO₃) |
| Ammonia (NH₃) | Hold damp red litmus paper near | Paper turns blue |
| Chlorine (Cl₂) | Hold damp litmus paper near | Paper is bleached to white |
| Sulfur dioxide (SO₂) | Pass through acidified potassium dichromate(VI) | Orange dichromate turns green |
Equations for selected reactions:
Both ammonia and chlorine affect litmus — but in opposite directions and for different reasons. Ammonia is alkaline so it turns red litmus blue. Chlorine is an oxidising agent that destroys the dye and bleaches (decolourises) litmus entirely. The distinction is important.
Adding sodium hydroxide solution precipitates metal hydroxides. The colour and solubility of the precipitate in excess NaOH are diagnostic.
| Ion | Observation with NaOH (drop by drop) | Observation with excess NaOH |
|---|---|---|
| Copper (Cu²⁺) | Light blue precipitate | Precipitate remains (insoluble) |
| Iron(II) (Fe²⁺) | Green precipitate (turns brown/rust at surface on standing in air) | Precipitate remains |
| Iron(III) (Fe³⁺) | Reddish-brown precipitate | Precipitate remains |
| Zinc (Zn²⁺) | White precipitate | Dissolves in excess (amphoteric — forms zincate ion) |
| Aluminium (Al³⁺) | White precipitate | Dissolves in excess (amphoteric — forms aluminate ion) |
| Calcium (Ca²⁺) | Slight white precipitate | Precipitate remains (slightly soluble) |
| Ammonium (NH₄⁺) | No precipitate; ammonia gas evolved on warming | Smell of ammonia; turns damp red litmus blue |
Key equations:
Ammonia solution can also precipitate metal hydroxides, but the results differ for ions that form soluble ammonia complexes.
| Ion | With dilute ammonia (drop by drop) | With excess ammonia |
|---|---|---|
| Copper (Cu²⁺) | Light blue precipitate | Precipitate dissolves to give a deep blue solution (tetraamminecopper(II) complex) |
| Zinc (Zn²⁺) | White precipitate | Precipitate dissolves (forms colourless zincammine complex) |
| Iron(II) (Fe²⁺) | Green precipitate | Precipitate does not dissolve |
| Iron(III) (Fe³⁺) | Reddish-brown precipitate | Precipitate does not dissolve |
The deep blue formed from Cu²⁺ with excess ammonia is the most striking and recognisable result:
This reaction distinguishes Cu²⁺ from all other common ions and is itself a confirmatory test.
Add dilute hydrochloric acid (or dilute H₂SO₄). A carbonate effervesces, releasing a colourless gas. The gas is then bubbled through limewater to confirm it is CO₂.
Acidify the solution with dilute nitric acid (to dissolve any sulfite or carbonate that might give a false positive), then add barium chloride solution. A white precipitate of barium sulfate forms immediately:
The precipitate is insoluble in dilute nitric acid — this distinguishes it from barium sulfite (which would dissolve).
Acidify with dilute nitric acid (to destroy carbonate and sulfite ions that would give false positives), then add silver nitrate solution. The colour of the precipitate identifies the halide:
| Ion | Precipitate colour | Solubility in dilute ammonia |
|---|---|---|
| Chloride (Cl⁻) | White (AgCl) | Dissolves readily |
| Bromide (Br⁻) | Cream (AgBr) | Dissolves in concentrated ammonia only |
| Iodide (I⁻) | Yellow (AgI) | Insoluble — does not dissolve in ammonia |
Always acidify with dilute nitric acid before adding silver nitrate — never hydrochloric acid (which would add Cl⁻ and give a false positive for chloride) or sulfuric acid (which would give a BaSO₄ precipitate if Ba²⁺ were used later).
The presence of water (or any aqueous solution) is shown by two tests:
| Test | Observation indicating water |
|---|---|
| Add anhydrous copper(II) sulfate (white powder) | Turns blue — water of crystallisation absorbed to form CuSO₄·5H₂O |
| Use cobalt(II) chloride paper (blue when dry) | Paper turns pink — CoCl₂ absorbs water to form the hydrated pink complex |
These tests detect the presence of water, but do not prove it is pure water. To confirm purity, measure the boiling point (100 °C at 1 atm) or the freezing point (0 °C).
| What you observe | Likely ion or substance |
|---|---|
| Yellow-orange flame | Na⁺ |
| Lilac flame | K⁺ |
| Brick-red flame | Ca²⁺ |
| Blue-green flame | Cu²⁺ |
| Squeaky pop with lit splint | H₂ gas |
| Relights glowing splint | O₂ gas |
| Turns limewater milky | CO₂ gas |
| Turns damp red litmus blue | NH₃ gas |
| Bleaches damp litmus | Cl₂ gas |
| Orange dichromate turns green | SO₂ gas |
| Light blue precipitate with NaOH, deep blue with excess NH₃ | Cu²⁺ |
| Green precipitate with NaOH | Fe²⁺ |
| Reddish-brown precipitate with NaOH | Fe³⁺ |
| White precipitate, dissolves in excess NaOH or NH₃ | Zn²⁺ (or Al³⁺ — Al³⁺ does not dissolve in NH₃) |
| Effervescence with acid, CO₂ confirmed by limewater | CO₃²⁻ |
| White precipitate with acidified Ba²⁺, insoluble in acid | SO₄²⁻ |
| White precipitate with acidified Ag⁺ | Cl⁻ |
| Cream precipitate with acidified Ag⁺ | Br⁻ |
| Yellow precipitate with acidified Ag⁺ | I⁻ |
| Anhydrous CuSO₄ turns blue | Water present |
| Cobalt chloride paper turns pink | Water present |