4.5 - Tests for Cations
The flame test
- A cation is a positively charged ion.
- When a metal compound is heated in a flame, it gives off a characteristic colour that identifies the metal ion.
Carrying out the test
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Clean the wire. Dip a nichrome or platinum wire loop in hydrochloric acid, then hold it in a blue Bunsen flame until it burns with no colour. Any residue left from an earlier test would mask the sample's colour. Hydrochloric acid is used because chloride ions give no flame colour. A fresh damp wooden splint can be used instead of the wire and discarded after one test.
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Pick up the sample. Dip the clean wire loop or damp splint into the solid sample so a little clings to it.
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Hold it in the flame and record the colour. Use the roaring blue flame (air hole open), which is the hottest part and won't add any yellow colour of its own.
Flame colours
| Metal ion | Flame colour |
|---|---|
| Lithium, Li+ | crimson (red) |
| Sodium, Na+ | yellow (yellow-orange) |
| Potassium, K+ | lilac |
| Calcium, Ca2+ | orange-red (brick red) |
| Barium, Ba2+ | light green (green or apple green) |
| Copper(II), Cu2+ | blue-green |
Where a question describes lithium's flame as crimson, a flame it calls simply red is calcium's.
What the test cannot do
- A flame test only works reliably on a single metal ion.
- If a sample contains a mixture of ions, the colours blend and a strong colour masks a weaker one (e.g. sodium's yellow completely masks potassium's lilac).
Precipitates with sodium hydroxide
- Adding sodium hydroxide solution to a solution of a metal compound identifies the positive ion (cation) it contains.
- Most metal hydroxides are insoluble in water. When mixed, the insoluble metal hydroxide forms as a solid suspended in the liquid - a precipitate.
- Method - Dissolve the sample in water, add sodium hydroxide solution a few drops at a time, and record the colour of the precipitate.
| Metal ion | Precipitate formed | Colour |
|---|---|---|
| Copper(II), Cu2+ | copper(II) hydroxide, Cu(OH)2 | blue |
| Iron(II), Fe2+ | iron(II) hydroxide, Fe(OH)2 | green |
| Iron(III), Fe3+ | iron(III) hydroxide, Fe(OH)3 | brown |
| Aluminium, Al3+ | aluminium hydroxide, Al(OH)3 | white |
| Calcium, Ca2+ | calcium hydroxide, Ca(OH)2 | white |
| Magnesium, Mg2+ | magnesium hydroxide, Mg(OH)2 | white |
Telling the white precipitates apart
Add sodium hydroxide solution until it is in excess.
| White precipitate | With sodium hydroxide in excess |
|---|---|
| Aluminium, Al3+ | dissolves to give a colourless solution |
| Calcium, Ca2+ | does not dissolve |
| Magnesium, Mg2+ | does not dissolve |
- Only aluminium hydroxide dissolves in excess sodium hydroxide. Calcium and magnesium precipitates remain unchanged.
- This test cannot identify Group 1 metals. Lithium, sodium, and potassium hydroxides are soluble in water, so no precipitate forms. These are identified using flame tests instead.
