TEST FOR
Test for Carbonate ion CO32- or
hydrogencarbonate HCO3-
Acid is added to the solid
carbonate in a test tube. You
could also collect a sample of gas
from a heated carbonate, i.e. the
solid is where the liquid is in the
left hand test tube.
Methods of gas preparation are
described in more detail on
another page.
5. INORGANIC Qualitative TESTS Anions and Alkalis
TEST METHOD
OBSERVATIONS
TEST CHEMISTRY
(i) Add any dilute strong
(i) Fizzing - colourless
(i) Any carbonate/hydrogencarbonate + acid ==> salt +
acid to the suspected solid
gas which turns limewater water + carbon dioxide, then white precipitate with
carbonate - if colourless
milky - cloudy fine white
limewater. The ionic equations are for carbonate ...
gas given off, test
precipitate (see above CO2).
with limewater.
CO32-(s) + 2H+(aq) ==> H2O(l) + CO2(g)
(ii) There might be colour
(ii) Effect of fairly strong
changes in the solid, but you and for hydrogencarbonate ...
heating and testing for any
need to collect a sample of
carbon dioxide given off.
gas from just above the
2HCO3-(s) + 2H+(aq) ==> H2O(l) + CO2(g)
heated solid to see it gives a
white precipitate with
Test (ii)
(ii) The thermal decomposition equations are for carbonates
limewater.
will distinguish sodium
hydrogencarbonate
(NaHCO3 readily
Apart from hydrated sodium MCO3(s) ==>MO(s) + CO2(g)
decomposes - 'baking
carbonate, sodium
powder') from anhydrous
hydrogencarbonate is one of e.g. M = Mg, Zn, CuO and note that some give clear colour
sodium carbonate (Na2CO3, the few common carbonates changes in the solid which might be useful to identify the
metal (see heating carbonates in metal cation section)
thermally very stable).
to give off water on heating
and condenses on side of test
tube, but basic carbonates
and for sodium hydrogencarbonate ...
will also give off H2O as well
as CO2.
2NaHCO3(s) ==> Na2CO3(s) +
H2O(l) + CO2(g)
Sulphate ion or sulphate(VI) ion (i) To a solution of the
SO42suspected sulfate add dilute
hydrochloric and a few drops
ofbarium chloride/
If the solution also contains the
chloride ion, you test with barium nitrate solution.
ions 1st, filter off any barium
sulphate precipitate and then test (ii) Add lead(II) nitrate
for chloride ion. This is because solution.
silver sulphate is also ~insoluble.
(i) A white precipitate of
barium sulfate.
(ii) A white precipitate of
lead(II) sulphate.
(i) Ba2+(aq) + SO42-(aq) ==> BaSO4(s)
Any soluble barium salt + any soluble sulphate forms a white
dense barium sulphate precipitate.
(ii) Pb2+(aq) + SO42-(aq) ==> PbSO4(s)
Test (i) is more definitive.
Neither white precipitate is soluble in excess hydrochloric
acid.
Sulphite ion or sulphate(IV) ion (i) Add dilute hydrochloric
SO32acid to the suspected sulfite.
Test (iii) is easily unreliable, the
sulphite ion is oxidised by air
(dissolved oxygen) to give the
sulphate ion, so you will lucky to
obtain a clear solution after
adding excess acid.
Sulphide ion S2-
(i) Acrid choking
sulfur dioxide gas
formed.
(ii) Test any gas evolved with
(ii) The dichromate
fresh potassium
dichromate(VI) paper.
paper turns
from orangeto green.
(iii) Add barium chloride or
(iii) A white ppt. of barium
barium nitrate solution.
sulphite which dissolves in
excess hydrochloric acid to
give a clear colourless
solution.
(i) If soluble, add a few drops (i) Black precipitate of lead
lead(II) ethanoate solution.
sulphide.
In test (ii) dangerous hydrogen
sulphide is formed.
(ii) If solid, add dil. HCl(aq)
(ii) Rotten egg smell of
acid, test smelly gas with
hydrogen sulphide and the
damp lead(II) ethanoate
H2S gas turns lead(II)
paper (old name lead
ethanoate paper black.
acetate).
