Tests for cations
- Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide
- If only a small amount of NaOH is used then normally the metal hydroxide precipitates
- The table below contains the results for each of the cations included in the syllabus
- If a precipitate is formed from NaOH then the hydroxide is insoluble in water
| Metal ion | Effect of adding NaOH |
|
Cu2+ |
Light blue precipitate formed |
|
Fe2+ |
Green precipitate formed |
|
Fe3+ |
Brown precipitate formed |
|
NH4+ |
Ammonia gas is produced which turns damp red litmus paper blue |
Examiner Tip
Sometimes you may not see much of a precipitate because the cation you are testing is present is very small amounts. However, every a slight cloudiness or colour change can indicate a positive test result.
