AQA GCSE Chemistry

Revision Notes

8.3.8 Flame Emission Spectroscopy

The Emission Spectrum

  • This technique is used to analyse metal ions in solution
  • When substances are heated they often emit energy in the form of light
  • This is due to electrons falling back to their original energy levels after becoming excited which causes them to jump up one or more energy levels
  • Flame emission spectroscopy works by exposing the sample to a very hot flame and then measuring the intensity and wavelength of the light emitted
  • The output is an emission spectrum in which different elements produce lines in different parts of the spectrum

Emission-Spectrum-of-Mercury, IGCSE & GCSE Chemistry revision notes

Diagram of an emission spectrum for mercury obtained from flame photometry

  • The emission spectrum consists of brightly coloured thin lines on a dark background and each element ion produces a unique spectrum
  • Flame emission spectroscopy also works for mixtures of ions
  • This is a major advantage over flame testing which can only analyze one ion at a time
  • The intensity of the light produced is proportional to the number of ions vaporised, so the technique can be used to determine the concentration of metal ions in a solution by reference to a standard solution of known concentration

Using Reference Data

  • Ions in unknown samples can be identified by comparing the sample spectrum to reference spectra
  • This is particularly useful if the sample contains a number of different ions
  • The following flame spectrum for example was obtained for a solution containing an unknown metal:

Sodium-Spectrum, IGCSE & GCSE Chemistry revision notes

Flame spectrum for an unknown metal 

  • When compared to the reference spectra below we can see that the solution must contain sodium ions:

Reference-Spectra, IGCSE & GCSE Chemistry revision notes

Diagram of a reference spectra including an emission spectrum for sodium

Exam Tip

By comparison with the emission spectrum of the sample, reference spectra allow us to quickly identify ions and mixtures of ions.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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