flame test colors

Sprinkle a little table salt onto a gas flame and the flame flushes orange-yellow; a pinch of a potassium salt tints it lilac; copper turns it green; strontium burns red. This is the flame test, the simplest spectroscopy there is, and it is the reason fireworks come in colours. The light is the element itself announcing its identity.

Here is the mechanism in plain steps. The heat of the flame is energetic enough to vaporize the salt and excite its metal atoms, kicking outer electrons up into higher-energy orbitals. An excited electron cannot stay up there; it falls back down, and as it drops it releases the energy difference as a photon of light. Because each element's energy levels are uniquely spaced, the photons come out at characteristic wavelengths — characteristic colours. Sodium emits an intense yellow at 589 nanometres; potassium gives a faint lilac; lithium a crimson red; caesium a blue-violet; calcium an orange-red; barium green. The colour is a fingerprint of the element's electronic structure.

Flame tests matter as a quick, cheap qualitative identification of s-block (and some other) metal ions, and the same physics underpins the analytical technique of flame emission spectroscopy and the colours of fireworks. The honest caveat: sodium's yellow is so strong that even a trace contaminant drowns out everything else, so chemists view potassium flames through blue cobalt glass to filter out the sodium. And a flame test confirms an element is present but cannot tell you how much — it is qualitative, not quantitative, on its own.