Chloride ion
(i) If the chloride is soluble,
(i) white precipitate of silver
add dilute nitric acid and
chloride soluble in dilute
silver
nitrate
solution.
The
ammonia.
Cl
silver nitrate is acidified with
dilute nitric acid to prevent the
(ii) You get nasty
If the solution also contains the
precipitation of other nonfumes of hydrogen
sulphate ion, you test with barium
halide silver salts.
chloride
which turn
ions 1st, filter off any barium
bluelitmus
red and give
sulphate precipitate and then test
(ii)
If
insoluble
salt,
add
conc.
a
white
precipitate
with
for chloride ion. This is because
silver nitrate solution.
silver sulphate is also ~insoluble, sulphuric acid, warm if
necessary then test gas as
so the two precipitates of silver
(iii) A white ppt. of lead(II)
sulfate and silver chloride could for HCl.
not be distinguished
chloride is formed.
(iii) Add lead(II) nitrate
(i) Any sulphite salt + hydrochloric acid ==> chloride salt
+ sulphur dioxide.
(ii) The sulphur dioxide reduces the dichromate(VI) to
chromium(III). Note: sulphites do not give ppt. with acidified
barium chloride/nitrate because sulphites dissolve in acids.
(iii) Ba2+(aq) + SO32-(aq) ==> BaSO3(s)
BaSO3(s) + 2HCl(aq) ==> BaCl2(aq) + H2O(l) + SO2(aq)
(i) Pb2+(aq) + S2-(aq) => PbS(s)
(ii) MS(s) + 2H+(aq) => M2+(aq) + H2S(g) (e.g. M = Pb, Fe, Cu, Ni
etc.) Then reaction (i) above occurs on the lead(II) ethanoate
paper (old name lead acetate).
(i) Ag+(aq) + Cl-(aq) ==> AgCl(s)
Any soluble silver salt + any soluble chloride gives a white
silver chloride precipitate, that darkens in light.
(ii) Cl-(s) + H2SO4(l) ==> HSO4-(s) + HCl(g) ,
then Ag+(aq) + Cl-(aq) ==> AgCl(s)
(iii) Pb2+(aq) + 2Cl-(aq) ==> PbCl2(s)
solution. Not a very specific
test - test (i) is best.
Bromide ion
Br-
Fluoride Ion
FFluoride and
hydrogen fluoride
gas are harmful,
irritating and
corrosive substances.
Iodide ion
I-
(i) Cream precipitate of
(i) Ag+(aq) + Br-(aq) ==> AgBr(s)
silver bromide, only soluble in
concentrated ammonia.
Any soluble silver salt + any soluble bromide gives a cream
silver bromide precipitate.
(ii) Orange
vapour of bromine (ii) The bromide ion is oxidised to bromine and the sulphuric
and pungent fumes of
acid is reduced to sulphur dioxide.
SO2, test for sulphur
(ii) If insoluble salt, add conc. dioxide.
(iii) Pb2+(aq) + 2Br-(aq) ==> PbBr2(s)
sulphuric acid, warm if
necessary.
(iii) A white ppt. of lead(II)
bromide is formed.
(iii) Add lead(II) nitrate
solution. Not a very specific
test - test (i) is best.
(i) If the suspected fluoride is (i) There is NO precipitate! (i) Silver fluoride, AgF, is moderately soluble so this test
soluble add dilute nitric acid
proves little except that it isn't chloride, bromide and iodide!
and silver nitrate solution.
(ii) Look for etching
effects on the surface of the (ii) Hydrogen fluoride gas is produced by displacement
(ii) You can warm a solid
glass rod.
fluoride with conc. sulphuric
F- + H2SO4 ==> HSO4- + HF which reacts with the glass silica
acid and hold in the fumes
to form silicic acid, silicon oxyfluoride, silicon fluoride. The
(ONLY!) a glass rod with a
chemistry is messy and complex BUT the glass rod is
drop of water on the end.
clearly etched.
