About this experiment
Dip a clean wire loop into a metal salt, hold it in a hot flame and the flame turns a colour that identifies the metal: crimson for lithium, yellow for sodium, lilac for potassium, green for barium. The heat lifts electrons to higher energy levels, and when they fall back they give out light of exact wavelengths. Test seven salts, look at their light through a spectroscope, see how a trace of sodium hides other colours and how cobalt glass rescues the test, then identify mystery samples.
Learning goals
- Carry out and describe a flame test safely and fairly
- Identify Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺ and Cu²⁺ from their flame colours
- Explain flame colours in terms of electrons moving between energy levels
- Link the wavelength of a line to the energy of a photon
- Explain why sodium contamination is a problem and how cobalt blue glass helps
How to use it
- 1Hold each salt in the flame in turn and record the flame colour in a table.
- 2Compare lithium and strontium. They look similar. How can the spectroscope tell them apart?
- 3Choose KCl and raise the sodium impurity to 10 %. What colour does the flame look now?
- 4Switch on the cobalt blue glass. Which part of the spectrum disappears, and what can you see again?
- 5Read the photon energy of the sodium line. Which is more energetic, a red or a violet photon?
- 6Switch to Mystery sample and identify five samples in a row. Which ones were hardest?
The model
Assumptions
- Lithium, sodium and potassium colours come from atomic lines, at wavelengths from the NIST database.
- Calcium, strontium, barium and copper colours come mostly from molecules formed in the flame, such as CaOH, SrOH, BaOH and CuCl. Their bands are drawn at approximate centres and widths.
- Relative brightness within one salt is approximate. Real intensities depend on the flame temperature, the concentration and the anion.
- The flame colour is computed from the spectrum with the CIE 1931 colour matching functions, so it is the colour of the light, shown as closely as a screen can.
- Mystery samples come from a fixed shuffle, so each salt appears once in every seven rounds.
Try this in class
- Q1A student sees a yellow flame for every sample. What is the most likely cause, and how should the wire be cleaned?
- Q2Calculate the energy of a photon from the lithium line at 670.8 nm in joules and in electronvolts.
- Q3Why do different metals give different colours?
- Q4Why can a flame test not identify the metals in a mixture very well?
- Q5Fireworks use strontium and barium salts. Which colours do they produce?
Frequently asked questions
Why is the wire loop made of nichrome or platinum?
These metals are unreactive and do not colour the flame themselves. The loop is cleaned by dipping it in concentrated hydrochloric acid and heating it until the flame shows no colour.
Why is the spectrum made of lines and not a rainbow?
Electrons in an atom can only have certain energies, so only certain energy drops, and therefore certain wavelengths, are possible. A hot solid such as the glowing wire gives a continuous spectrum instead.
Is a flame test enough to identify a metal?
It is a quick first test. Some colours are similar, such as lithium and strontium, and sodium can hide others. Chemists confirm with other tests or with an instrument such as a flame emission or atomic absorption spectrometer, which measures the exact lines.