Upper secondary · 10 min

Flame tests: the color of metal ions

Hold a salt in a Bunsen flame and identify the metal from the color of the flame.

Goal

Link flame colors to the emission of light by excited electrons and learn which metals are easy to tell apart.

Apparatus and reagents

A Bunsen burner, a clean platinum or nichrome wire loop, and chloride salts of Li, Na, K, Ca, Sr, Ba and Cu moistened with dilute hydrochloric acid.

Procedure

Step through the seven metals one by one and note the color of each flame.
Change the amount of salt on the wire and see how the intensity changes.
Compare Li with Sr, and Na with K: which pairs are hard to tell apart by eye?

What to observe

Li and Sr give red flames, Ca orange-red, Na intense yellow-orange, K lilac, Ba pale green and Cu blue-green (approximate perceived colors).
A trace of sodium is enough to hide other colors; potassium is usually judged through cobalt-blue glass that filters out the yellow.

Explanation

Heat gives valence electrons enough energy to jump to higher levels. When they fall back they emit photons with energy $E = h\nu = hc/\lambda$ equal to the gap between levels. Each element has its own set of gaps, so its own line spectrum, and the strongest lines in the visible range set the color we see: for sodium the doublet near 589 nm is yellow. The same principle, measured accurately with a spectrometer instead of by eye, underlies atomic emission and absorption analysis (AAS, ICP).

History of the experiment

Chemists had long noticed coloured flames — Kircher and Marggraf wrote about the orange of sodium and the lilac of potassium — but salts were hard to distinguish because every burner was different. Robert Bunsen's non-luminous gas burner (1855) gave a clean, hot flame, and with Kirchhoff he turned the test into spectroscopy, discovering caesium (1860) and rubidium (1861) by their unseen spectral lines. The same experiment in this lab is still the quickest qualitative check for Na⁺, K⁺, Ca²⁺ and friends.

Chemists behind it

Robert Bunsen