Testing of aqueous cations with aqueous sodium hydroxide and aqueous ammonia:
| Cation | When aqueous sodium hydroxide is added | When aqueous ammonia is added |
| aluminium, Al³⁺ | White ppt formed, soluble in excess aqueous sodium hydroxide, to give a colourless solution. | White ppt formed, insoluble in excess aqueous ammonia. |
| ammonium, NH₄⁺ | Ammonia gas is liberated when a solution containing ammonium (NH₄⁺) is heated with aqueous sodium hydroxide. | – |
| calcium, Ca²⁺ | White ppt formed, insoluble in excess aqueous sodium hydroxide. | No ppt or slight white ppt observed. |
| chromium (III), Cr³⁺ | Green ppt formed, soluble in excess aqueous sodium hydroxide, to give a green solution. | Green ppt formed, insoluble in excess aqueous ammonia. |
| copper(II), Cu²⁺ | Light blue ppt formed, insoluble in excess aqueous sodium hydroxide. | Light blue ppt formed, soluble in excess aqueous ammonia to give a dark blue solution. |
| iron (II), Fe²⁺ | Green ppt, insoluble in excess aqueous sodium hydroxide. Green ppt turns brown near surface on standing. | Green ppt, insoluble in excess aqueous ammonia. Green ppt turns brown near surface on standing. |
| iron (III), Fe³⁺ | Red-brown ppt, insoluble in excess aqueous sodium hydroxide. | Red-brown ppt, insoluble in excess aqueous ammonia. |
| zinc, Zn²⁺ | White ppt formed, soluble in excess aqueous sodium hydroxide, to give a colourless solution. | White ppt formed, soluble in excess aqueous ammonia, to give a colourless solution. |
**ppt is the abbreviation for precipitate. A pre (a solid
Watch the video below on the test for the various cations.
In the video above, notice that a “white tile” is used to help us see clearly whether the precipitate has dissolved or not. During your practical, if you do not have a white tile, use a white piece of paper instead.
** Note that the above vide does not have the test for chromium ions.
Watch this video here for the test on chromium (III) ions: