19.4 Testing for Cations

Lithium Flame

Li+, red

Sodium Flame

Na+, yellow

Potassium Flame

K+, lilac

Calcium Flame

Ca2+, orange-red

Barium Flame

Ba2+, light green

Copper Flame

Cu2+, blue-green

Ammonium Test

NH4+, add dilute aqueous sodium hydroxide and heat gently. Ammonia gas should be given off.
NH4+ + OH- → NH3 + H2O

Iron

Fe2+ add dilute aqueous sodium hydroxide or aqueous ammonia. A pale green precipitate forms.
Fe23+ add dilute aqueous sodium hydroxide or aqueous ammonia. A red-brown precipitate forms.

Zinc

Zn2+ add dilute aqueous sodium hydroxide or aqueous ammonia. A white solution forms and dissolves in Excess of either.

General Formula for metal reactions with dilute aqueous sodium hydroxide

The general reaction: Metaln+ + nOH- → Metal(OH)n

The reason why metals dissolve in excess dilute aqueous sodium hydroxide

This is because the metal hydroxide is amphoteric.

The reason why metals dissolve in excess ammonia

This is because it forms complex ions with ammonia that dissolves.

Aluminum

Al3+, DASH makes a white precipitate form. It dissolves in excess forming a colourless solution. Ammonium makes a white precipitate form, it does not dissolve in excess.

Calcium

Ca2+, with DASH this forms a white precipitate that is not dissolved by excess.
With ammonnium a very slight white or no precipitate forms.

Copper

Cu2+, DASH reacts with it to form a pale blue precipitate that does not dissolve in excess.
In ammonium a pale blue precipate forms and gives a dark blue solution when excess is added.

Chromium

Cr3+, DASH creates a green precipitate. It dissolves in excess to give a clear solution. In ammonnia a grey-green precipate forms and excess has no effect.