Flame Tests
- Metal ions produce a colour if heated strongly in a flame
- Ions from different metals produce different colours
- The flame test is thus used to identify metal ions by the colour of the flame they produce
- Dip the loop of an unreactive metal wire such as nichrome or platinum in dilute acid, and then hold it in the roaring blue flame of a Bunsen burner until there is no colour change
- This cleans the wire loop and avoids contamination
- This is an important step as the test will only work if there is just one type of ion present
- Two or more ions means the colours will mix, making identification erroneous
- Dip the loop into the solid sample and place it in the edge of the blue Bunsen flame
- Avoid letting the wire get so hot that it glows red otherwise this can be confused with a flame colour
Diagram showing the technique for carrying out a flame test
- The colour of the flame is observed and used to identify the metal ion present. Some common examples are:
Diagram showing the colours formed in the flame test for metal ions
Examiner Tip
The sample needs to be heated strongly, so the Bunsen burner flame should be on a roaring blue flame. Blue flames are not actually very blue, so the colour of the flame is not masked as it would be in the yellow safety flame.
Remember that flame tests often need to be followed up by precipitation reactions in order to confirm the identify of the ion.
The lilac flame of the K+ ion is easily masked by the bright yellow of the sodium flame, so a yellow flame test in particular can easily conceal a second ion present. Check the question to see if this is implied!
