Testing for Cations

Why are aqueous hydroxide ions used to test for cations?

I know the results but I need to know why the hydroxide ions SPECIFICALLY are used

This is a bit of a complicated answer. For transition metals, a ligand exchange reaction is needed in order to properly test and identify cations. When sodium hydroxide is used to test cations, it engages and reacts with metal aquo complex. This interaction then requires the sodium hydroxide to replace one or more of the ligands. Once the sodium hydroxide has replaced the ligand, metal hydroxide is formed. Hydroxides of metals other than group 1 metals are insoluble and form precipitates.This also explains why the hydroxides of non-transition metals appear without color.

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