text: 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  or .
  Aluminium hydroxide is used as a gelatinous 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.