A test is only useful if its result is unique

1. A test identifies an ion by giving a result that no other ion gives.
2. A result shared by two ions leaves the answer ambiguous.
3. A white precipitate on its own identifies nothing, because several ions give one.
4. What makes a test unique is the combination of reagent and observation.
5. Where one test cannot separate two ions, a second test is used alongside it.

A flame test identifies a metal ion by colour

1. A small sample of the solid is held in a flame and the colour is observed.
2. The wire holding the sample is cleaned first, usually by dipping it in acid.
3. A clean wire is checked by holding it in the flame until it gives no colour.
4. The flame used is a blue Bunsen flame, since a yellow flame would hide the result.
5. The colour comes from the metal ion, so the same ion gives the same colour in any compound.

The five flame colours

1. Lithium, ${Li}^{+}$, gives a red flame.
2. Sodium, ${Na}^{+}$, gives a yellow flame.
3. Potassium, $K^{+}$, gives a lilac flame.
4. Calcium, ${Ca}^{2+}$, gives an orange-red flame.
5. Copper, ${Cu}^{2+}$, gives a blue-green flame.

What a flame test cannot do

1. Only one ion can be identified at a time, so a mixture gives a confused result.
2. Sodium's strong yellow colour masks the paler colours of other ions in the same sample.
3. Lithium's red and calcium's orange-red are similar, so they can be confused.
4. A flame test says nothing about the negative ion in the compound.
5. A full identification therefore needs a separate test for the negative ion.