Testing for ions
Chemists are often asked to test a substance to find out if it contains specific ions or compounds.
A positive ion. in metal compounds can be identified using a flame test.
Most metal ions produce a strong colour when put into a blue Bunsen burner flame.
The colour of the flame can be used to identify which metal cation are present in an unknown sample.
Method:
- make a small loop in the end of a clean nichrome wire.
- dip the loop into concentrated hydrochloric acid and then into the sample.
- hold the loop in a blue Bunsen flame and record the first colour you observe.
Different metal ions produce different coloured flames.
| Ion present | Flame colour |
| Lithium, Li+ | Crimson |
| Sodium, Na+ | Yellow / orange |
| Potassium , K+ | Lilac |
| Calcium, Ca2+ | Brick red |
| Copper, Cu2+ | Blue-green / green-blue |
| Ion present | Lithium, Li+ |
|---|---|
| Flame colour | Crimson |
| Ion present | Sodium, Na+ |
|---|---|
| Flame colour | Yellow / orange |
| Ion present | Potassium , K+ |
|---|---|
| Flame colour | Lilac |
| Ion present | Calcium, Ca2+ |
|---|---|
| Flame colour | Brick red |
| Ion present | Copper, Cu2+ |
|---|---|
| Flame colour | Blue-green / green-blue |
It is important to clean the nichrome wire in between testing different samples.
To do this, dip the wire into concentrated acid and place it in the blue Bunsen flame until no colour change is observed.