Flame Tests Experiment

Aim

To identify metal ions (cations) in unknown compounds using flame tests.

Apparatus

• Nichrome wire loop (or platinum wire loop)
• Bunsen burner
• Heatproof mat
• Concentrated hydrochloric acid (in a watch glass)
• Watch glass or dimple tile (for solid samples)
• Blue cobalt glass (for distinguishing potassium from sodium)
• Samples of known metal compounds for testing: lithium chloride, sodium chloride, potassium chloride, calcium chloride, copper(II) chloride (or sulfate)
• Unknown compounds for identification

Method

1. Set the Bunsen burner to a roaring blue flame (air hole fully open). A blue flame is essential because a yellow safety flame would mask the colour of the metal ions.
2. Clean the nichrome wire loop: dip it into concentrated hydrochloric acid, then hold the loop in the hottest part of the blue flame. Repeat until the wire produces no colour in the flame. This step is critical to avoid contamination from previous tests.
3. Dip the clean wire loop into the first sample (either a solid or a solution in a watch glass).
4. Hold the wire loop at the edge of the blue flame and observe the colour produced.
5. Record the flame colour.
6. Clean the wire loop again (dip in HCl, burn clean) before testing the next sample.
7. Repeat for all samples.
8. If a yellow flame is observed and potassium is suspected, view the flame through blue cobalt glass. The glass absorbs the yellow sodium colour and reveals the lilac colour of potassium if present.

Expected results

Metal ion	Flame colour
Lithium (Li+)	Crimson / red
Sodium (Na+)	Yellow / golden yellow
Potassium (K+)	Lilac / purple
Calcium (Ca2+)	Orange-red / brick red
Copper (Cu2+)	Blue-green / green

The colours are produced because the heat energy from the flame promotes electrons in the metal ions to higher energy levels. When the electrons fall back to their ground state, they emit light of specific wavelengths corresponding to characteristic colours.

Analysis

Flame tests identify the metal cation in a compound. The anion does not affect the flame colour — lithium chloride and lithium sulfate both give a crimson flame because the lithium ion is responsible for the colour.

The technique is limited to metal ions that produce visible colours. Most transition metals and some other metals do not produce distinctive flame colours at Bunsen burner temperatures. For these, use the NaOH precipitation test instead.

Chloride salts are preferred for flame tests because they are generally more volatile than sulfates or nitrates, making the metal ions easier to vaporise in the flame.

Sources of error

• Wire not cleaned properly: Residual metal ions from a previous test produce the wrong colour. Always clean until the flame shows no colour.
• Sodium contamination: Sodium is present in glass, sweat, and many common materials. Its intense yellow flame can mask other colours, especially potassium’s lilac. Always use blue cobalt glass when potassium is suspected.
• Yellow vs orange-red confusion: Sodium (yellow) and calcium (orange-red) can look similar if the wire is held in the wrong part of the flame. Observe in the edge of the flame for the clearest colour.
• Mixed compounds: If a sample contains two metal ions, both colours may appear simultaneously, making identification difficult. Dissolve the sample and test separately, or use other tests (NaOH precipitation) to confirm.

Exam tips

Flame test colours must be learned precisely. Examiners accept the specific terms listed above but reject vague descriptions: “red” for lithium is accepted but not for calcium (which is “orange-red” or “brick red”). “Purple” for potassium is accepted, but “blue” is not.

A common exam question format presents a table of unknown compounds with flame test results and asks you to identify the cation. Another format asks why concentrated hydrochloric acid is used to clean the wire (it removes traces of previous metal ions as volatile metal chlorides).

The explanation of why flame tests work (electrons excited to higher energy levels, then emit light as they return to ground state) is required at supplement level. Different metals have different electron configurations, so they emit different wavelengths and produce different colours.

If asked for a limitation of flame tests, state that some metals do not produce a distinctive colour and that sodium contamination can mask other colours. For these cases, the NaOH precipitation test provides more reliable identification.