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Why do metals burn with different colours?

The GCSE picture: flame test colours, and where the light comes from.

  1. A firework100 m
  2. A flame5 cm
  3. A hot atom0.3 nm
  4. One electron fallinga single photon
ΔE = hf = hc / λ
What the simulation shows, in words

Six dark strips, one for each element, with bright coloured lines where that element gives out light, on a scale from 380 to 780 nanometres. Sodium has a strong pair of yellow-orange lines close together near 589 nm. Lithium has a strong red line at 671 nm. Potassium has deep red lines near 770 nm and a violet one near 404 nm. Calcium has a strong violet line at 423 nm and several orange-red ones. Copper has a group of green lines near 510 to 522 nm. Hydrogen has four lines, red, blue-green and two violet. In the second mode, a mystery sample appears at the top and you pick which metals it contains.

Light with a fingerprint

Hold a compound of lithium in a hot flame and the flame turns crimson. Sodium turns it yellow, potassium lilac, calcium orange-red and copper green. Chemists use this as a quick test for which metal ion is in a compound: a flame test.

The colour comes from the electrons. Heat gives some electrons enough energy to jump up to a higher energy level, further from the nucleus. They don’t stay there. They fall back down, and each one gives out the energy it gained as a tiny packet of light.

Why each element has its own colour

An electron can only sit in certain energy levels, and the gaps between them are different in every element. So each element can only give out light of certain exact colours, its own set of wavelengths. Spread the light out with a prism or a grating and instead of a rainbow you see a few bright lines. That pattern is an emission spectrum, and no two elements have the same one.

The simulation shows the visible lines of six elements, measured by NIST. Sodium’s light is almost all in two lines very close together near 589 nm. That is why old sodium street lamps glowed such a pure orange-yellow.

Seeing what you can’t reach

Because every element has its own lines, light can tell you what something is made of without touching it.

  • Astronomers find the elements in stars from the lines in their light. Helium was found in the Sun’s spectrum in 1868, 27 years before anyone found it on Earth, and was named after the Greek for Sun.
  • Laboratories use flame emission spectroscopy: an instrument measures the lines from a sample in a flame. It can pick out several metals in a mixture at once and measure how much of each is there, which a flame test by eye can’t do.

Try the second mode in the simulation. A mystery sample shows up at the top, and you match its lines to the elements below.

Safety

Flame tests use a hot flame and compounds that can be toxic. They are done in a school lab with a teacher, eye protection and a risk assessment. Don’t try them at home, and never burn chemicals in a fire to see the colours.