A flame test is a common method used to identify metal ions, as most metal ions will produce a distinct colour when exposed to a consistent flame. By analysing the colour of the flame, we can identify the specific metal cations present in an unknown sample.
Having unique results for a test is essential. If multiple ions give the same result, it becomes difficult to determine the specific ion present in the sample, which can create confusion.
To perform a flame test, follow these steps:
1. Make a small loop at the end of a clean nichrome wire
2. Dip the loop into concentrated hydrochloric acid and then into the solid sample
3. Hold the loop in a blue Bunsen burner flame and record the first colour you observe
Different metal ions produce different coloured flames. The table below shows the expected results of the experiment:
|Metal Ion||Flame colour|
|Lithium – Li⁺||Crimson-red ◼|
|Sodium – Na⁺||Yellow ◼|
|Potassium – K⁺||Lilac ◼|
|Calcium – Ca²⁺||Orange-red ◼|
|Copper – Cu²⁺||Blue-green ◼|
This test only works if there is one metal ion present. If there are two or more metal ions in the sample, the colours will mix. This will make it more difficult to identify the ions.
Also, be careful not to let the wire get too hot, as it will glow red, which can be mistaken for the flame colour. It’s important to observe the colour of the flame as soon as it appears, before the wire has a chance to glow red.