TEST FOR CATIONS
Reagents needed:
Sodium hydroxide (NaOH) and Aqueous ammonia [NH3(aq)]
Cation
Insoluble
NH3(aq)
dropwise
Green ppt
NH3(aq)
excess
Insoluble
Red-brown
ppt
Insoluble
Red-brown ppt
Insoluble
Fe3+(aq) + 3OH- (aq) → Fe(OH)3 (s)
Copper (II) ion
Cu2+
Pale blue ppt
Insoluble
Pale blue ppt
Cu2+(aq) + 2OH- (aq) → Cu(OH)2 (s)
Chromium (III) ion
Cr3+
Gray-green
ppt
Dissolves in
excess to
produce a dark
green solution
Gray-green ppt
Dissolves in
excess to
produce a
dark blue
solution
Insoluble
Zinc ion
Zn2+
White ppt
Dissolves in
excess to
produce a
colourless
solution
White ppt
Dissolves in
excess to
produce a
colourless
solution
Zn2+(aq) + 2OH- (aq) → Zn(OH)2 (s)
Aluminium ion
Al3+
White ppt
Dissolves in
excess to
produce a
colourless
solution
White ppt
Insoluble
Al3+(aq) + 3OH- (aq) → Al(OH)3 (s)
Calcium ion
Ca2+
White ppt
Insoluble
No visible
change
No visible
change
Ca2+(aq) + 2OH- (aq) → Ca(OH)2 (s)
Ammonium ion
NH4+
No visible
change
Upon heating,
a pungent
smelling
ammonia gas
evolves that
turns damp red
litmus paper
blue.
Iron (II) ion
Fe2+
Iron (III) ion
Fe3+
NaOH
dropwise
Green ppt
NaOH excess
Ionic Equation
Fe2+(aq) + 2OH- (aq) → Fe(OH)2 (s)
Cr3+(aq) + 3OH- (aq) → Cr(OH)3 (s)
NH4+ (aq) + OH- (aq) → NH3 (g)+ H2O(l)