Flame tests

When pasta water boils over on a gas hob, the blue flame flares bright yellow for a moment. The salt in the water is to blame. The same effect is behind every firework: the red, green and blue bursts in the sky are metal salts glowing in the heat. Chemistry turns this into a test. The flame colour reveals which metal is in a sample, and all you need is a flame and a sharp eye.

Why the flame takes on colour

A burner flame gets hotter than 1000 degrees Celsius. That energy lifts the electrons of the metal atoms onto a higher level. They only last there for a tiny moment before dropping back. The energy released is given off as light, in very specific colours. Because every element has its levels in different places, every element glows in its own mixture of colours. The pattern is as unmistakable as a fingerprint.

The colours that matter most

A few colours are worth remembering: sodium glows an intense yellow, calcium brick red, strontium deep red, barium green and copper blue-green. Potassium tints the flame a delicate violet, but you rarely get to see that colour, because even traces of sodium drown everything in yellow. That is why a potassium test is viewed through blue cobalt glass, which filters out the yellow part. One more important point: the colour comes from the metal, not from its partner in the salt. Sodium chloride and sodium carbonate both glow yellow.

From the burner to the stars

Around 1860 Robert Bunsen and Gustav Kirchhoff built a device that fans flame light out into its individual colours: the spectroscope. Instead of one washed out mixed colour they now saw sharp coloured lines, a separate pattern for every element. With it they discovered two new elements and named them after their lines: caesium after its sky blue one, rubidium after its dark red one. Soon afterwards the same line patterns were found in the light of the Sun and the stars. To this day astronomers read from them which elements a star is made of, even though nobody has ever fetched a sample from there.