Sometimes we can use the reactions of the unknown compounds with Sodium Hydroxide Solution to help us with our identifcation. Aluminium ions, Calcium ions and Magnesium ions all form white precipitates with Sodium Hydroxide solution. So if we add Sodium Hydroxide to an unknown compound and a white precipitate forms we know it contains either Aluminium, Calcium or Magnesium ions.
If we add more and more Sodium Hydroxide then the precipitate formed with Aluminium ions dissolves - but a precipitate formed with Calcium or Magnesium ions will not. Calcium and Magnesium ions can be distinguished by a flame test. Calcium ions give a brick red flame, but Magnesium ions produce no colour at all.
Some metal ions form coloured precipitates with Sodium Hydroxide.
- If we add Sodium Hydroxide solution to a substance containing Copper ions a light blue precipitate appears
- If the substance contains Iron(II) ions a dirty green precipitate is produced when Sodium Hydroxide solution is added.
- When Sodium Hydroxide solution is added to Iron(III) ions, a reddish brown precipitate is produced.
Comments
Report