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You need to learn how to test for carbonate ions (CO₃^2-), halide ions (Cl^-, Br^-, I^-) and sulfate ions (SO₄^2-).

The carbonate ion, CO₃^2- will react with a dilute acids to release carbon dioxide gas. Limewater is used to test for carbon dioxide, it turns milky or chalky when CO₂ is bubbled through it. The set-up is shown opposite, approximately 10ml of dilute hydrochloric acid is added to a few granules of the suspected metal carbonate. If carbonate ions are present then CO₂ gas will be released, this will bubble through the limewater and turn it a milky/chalky colour, e.g if the carbonate is calcium carbonate then the following reaction will occur:

calcium carbonate_(s) + hydrochloric acid_(aq) → calcium chloride_(aq) + carbon dioxide_(g) + water_(l)

CaCO_3(s) + 2HCl_(g) → CaCl_2(aq) + CO_2(g) + H₂O_(l)

If a solution contains sulfate ions and a few drops of barium chloride solution are added an insoluble white precipitate of barium sulfate will form. However if the solution also contains carbonate ions then a white precipiate of barium carbonate will also form. So it might seem that this is not really a very good test for sulfate ions as it also gives a positive test for carbonate ions, but if a few drops of hydrochloric acid are added then any carbonate present will react with the acid to release carbon dioxide gas, as in the above test for carbonate. However no fizzing is seen if only the sulfate ion is present. An outline of the test is shown below:

Testing for the halide ions, chloride, bromide and iodide ions.

Halide ions are chloride, bromide and iodide ions. Most compounds containing the chloride, bromide and iodide ions that you will meet are likely to be soluble, but silver chloride, bromide and iodide are insoluble and have different colours. Silver chloride is white, silver bromide is cream and silver iodide is yellow. To test for the presence of chloride (Cl-), bromide(Br-) and iodide (I-) ions in solution simply add a few drops of nitric acid (the acid is needed in case carbonate ions are present) and then add a few drops of the silver nitrate solution. If the halides are present then a solid insoluble precipitate will be produced. This is summarised in the diagram below.

The equations below may help you to understand these reactions in a little more detail.

sodium chloride_(aq) + silver nitrate_(aq) → silver chloride_(s) + sodium nitrate dioxide_(aq)

NaCl_(aq) + AgNO_3(aq) → AgCl_(s) + NaNO_3(aq)

sodium bromide_(aq) + silver nitrate_(aq) → silver bromide_(s) + sodium nitrate dioxide_(aq)

NaCl_(aq) + AgNO_3(aq) → AgBr_(s) + NaNO_3(aq)

sodium iodide_(aq) + silver nitrate_(aq) → silver iodide_(s) + sodium nitrate dioxide_(aq)

NaI_(aq) + AgNO_3(aq) → AgI_(s) + NaNO_3(aq)

Key Points

• Carbonate ions react with acid, dilute hydrochloric is usually used, to release carbon dioxide gas. Carbon dioxide gas can be detect by bubbling it through limewater. Limewater turns milky or chalky when CO ₂ is bubbled through it.
• Sulfate ions form an insoluble colourless (white) precipitate of barium chloride, this is used to test for the presence of sulfate ions. Acidify the unknown solution by adding hydrochloric acid, then add barium chloride solution. If an insoluble white precipiate forms then this will confirm the presence of sulfate ions. There should be no fizzing. If the solution fizzes then carbonate ions are also present. You should never use sulfuric acid to acidify the unknown solution as this contains sulfate ions.
• The silver nitrate test is used to identify chloride, bromide and iodide ions. The unknown sample can be dissolved in nitric acid and then a few drops of silver nitrate are added. A white precipitate will show the presence of chloride ions, a cream precipitate bromide ions and a yellow precipitate will confirm the presence of iodide ions. It is worth noting that silver chloride, silver bromide and silver iodide are all light sensitive chemical and that if left exposed to light then a black precipiate of silver ions will form. So record any colour changes after the addition of silver nitrate solution to your test solution should be observed quickly and the solutions disposed of quickly.

Check your understanding - Questions on tests for anions

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