Flame Tests

Flame tests are conducted to determine the identity of the metal present in a salt sample. Different metal cations produce different coloured flames, and their identity can be determined by looking up a reference table.

Please refer to the video for the procedure.

The theory behind the flame test is the same one that explains emission spectra and is an extension of the quantum theory which explains the quantum structure of atoms and ions.

When the salt is heated, the electrons in the metal ions in the salt sample are 'excited', i.e. they gain energy and are promoted to a higher energy level in the ion's electronic structure. These electrons however, are not in a stable state due to their high energy and they consequently release that extra energy after a brief moment, and return back to a lower energy level. The energy released leaves the ions in the form of photons, i.e. light, which causes the salt sample to emit a coloured flame.

The higher the energy released, the higher the frequency of the light emitted, with the lowest frequency being red and the highest violet (or even ultraviolet), following the sequence of the colours of the rainbow.

Different amounts of energy are released depending on the electronic structure of the ion, which is unique to the element of that metal ion. Hence, each metal ion produces its own characteristic colour, which allows us to use the flame test to identify them.

Some common metal ions and their flame colours are listed below:

Lithium -  red
Sodium - orange
Potassium - lilac
Calcium - brick-red
Barium - pale green/apple green
Copper (II) - bluish-green with white flame centre if heated at high temperature
Lead - greyish-white
Magnesium - white
Zinc - blue

Some metal ions do not produce flame colours; rather, they produce 'sparks'.
One example is iron (III), which produces gold sparks.