Editing Sodium hydroxide (section)

====Precipitant====
Unlike sodium hydroxide, which is soluble, the hydroxides of most transition metals are insoluble, and therefore sodium hydroxide can be used to [[precipitate]] transition metal hydroxides. The following colours are observed:
* Copper - blue
* Iron(II) - green
* Iron(III) - yellow / brown

Zinc and lead salts dissolve in excess sodium hydroxide to give a clear solution of {{chem2|Na2ZnO2}} or {{chem2|Na2PbO2}}.

[[Aluminium hydroxide]] is used as a gelatinous [[Flocculation#Flocculants|flocculant]] to filter out particulate matter in [[water treatment]]. Aluminium hydroxide is prepared at the treatment plant from [[aluminium sulfate]] by reacting it with sodium hydroxide or bicarbonate.

:{{chem2|Al2(SO4)3 + 6 NaOH → 2 Al(OH)3 + 3 Na2SO4}}
:{{chem2|Al2(SO4)3 + 6 NaHCO3 → 2 Al(OH)3 + 3 Na2SO4 + 6 CO2}}