C7.10 Precipitation reactions and tests for ions
Compounds containing a metal or ammonium in the name are ionic. These compounds contain ions
which can be worked out from the name. Eg. aluminium sulfate contains an aluminium Al3+ and
sulfate ions SO42-.
ion name
ion formula
example compound with ion
hydrogen
H+
sodium Na+
sodium bromide NaBr
potassium
K+
potassium iodide KI
aluminium
Al3+
aluminium sulfate Al2(SO4)3
calcium
Ca2+
calcium sulfate CaSO4
copper Cu2+
copper sulfate CuSO4
iron II Fe2+
iron II sulfate FeSO4
iron III Fe3+
iron III sulfate Fe2(SO4)3
ammonium
NH4+ ammonium chloride NH4Cl
carbonate
CO32- potassium carbonate K2CO3
any acid e.g. hydrochloric acid HCl
sulfite SO32- calcium sulfite CaSO3
sulfate SO42- magnesium sulfate MgSO4
chloride
Cl-
lead chloride PbCl2
bromide
Br-
silver bromide AgBr
iodide I-
sodium iodide NaI
hydroxide
OH-
sodium hydroxide NaOH
If any of the above ions that can combine to make an insoluble compound are present in a mixture
of ionic compounds in solution then they will do so. The result will be the formation of a precipitate.
e.g. You can work out that iron II hydroxide is insoluble and that iron II sulfate and sodium hydroxide
are soluble. When mixed that following ions are present; iron II, hydroxide, sulfate, and sodium.
The result will be a precipitate of iron II hydroxide.
learning activity - name both of the ions in each of the compounds in the table above.
list the soluble and insoluble compounds in the table
name pairs of soluble compounds from the table and name the precipitate formed
ests for common ions
name of ion
test
result of positive test
indicator turns red
hydrogen
add universal indicator
hydrogen
add carbonate
effervescence
carbon dioxide detected
effervescence hydrogen
hydrogen
add magnesium
sodium
detected
flame test
yellow flame
potassium
flame test
calcium
flame test
copper
flame test
aluminium
calcium
copper
iron II
lilac flame
brick red flame
blue/green flame
white precipitate forms
add drop aqueous sodium hydroxide
white precipitate
add excess aqueous sodium hydroxide
redissolves
fine white precipitate forms
add drop aqueous sodium hydroxide
precipitate does not
add excess aqueous sodium hydroxide
dissolve
add drop aqueous sodium hydroxide
blue precipitate form
add drop aqueous sodium hydroxide
green precipitate forms
add drop aqueous sodium hydroxide
brown precipitate forms
ammonium
add 5cm3 of sodium hydroxide and
warm
ammonia detected
carbonate
add a little dilute acid
effervescence
carbon dioxide detected
iron III
sulfite
add a little dilute hydrochloric acid,
sulfur dioxide detected
warm
add dilute hydrochloric acid then a few
white precipitate formed
drops of aqueous barium chloride
sulfate
add dilute nitric acid then a few drops
white precipitate forms
chloride
bromide
iodide
of silver nitrate
add dilute nitric acid then a few drops
of silver nitrate
add dilute nitric acid then a few drops
of silver nitrate
buff precipitate forms
yellow precipitate form
indicator turns blue
hydroxide
add universal indicator
hydroxide
add ammonium compound and warm
ammonia detected
C7.12 explaining ion tests with equations
hydrogen ions
hydrochloric acid + magnesium ---> magnesium chloride + hydrogen
2HCl(aq) + Mg(s) ---> MgCl2(aq) + H2(aq)
2H+(aq) + Mg(s) ---> Mg2+(aq) + H2(g)
nitric acid + sodium carbonate --> sodium nitrate + water + carbon dioxide
2HNO3(aq) + Na2CO3(aq) ---> 2NaNO3(aq) +H2O(l) + CO2(g)
2H+(aq) + CO32-(aq) ---> H2O(l) + CO2(g)
Aluminium ions
aluminium sulfate + sodium hydroxide ---> aluminium hydroxide + sodium
sulfate
Al2(SO4)3(aq) + 6NaOH(aq) ---> 2Al(OH)3(s) + 3Na2SO4(aq)
2Al3+(aq) + 6OH- ---> 2Al(OH)3(s)
aluminium hydroxide + sodium hydroxide ---> sodium aluminate
Al(OH)3(s) + NaOH(aq) ---> NaAl(OH)4(aq)
Al(OH)3(s) + OH-(aq) ---> Al(OH)4-(aq)
calcium ions
calcium nitrate + sodium hydroxide ---> calcium hydroxide + sodium nitrate
Ca(NO3)2(aq) + 2NaOH(aq) ---> Ca(OH)2(s) + 2NaNO3(aq)
Ca2+(aq) + 2OH-(aq) ---> Ca(OH)2(s)
copper ions
copper chloride + sodium hydroxide ---> copper hydroxide + sodium chloride
CuCl2(aq) + NaOH(aq) ---> Cu(OH)2(s) + NaCl(aq)
Cu2+(aq) + OH-(aq) ---> Cu(OH)2(s)
iron II ions
iron II chloride + sodium hydroxide ---> iron II hydroxide + sodium chloride
FeCl2(aq) + NaOH(aq) ---> Fe(OH)2(s) + NaCl(aq)
Fe2+(aq) + OH-(aq) ---> Fe(OH)2(s)
iron III ions
iron III sulfate + sodium hydroxide ---> iron III sulfate + sodium sulfate
Fe2(SO4)3(aq) + 6NaOH(aq) ---> 2Fe(OH)3(aq) + 3Na2SO4(aq)
Fe3+(aq) + 3OH-(aq) ---> Fe(OH)3(aq)
ammonium ions or hydroxide ions
ammonium chloride + sodium hydroxide ---> sodium chloride + water +
ammonia
NH4Cl(aq) + NaOH(aq) ---> NaCl(aq) + H2O(l) + NH3(g)
NH4+(aq) + OH-(aq) ---> H2O(l) + NH3(g)
carbonate ions
sodium carbonate + nitric acid --->sodium nitrate + water + carbon dioxide
Na2CO3(aq) + 2HNO3(aq) ---> 2NaNO3(aq) + H2O(aq) + CO2(aq)
CO32-(aq) + 2H+(aq) --- > H2O(aq) + CO2(aq)
sulfite
sodium sulfite + hydrochloric acid ---> sodium chloride + water + sulfur
dioxide
Na2SO3(aq) + 2HCl ---> 2NaCl(aq) + H2O(aq) + SO2(aq)
SO32-(aq) + 2H+(aq) ---> H2O(aq) + SO2(aq)
sulfate
sodium sulfate + barium chloride ---> barium sulfate + sodium chloride
Na2SO4(aq) + BaCl2(aq) ---> BaSO4(s) + 2NaCl(aq)
SO42-(aq) + Ba2+(aq) ---> BaSO4(s)
chloride
sodium chloride + silver nitrate ---> silver chloride + sodium nitrate
NaCl(aq) + AgNO3(aq) ---> AgCl(s) + NaNO3(aq)
Cl-(aq) + Ag+(aq) ---> AgCl(s)
bromide
sodium bromide + silver nitrate ---> silver bromide + sodium nitrate
NaBr(aq) + AgNO3(aq) ---> AgBr(s) + NaNO3(aq)
Br-(aq) + Ag+(aq) ---> AgBr(s)
iodide
sodium iodide + silver nitrate ---> silver iodide + sodium nitrate
NaI(aq) + AgNO3(aq) ---> AgI(s) + NaNO3(aq)
I-(aq) + Ag+(aq) ---> AgI(s)