Identifying unknown compounds by their ions is a classic chemistry skill. Flame tests reveal metal cations by the colour they give to a Bunsen flame; adding sodium hydroxide or silver nitrate solution to solutions produces distinctive precipitates that identify both positive and negative ions.
What are cations and anions?
Ions carry an electrical charge. Cations are positively charged (they have lost electrons) and anions are negatively charged (they have gained electrons). When you dissolve an ionic compound in water, it splits into its constituent ions, and those ions can then be identified by their characteristic chemical reactions.
At GCSE, the common cations to identify are metal ions (Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Al³⁺) and the ammonium ion (NH₄⁺). Common anions include chloride (Cl⁻), bromide (Br⁻), iodide (I⁻), sulfate (SO₄²⁻), carbonate (CO₃²⁻), and hydroxide (OH⁻).
How do flame tests identify metal cations?
When a metal compound is placed in a hot Bunsen flame, electrons in the metal ions absorb energy and jump to higher energy levels. When they fall back to lower levels, they release the absorbed energy as visible light of characteristic wavelengths — producing characteristic flame colours.
Flame test procedure:
- Clean a nichrome wire loop by dipping it in hydrochloric acid and heating until no colour is produced.
- Dip the clean loop in the compound to be tested (dissolved in a few drops of HCl helps).
- Hold the loop in the hottest part of the Bunsen flame (the blue cone tip).
- Observe and record the colour.
| Metal ion | Symbol | Flame colour |
|---|---|---|
| Lithium | Li⁺ | Crimson / scarlet red |
| Sodium | Na⁺ | Persistent bright yellow |
| Potassium | K⁺ | Lilac / pale violet |
| Calcium | Ca²⁺ | Brick red / orange-red |
| Barium | Ba²⁺ | Apple green |
| Copper | Cu²⁺ | Blue-green / turquoise |
Limitation: Sodium gives such an intense yellow colour that it can mask other colours. If sodium is suspected, it is best to use dilute acid-treated compounds and observe through a blue cobalt glass filter, which absorbs yellow light, making potassium's lilac visible.
How does sodium hydroxide solution identify metal cations?
Adding dilute sodium hydroxide (NaOH) solution to a solution containing a metal cation usually produces a coloured precipitate of the metal hydroxide. The precipitate forms because the metal cation reacts with OH⁻ ions:
General equation: Metal²⁺(aq) + 2OH⁻(aq) → Metal(OH)₂(s)
| Ion tested | Precipitate colour | Dissolves in excess NaOH? |
|---|---|---|
| Al³⁺ (aluminium) | White | Yes — dissolves to give colourless solution |
| Ca²⁺ (calcium) | White | No |
| Cu²⁺ (copper) | Blue | No |
| Fe²⁺ (iron(II)) | Green | No |
| Fe³⁺ (iron(III)) | Brown / rust | No |
| NH₄⁺ (ammonium) | No precipitate — pungent gas (NH₃) given off on warming | — |
Key distinguishing tests:
- Al³⁺ and Ca²⁺ both give white precipitates, but Al³⁺ dissolves in excess NaOH (it forms the aluminate ion [Al(OH)₄]⁻), while Ca²⁺ does not.
- Fe²⁺ and Fe³⁺ both give coloured precipitates that do NOT dissolve in excess — the colours distinguish them (green vs brown).
- NH₄⁺ produces ammonia gas when NaOH is added and warmed. Ammonia has a pungent smell and turns damp red litmus paper blue.
How do you test for halide anions?
To test for chloride, bromide, and iodide ions, use silver nitrate solution (AgNO₃) acidified with dilute nitric acid:
- Acidify the sample with dilute nitric acid (this removes carbonate and hydroxide ions that would also precipitate with silver nitrate, giving false positives).
- Add a few drops of silver nitrate solution.
- Observe the colour of any precipitate formed.
| Halide ion | Precipitate | Colour | Solubility in ammonia solution |
|---|---|---|---|
| Cl⁻ (chloride) | AgCl | White | Dissolves in dilute ammonia |
| Br⁻ (bromide) | AgBr | Cream | Dissolves only in concentrated ammonia |
| I⁻ (iodide) | AgI | Yellow | Insoluble in ammonia solution |
The ammonia solubility test is used when two precipitates look similar in colour.
How do you test for sulfate and carbonate anions?
Sulfate (SO₄²⁻): Add dilute hydrochloric acid (to remove carbonate interference) followed by barium chloride solution (BaCl₂). A white precipitate of barium sulfate (BaSO₄) confirms sulfate. BaSO₄ is insoluble in dilute acid, distinguishing it from barium carbonate.
Carbonate (CO₃²⁻): Add dilute hydrochloric acid to the solid or solution. Effervescence (fizzing) occurs, producing carbon dioxide gas. Bubble the gas through limewater: if it turns milky, CO₂ is confirmed and therefore carbonate is present.
Frequently asked questions
Why do different metal ions produce different flame colours?
Each metal element has a unique set of electron energy levels — this is a consequence of quantum mechanics and the number of protons in the nucleus. When heat energy from the Bunsen flame excites electrons, they absorb specific amounts of energy to jump to higher levels. When they fall back, they release that energy as photons of light at specific wavelengths. Because the energy levels are unique to each element, the wavelengths of light emitted are unique, producing the characteristic colours. Sodium's yellow is so intense because the two 3s electrons fall through one of the highest-energy transitions in the visible range.
Why must you acidify the sample before adding silver nitrate?
Without acidification, the silver nitrate would also react with carbonate (CO₃²⁻), phosphate (PO₄³⁻), and hydroxide (OH⁻) ions present in many solutions, producing white or cream precipitates that could be mistaken for silver chloride or bromide. Adding dilute nitric acid first decomposes carbonate (it fizzes off as CO₂) and neutralises hydroxide, leaving only the halide ions to react with silver nitrate and produce the correct, unambiguous result.
How do you tell aluminium ions from calcium ions in a NaOH test?
Both Al³⁺ and Ca²⁺ initially produce a white precipitate with sodium hydroxide. The key distinction is adding excess sodium hydroxide solution: the white aluminium hydroxide precipitate dissolves in excess NaOH, forming the soluble tetrahydroxoaluminate ion [Al(OH)₄]⁻, so the solution becomes clear again. Calcium hydroxide does not dissolve in excess NaOH — the white precipitate remains. This dissolve-in-excess step is a classic distinguishing test at GCSE.
Can flame tests identify all metal ions?
No. Flame tests are reliable for the alkali metals (Li, Na, K), alkaline earth metals (Ca, Ba), and copper, but some metals give no characteristic colour in a standard Bunsen flame — iron, aluminium, and zinc, for example, do not produce colours visible to the naked eye. For these, the sodium hydroxide precipitate test or more sophisticated instrumental analysis (flame emission spectroscopy, which uses a spectrometer to analyse the precise wavelengths rather than just the colour) is more reliable. Sodium contamination is also a common problem, since even traces of sodium from glassware can mask other colours.
For particle-model-first GCSE chemistry with Professor Curie — reasoning from the ion to the observable colour or precipitate — visit aitutors.me.