(i) If iodide soluble, add dilute (i) Yellow precipitate of
(i) Ag+(aq) + I-(aq) ==> AgI(s) , any soluble silver salt + any
nitric acid and silver nitrate
silver iodide insoluble in
soluble iodide ==> silver iodide precipitate,
solution. The silver nitrate is concentrated ammonia.
acidified with dilute nitric acid
(ii) iodide ion is oxidised to iodine and the sulphuric acid is
to prevent the precipitation of (ii) purple vapour and rotten reduced to 'rotten eggs' smelly hydrogen sulphide,
other non-halide silver salts.
(i) If bromide soluble, add
dilute nitric acid and silver
nitrate solution. The silver
nitrate is acidified with dilute
nitric acid to prevent the
precipitation of other nonhalide silver salts.
(ii) If insoluble salt can heat
with conc. sulphuric acid, (ii)
get purple fumes of iodine
and very smelly hydrogen
sulphide.
(iii) If iodide soluble, add
lead(II) nitrate solution.
Nitrate ion or nitrate(V) (i) Boil the suspected nitrate
ion NO3with sodium hydroxide
solution and fine aluminium
powder (Devarda's Alloy) or
aluminium foil.
(ii) Add iron(ii) sulphate
solution and then conc.
sulphuric acid (the 'brown
ring' test)
egg smell!
(iii) insoluble lead(II) iodide formed
(iii) Yellow precipitate of
lead(II) iodide. Not too
definitive -Test (i) best.
Pb2+(aq) + 2I-(aq) ==> PbI2(s)
(i) the fumes contain (i) The aluminium powder is a powerful reducing agent and
converts the nitrate ion, NO3-, into ammonia gas, NH3
ammonia, which
turns red litmus
blue, seeammonia t (ii) NO complex of iron(II) formed
est details
(ii) Where the liquids meet
a brown ringforms
(iii) Nasty brown
gas (beware!) of nitrogen
(IV) oxide (nitrogen dioxide)
(iii) a general thermal decomposition equation for this
reaction is
2M(NO3)2(s) ==> 2MO(s) + 4NO2(g) + O2(g)
where M = Pb, Zn, Mg, Cu etc.
(iii) Strongly heating nitrates
of M2+ salts.
Nitrite ion or nitrate(III) No simple test to clearly i.d. it, (i) in acid solution it decomposes to give colourless NO gas which rapidly oxidises to
ion NO2nasty brown fumes of NO2, (ii) it decolourises (purple ==> colourless) acidified potassium manganate(VII), (iii) it liberates
iodine from acidified potassium iodide solution, (iv) forms ammonia with hot Al powder-foil/NaOH(aq) (see nitrate test) and
gives 'brown ring' test - see nitrate tests above.
Alkali: Hydroxide
(i) Litmus or universal
(i) It turns litmus blue,
(i) A pH meter gives a value of more than 7, the higher the
ion i.e. a soluble base indicator or pH meter.
variety of colours univ. ind.
pH number the stronger the alkali, the higher the OH(alkali) which forms the
dark green - violet for weak - concentration, (ii) ammonia gas is evolved:
OH- ion in water (note: (ii) Add a little of an
strong.
to completely identify alkalis
(ii) Ammonia released from the salt.
ammonium salt.
you need to test for the cation
(ii) If strongly alkaline
e.g. sodium for NaOH etc.)
ammonia should be released, NH4+(aq) + OH-(aq) ==> NH3(g) + H2O(l)
see ammonia test for rest of
details
Chromate(VI) ion
(i) Add dilute sulphuric acid.
CrO42- (yellow)
(ii) Add barium
chloride/nitrate solution.
These tests are not very
definitive, but collectively they are (iii) Add lead(II) nitrate
a good 'pointer'!
solution.
(i) The yellow solution
(i) CrO42-(aq) + 2H+(aq) ==> Cr2O72-(aq)
turns orange as the
dichromate(VI) ion is formed. (ii) Ba2+(aq) + CrO42-(aq) ==> BaCrO4(s)
(ii) A yellow precipitate of
barium chromate(VI) is
formed.
(iii) A yellow precipitate of
lead(II) chromate(VI) is
formed. 'lead chromate'
(iii) Pb2+(aq) + CrO42-(aq) ==> PbCrO4(s)