This is also a used as a specific test for ammonium
ions.
Alkalis contain hydroxide ions
that are used to precipitate out insoluble metal hydroxides.
Aqueous sodium hydroxide and aqueous ammonia are specific
reagents used to test for cations.
Zn2+
(aq) + 2OH-
(aq)----->Zn(OH)2
(s)
Cu2+
(aq) + 2OH-
(aq)-----> Cu(OH)2
(s)
Metal hydroxides that are amphoteric
in nature e.g. zinc hydroxide, aluminium hydroxide
and lead(II) hydroxide reacts with excess sodium hydroxide
to form complex ions that are soluble.
Zn(OH)2
(s) + 2OH-
(aq)----->Zn(OH)42-
(aq)
Colourless
Complex Ion
When excess aqueous ammonia is added to a blue precipitate
of copper(II) hydroxide, ammonia reacts with the blue
precipitate to form a dark blue solution.
