The anions SO
42– (sulfate), chloride (Cl
–), iodide (I
–), carbonate (CO
32–) and OH
– (hydroxide) can be distinguished from one another in aqueous solution by
dipping red litmus paper into the solution
CO32– and OH– are basic and turn red litmus blue
adding aqueous acid (if the litmus has turned blue)
CO32– reacts with H+ to form H2CO3 which decomposes to H2O and CO2 which bubbles out of solution
adding acidified barium chloride to a new sample (if the litmus stayed red)
SO42– forms a white precipitate of BaSO4
adding aqueous silver nitrate to a new sample (if none of the above tests were positive)
Cl– and I– form insoluble precipitates AgCl (white) and AgI (cream to yellow)
AgCl dissolves on addition of aqueous ammonia to form Ag(NH3)2+, AgI remains undissolved.