Reagents: Sodium Hydroxide, Aqueous ammonia.
all cations give precipitate with these 2 reagents except for K+, Na+, and NH4+
THE MOST IMPORTANT TABLE TO MEMORISE:
| Cation | Sodium Hydroxide | Aqueous Ammonia |
|---|---|---|
| Al 3+ | On adding a few drops: White Precipitate of Aluminum Hydroxide when added in excess: Precipitate dissolves in excess to form a colorless solution | On adding a few drops: White precipitate of Aluminum Hydroxide. when added in excess: Precipitate is insoluble |
| Zn2+ | on adding a few drops: white precipitate of zinc hydroxide when excess is added: the precipitate dissolves in excess to form a colorless solution | on adding a few drops: white precipitate of zinc hydroxide when excess is added: the precipitate dissolves in excess to form a colorless solution. |
| Ca2+ | on adding few drops: White precipitate of Calcium Hydroxide when excess is added: precipitate is insoluble in excess | upon adding a few drops: no precipitate when excess is added: No change is observed |
| Cu2+ (copper(II)) | on adding few drops: light blue precipitate of copper(II)hydroxide when excess is added: precipitate is insoluble in excess | on adding few drops: light blue precipitate of copper(II)hydroxide when excess is added: precipitate dissolves to form a clear blue/dark blue solution |
| Fe2+ (Iron(II)) | on adding few drops: green precipitate of iron(II)hydroxide when excess is added: precipitate is insoluble in excess. it turns brown on standing | on adding few drops: dirty green precipitate of iron(II)hydroxide when excess is added: precipitate is insoluble in excess. it turns brown on standing. |
| Fe3+ (iron(III)) | on adding few drops: Reddish brown precipitate of iron(III)hydroxide when excess is added: precipitate is insoluble in excess | on adding few drops: Reddish brown precipitate of iron(III)hydroxide when excess is added: precipitate is insoluble in excess |
Concept Behind this
aqueous ammonia and sodium hydroxide - contain hydroxide ions - that will combine with the metal ions - to form selected metal hydroxides with distinct colors. → known as precipitation
This works because most metal hydroxides are insoluble.
maybe to memorize:
- no precipitate is soluble except for: Al3+ in excess sodium hydroxide Zn2+ in both excess sodium hydroxide and excess aqueous ammonia. Cu2+ in excess aqueous ammonia
Formation of Precipitates upon addition of reagents
precipitate formed in each of the reactions (in table above) → is the hydroxide of the metal ion.
most hydroxides → are insoluble in water except for group 1 hydroxides (eg: NaOH) and aqueous ammonia, NH3(aq)
example → when aqueous sodium hydroxide or aqueous ammonia is added to a solution containing iron(III)ions → the red brown precipitate formed is iron (III)hydroxide, Fe(OH)3
iron(III) ion + hydroxide ion → Iron(III)hydroxide
Fe3+ (aq) + 3OH-(aq) → Fe(OH)3(s)
How to write the ionic equation for formation of precipitates Whatever the cation is, just put down the number of OH- ions so that it balances out. Example: