Select a metal salt — watch the characteristic flame color appear. Used in qualitative analysis to identify metal ions by their emission spectra.
The flame test is a qualitative analytical technique used in chemistry to identify the presence of certain metal ions in a sample. When a metal salt is introduced into a flame, the heat excites the outer electrons of the metal ions to higher energy levels. As these electrons fall back to their ground state, they release energy in the form of visible light — each metal emitting a characteristic colour unique to its electronic structure.
This technique is widely used in inorganic qualitative analysis, fireworks manufacture, and spectroscopic identification. In the Kerala University FYUGP Chemistry inorganic laboratory, the flame test is a standard procedure for the identification of cations like Ba²⁺, Sr²⁺, Ca²⁺, Na⁺, K⁺, Li⁺, and Cu²⁺.
| Metal | Ion | Flame Color | Wavelength | Key Use |
|---|---|---|---|---|
| Lithium | Li⁺ | ● Crimson Red | ~670 nm | Lithium detection, batteries |
| Sodium | Na⁺ | ● Golden Yellow | ~589 nm | Sodium, NaCl, streetlights |
| Potassium | K⁺ | ● Lilac / Violet | ~767 nm | Potassium salts, fertilisers |
| Calcium | Ca²⁺ | ● Brick Red | ~622 nm | Calcium, lime, cement |
| Strontium | Sr²⁺ | ● Crimson Red | ~606 nm | Red fireworks, flares |
| Barium | Ba²⁺ | ● Pearl Green | ~524 nm | Green fireworks, barium salts |
| Copper | Cu²⁺ | ● Blue-Green | ~515 nm | Copper compounds, fireworks |
| Iron | Fe³⁺ | ● Orange-Gold | ~580 nm | Iron detection, steelwork |
Procedure: A clean platinum or nichrome wire loop is dipped into concentrated hydrochloric acid and heated in a Bunsen burner flame until no colour is imparted (to remove contamination). The wire is then dipped into the metal salt sample and held in the hottest part of the flame — the blue cone. The colour of the resulting flame is observed and recorded.
Why HCl is used: Hydrochloric acid converts metal salts into their chlorides, which are more volatile and vaporise easily in the flame, producing a clearer and brighter colour emission.
Distinguishing similar colours: Sodium's intense golden yellow can mask other colours. Potassium's lilac flame is best viewed through a blue cobalt glass, which filters out sodium interference. Strontium (crimson) and Lithium (scarlet red) look similar but can be distinguished spectroscopically.
The flame test is a standard experiment in the inorganic qualitative analysis laboratory of the Kerala University Four Year Undergraduate Programme (FYUGP) Chemistry course. Students perform flame tests to identify Group 1 and Group 2 metal cations — particularly Ba²⁺ (pearl green), Sr²⁺ (crimson red), Ca²⁺ (brick red), Na⁺ (golden yellow), and K⁺ (lilac). Understanding the electronic basis of flame colours — electron excitation and emission — connects the lab observations to atomic structure theory covered in the FYUGP syllabus.