[darrenfoong]

I understand that amphoteric hydroxides like lead (II) hydroxide, aluminium hydroxide, and zinc hydroxide can dissolve in excess sodium hydroxide solution to form a complex (aka aluminates, zincates, and plumbates)

Wikipedia didn't provide very very reliable information for me, and so I would like to know the exact reaction of these three hydroxides with sodium hydroxide to form sodium [something]. I'm just asking this out of curiosity.

And BTW, why does zinc hydroxide dissolve in aqueous ammonia, but lead (II) hydroxide and aluminium hydroxide does not? (if possible, can provide reactions).

Thanks!

[AWK]

Formation of complexes is an intrinsic property of both: cation of metal and ligand.
Some cations of main groups of metals show relatively great affinity to hydroxide anions to form hydroxo complexes. Ammonia itself shows too low OH^- concentration to form a significant amount of hydroxocomplexes ot these metal cations.
Transion metal cations (d-block elements) very often show a great affinity to ammonia to form ammonia complexes.

Wikipedia is not a texbook on chemistry!

[darrenfoong]

Thanks for the information - I always knew Wikipedia wasn't very reliable in science.

I would like to ask another question: my teacher told me that to check for the presence of nitrate or nitrite anions, aqueous sodium hydroxide and aluminium is added. The resulting mixture is warmed, and the anions are proved to be present if ammonia is evolved.

However, he didn't explain why, and only said it was a "test for anions". Could anybody explain what exactly happens? I don't like knowing without understanding.

[AWK]

And hi is right. Reduction of nitrite or nitrate with zinc (or aluminium, or Devarda alloy) in NaOH solution gives ammonia.
This is a standard test fot these anions in qualitative inorganic analysis

[Borek]

Thanks for the information - I always knew Wikipedia wasn't very reliable in science.

It is not about reliability. In my experience Wikipedia in science is usually very reliable, as long as you remember to think when reading (same happens with every other source BTW).

However, it is not a textbook, it is an encyclopedia.