Test for Ions
Test for carbonate ions
You can test a solution to see if it contains carbonate ions (CO32-) by adding acid.
Acids react with carbonates to form a salt, water and carbon dioxide.
Then test for carbon dioxide by bubbling through limewater and seeing if it turns cloudy.
Test for sulfate ions
We test for the presence of sulfate ions, SO42-, in solution by adding barium chloride (BaCl2).
The sulfate ions displace the chloride ions, forming the insoluble salt barium sulfate (BaSO4). Barium sulfate is a white precipitate — so a positive result will be the appearance of a white solid.
Test for halide ions
We can test for the halide ions (chloride, bromide and iodide ions) using dilute nitric acid followed by silver nitrate.
We add dilute nitric acid to remove any carbonate or sulfate ions which would give a false positive result. Silver ions react with the halide ions to form precipitates of silver chloride, silver bromide or silver iodide which deepen in colour from white, cream then yellow.
The equations for these reactions are:
The issue with this test is that the colours of the precipitates are very similar and subjective. To confirm our result, we can carry out an additional test involving ammonia solution. The three precipitates have different solubilities in ammonia solution, so we can confirm their identity depending on how well they dissolve.
Silver chloride dissolves in dilute ammonia solution
Silver bromide will only dissolve in concentrated ammonia solution
Silver iodide will not dissolve in dilute or concentrated ammonia solution (it is insoluble)
Test for ammonium ions
You can test for ammonium ions, NH4+, by adding sodium hydroxide.
The ammonium ions react with the hydroxide ions to form ammonia gas and water. You then test for the presence of ammonia gas by placing a piece of damp red litmus paper at the open end of the test tube. Ammonia gas reacts with water to form an alkali, turning the red litmus paper blue.
