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Tests for NEGATIVE IONS
Gases given off by carbonates (CO32-) and sulphites (SO32-) reacting with hydrochloric acid (HCl) can
be used to identify negative ions.
Carbonates give off CO2 with HCl
With dilute hydrochloric acid, carbonates (CO32-) give off carbon dioxide.
CO32- (s) + 2H+ (aq) -> CO2 (g) + H2O (l)
You can test for carbon dioxide using limewater limewater will turn milky when carbon dioxide is
Sulphites give off SO2 with HCl
Sulphites (SO32-) give off sulphur dioxide when mixed with dilute hydrochloric acid.
SO32- (s) + 2H+ (aq) -> SO2 (g) + H2O (l)
You can test for sodium dioxide using damp potassium dichromate (VI) paper. The paper turns from
orange to green.
Test for sulphates with HCl and barium chloride (BaCl2).
Sulphate ions (SO42-) produce a white precipitate.
To test for a sulphate ion, add dilute hydrochloric acid, followed by barium chloride.
Ba2+ (aq) + SO42- (aq) -> BaSO4 (s)
A white precipitate of barium sulphate means the original compound was a sulphate. The hydrochloric
acid is added to get rid of any traces of carbonate or sulphite ions before you do the test. Both of
these would also produce a precipitate, so they would confuse the results.
Test for halides (Cl-, Br-, I-) with nitric acid and silver nitrate.
To test for chloride, bromide or iodide ions, add dilute nitric acid (HNO3), followed by silver nitrate
Ag+ (aq)+ Cl- (aq) -> AgCl (s) A chloride gives a white precipitate of silver chloride
Ag+ (aq)+ Br- (aq) -> AgBr (s) A bromide gives a cream precipitate of silver bromide
Ag+ (aq)+ I- (aq) -> AgI (s) An iodide gives a yellow precipitate of silver iodide
Again, the acid is added to get rid of carbonate or sulphite ions before the test. You use nitric acid in
this test instead of hydrochloric acid.