| Anion | Test | Observations for positive test and inference |
|---|---|---|
| CO32- | add any dilute acid bubble the produced gas into limewater | effervesence is observed gas given off forms a white precipitate of CaCO3 when bubbled in limewater. carbon dioxide is the gas produced. |
| Cl- | add dilute nitric acid, then add aqueous silver nitrate. | white precipitate of silver chloride, is formed. |
| I- | add dilute nitric acid, then add aqueous silver nitrate | yellow precipitate of silver iodide, is formed. |
| SO42- | add dilute nitric acid, then add aqueous barium nitrate | white precipitate of barium sulfate is formed. |
| NO3- | add aqueous sodium hydroxide, then add a piece of aluminum, warm the mixture carefully. test the gas given off with a piece of damp red litmus paper | Effervescence is observed. the gas given off turns damp red litmus paper blue ammonia gas is produced. |
Why is dilute nitric acid added?
added in test to identify chloride, iodide and sulfate ions. → removes presence of any carbonate or hydroxide ions - ensuring there is no error in the analysis obtained. (prevent precipitation of xxx carbonate/ xxx hydroxide)
Dilute HCl and dilute H2SO4 cannot be used → as they produce Cl- and SO42- ions, which could cause any unwanted precipitates.
Points:
- for test of Cl-, I-, and SOr2-, dilute nitric acid is added - to remove presence of any CO32- ions -which usually tend to form insoluble salts.
- sometimes in test for chloride/iodide - lead(II)nitrate is used. this is not the best reagent as both lead(II)sulfate and lead(II)chloride → are insoluble and white, which means results will not be conclusive.
- to test for sulfate ions - sometimes barium chloride can be used. White precipitate of barium sulfate will still form . (not good as aqueous barium nitrate as well as white precipitate of silver chloride may be formed if unknown happens to be aq silver nitrate)
NaOH + Warming = test for ammonium cation