Testing for cations

1. Most metal cations are identified by adding aqueous sodium hydroxide or aqueous ammonia to a solution of the sample.
2. Both reagents contain hydroxide ions, ${OH}^{-}$, and many metal hydroxides are insoluble in water, so they appear as a precipitate.

   1. For example, copper(II) sulfate solution reacts with aqueous sodium hydroxide to form light blue copper(II) hydroxide:

      $$\mathrm{CuS}{\mathrm{O}}_4(\mathrm{aq})+2NaOH(\mathrm{aq})\to \mathrm{Cu}(\mathrm{OH}{)}_2(\mathrm{s})+\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4(\mathrm{aq})$$
3. The colour of the precipitate narrows down which cation is present.
4. Adding the reagent in excess gives a second observation, because some precipitates dissolve in excess reagent and others do not.
5. The reagent is added a few drops at a time at first, then more is added until it is in excess, with the tube shaken after each addition.
6. A fresh portion of the sample is used for each reagent.

Aqueous sodium hydroxide

1. Aluminium, ${Al}^{3+}$: white precipitate, soluble in excess, giving a colourless solution.
2. Calcium, ${Ca}^{2+}$: white precipitate, insoluble in excess.
3. Chromium(III), ${Cr}^{3+}$: green precipitate, soluble in excess.
4. Copper(II), ${Cu}^{2+}$: light blue precipitate, insoluble in excess.
5. Iron(II), ${Fe}^{2+}$: green precipitate, insoluble in excess.
   1. On standing, the green precipitate turns brown near the surface, because oxygen in the air oxidises the iron(II) to iron(III).
6. Iron(III), ${Fe}^{3+}$: red-brown precipitate, insoluble in excess.
7. Zinc, ${Zn}^{2+}$: white precipitate, soluble in excess, giving a colourless solution.
8. Ammonium, ${NH}_4^{+}$: no precipitate forms, but ammonia gas is given off when the mixture is warmed.
   1. The ammonia turns damp red litmus paper blue.

Aqueous ammonia

1. Aluminium, ${Al}^{3+}$: white precipitate, insoluble in excess.
2. Calcium, ${Ca}^{2+}$: no precipitate, or only a very slight white precipitate.
3. Chromium(III), ${Cr}^{3+}$: green precipitate, insoluble in excess.
4. Copper(II), ${Cu}^{2+}$: light blue precipitate, soluble in excess, giving a dark blue solution.
5. Iron(II), ${Fe}^{2+}$: green precipitate, insoluble in excess, turning brown near the surface on standing.
6. Iron(III), ${Fe}^{3+}$: red-brown precipitate, insoluble in excess.
7. Zinc, ${Zn}^{2+}$: white precipitate, soluble in excess, giving a colourless solution.
8. There is no aqueous ammonia test for ammonium ions, because the reagent already contains ammonia.

Telling similar cations apart

1. Several cations give the same precipitate colour, so the result in excess reagent is what tells them apart.
2. White precipitates: aluminium, calcium and zinc.
   1. Calcium is the only one of the three whose precipitate stays in excess sodium hydroxide.
   2. Aluminium and zinc both dissolve in excess sodium hydroxide, but only zinc's precipitate dissolves in excess aqueous ammonia.
3. Green precipitates: chromium(III) and iron(II).
   1. Chromium(III) hydroxide dissolves in excess sodium hydroxide, while iron(II) hydroxide does not and slowly turns brown at the surface.
4. Copper(II) is the only cation in this set that gives a dark blue solution with excess aqueous ammonia.

Flame tests

1. Some metal cations give a characteristic colour to a flame, so a flame test can identify them.
2. The flame colours for the cations you need to know are:
   1. Lithium, ${Li}^{+}$: red.
   2. Sodium, ${Na}^{+}$: yellow.
   3. Potassium, $K^{+}$: lilac.
   4. Calcium, ${Ca}^{2+}$: orange-red.
   5. Barium, ${Ba}^{2+}$: light green.
   6. Copper(II), ${Cu}^{2+}$: blue-green.
3. Lithium, sodium, potassium and barium ions are not identified by the hydroxide tests above, so the flame test is the way to identify them.
4. Calcium and copper(II) ions can be confirmed by both a precipitate test and a flame test.
5. The yellow sodium flame is very bright, so even a trace of sodium on the wire can hide a paler colour such as the lilac of potassium.