Test for anions

Halide ion tests

Halide ions are the ions formed by Halogens (Group 7)

For the test to work it must be be acidic.

We use Nitric Acid to acidify the solutions because Hydrochloric acid would add Cl^-ions that would show up in the precipitate.

Sulphuric acid would add Sulphate ions which we also often test for.

Once the solution has been acidified a few drops of Silver Nitrate solution will form a differently coloured precipitate with each halide ion.

This happens because when Silver ions (Ag⁺) meet halide ions they form insoluble solids.

Ag⁺ (aq) + Cl^- (aq)--> AgCl_(s)_{Silver Chloride ppt =White}

Ag⁺ (aq) + Br^- (aq)--> AgBr_(s)Silver Bromide ppt = Cream

Ag⁺ (aq) + l^- (aq)--> Agl_(s)Silver Iodide ppt =Yellow

Sulphate ion test

This test also requires acidifying to work correctly.

Clearly there would be no point acidifying with Sulphuric acid as this contains Sulphate ions.

So, Hydrochloric acid is used.

A few drops of Barium Chloride will form a precipitate since Barium Sulphate is insoluble.

Ba²⁺ (aq) + SO₄^2- (aq)--> BaSO_4(s)_{Precipitate = White}

_{Carbonate ion test}

_{Carbonates react with dilute acids to produce Carbon Dioxide gas}

Carbonate + Acid --> Salt + Water + Carbon Dioxide

So adding a little Hydrochloric acid and allowing the gas produced to bubble through lime-water should produce a milky white precipitate.

Nitrate ion test

Nitrate reacts with aluminum (we use aluminum foil) in the presence of NaOH to produce ammonia.

When heated, if ammonia gas is given off, the unknown substance contains nitrate ions

8Al (s) + 3NO₃^- (aq) + 5OH^- (aq) +2 H₂O (l)--> 3NH_3(g)+ 8AlO₂^- (aq)

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Test for anions

Halide ion tests

Sulphate ion test

Nitrate ion test

Videos

Chloride ion test

Iodide ion test

Sulphate ion test

Carbonate ion test

Nitrate ion test