You need to learn how to test for carbonate ions (CO32-), halide ions (Cl-, Br-, I-) and sulfate ions
Testing for carbonate ions (CO32-)
The caronbate ion, CO32- will react with a dilute acid
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 milky/chalky, 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)
CaCO3(s) + 2HCl(g) → CaCl2(aq) + CO2(g) + H2O(l)
Test for sulfate ions (SO4)
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 Cl-, Br- and I- ions in solution
simply add a few drops of nitric acid 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) + AgNO3(aq) → AgCl(s) + NaNO3(aq)
and with bromide ions:
sodium bromide(aq) + silver nitrate(aq) → silver bromide(s) + sodium nitrate dioxide(aq)
NaCl(aq) + AgNO3(aq) → AgBr(s) + NaNO3(aq)
and with iodide ions
sodium iodide(aq) + silver nitrate(aq) → silver iodide(s) + sodium nitrate dioxide(aq)
NaI(aq) + AgNO3(aq) → AgI(s) + NaNO3(aq)
- 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.