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5. INORGANIC Qualitative TESTS Anions and Alkalis |
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| TEST FOR | TEST METHOD | OBSERVATIONS | TEST CHEMISTRY | |
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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.
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(i) Add
any dilute strong acid to the suspected solid carbonate - if colourless
gas given off, test with limewater. (ii) Effect of fairly strong heating and testing for any carbon dioxide given off. Test (ii) will distinguish sodium hydrogencarbonate (NaHCO3 readily decomposes - 'baking powder') from anhydrous sodium carbonate (Na2CO3, thermally very stable). |
(i) Fizzing
- colourless gas which turns limewater
milky cloudy
(see above CO2). (ii) There might be colour changes in the solid, but you need to collect a sample of gas from just above the heated solid to see it gives a white precipitate with limewater. Apart from hydrated sodium carbonate, sodium hydrogencarbonate is one of the few common carbonates to give off water on heating and condenses on side of test tube, but basic carbonates will also give off H2O as well as CO2. |
(i)
Any carbonate/hydrogencarbonate + acid ==> salt + water +
carbon dioxide,
then white precipitate with limewater. The ionic equations are for
carbonate ... CO32-(s) + 2H+(aq) ==> H2O(l) + CO2(g) and for hydrogencarbonate ... 2HCO3-(s) + 2H+(aq) ==> H2O(l) + CO2(g) (ii) The thermal decomposition equations are for carbonates MCO3(s) ==>MO(s) + CO2(g) e.g. M = Mg, Zn, CuO and note some give clear colour changes in the solid which might be useful to identify the metal (see heating carbonates in metal cation section) and for sodium hydrogencarbonate ... 2NaHCO3(s)
==> Na2CO3(s) + H2O(l) +
CO2(g) |
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Sulphate
ion or sulphate(VI) ion SO42- If the solution also contains the chloride ion, you test with barium ions 1st, filter off any barium sulphate precipitate and then test for chloride ion. This is because silver sulphate is also ~insoluble. |
(i) To
a solution of the suspected sulfate add dilute hydrochloric and a
few drops of barium chloride/ nitrate solution. (ii) Add lead(II) nitrate solution. |
(i)
A white
precipitate of barium sulfate. (ii) A white precipitate of lead(II) sulphate. Test (i) is more definitive. |
(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) Neither white precipitate is soluble in excess hydrochloric acid. |
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Sulphite
ion or sulphate(IV) ion
SO32- 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. |
(i)
Add
dilute hydrochloric acid to the suspected sulfite. (ii) Test any gas evolved with fresh potassium dichromate(VI) paper. (iii) Add barium chloride or barium nitrate solution. |
 (i)
Acrid
choking sulfur dioxide gas formed.(ii) The dichromate paper turns from orange to green. (iii) A white ppt. of barium sulphite which dissolves in excess hydrochloric acid to give a clear colourless solution. |
(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) |
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Sulphide
ion S2-
In test (ii) dangerous hydrogen sulphide is formed. |
(i)
If soluble, add a few drops lead(II) ethanoate solution.
(ii) If solid, add dil. HCl(aq) acid, test smelly gas with damp lead(II) ethanoate paper (old name lead acetate). |
(i)
Black ppt. of lead sulphide.
(ii) Rotten egg smell of hydrogen sulphide and the H2S gas turns lead(II) ethanoate paper black. |
(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). |
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Chloride
ion Cl- If the solution also contains the sulphate ion, you test with barium ions 1st, filter off any barium sulphate precipitate and then test for chloride ion. This is because silver sulphate is also ~insoluble, so the two precipitates of silver sulfate and silver chloride could not be distinguished |
(i)
If the chloride is soluble, add dilute nitric acid and silver nitrate
solution. The silver nitrate is acidified with dilute nitric acid to
prevent the precipitation of other non-halide silver salts. (ii) If insoluble salt, add conc. sulphuric acid, warm if necessary then test gas as for HCl. (iii) Add lead(II) nitrate solution. Not a very specific test - test (i) is best. |
(i)
white precipitate of silver chloride soluble in dilute ammonia.
(iii) A white ppt. of lead(II) chloride is formed. |
(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) |
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Bromide
ion
Br- |
(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 non-halide silver salts. (ii) If insoluble salt, add conc. sulphuric acid, warm if necessary. (iii) Add lead(II) nitrate solution. Not a very specific test - test (i) is best. |
(i) Cream precipitate of silver
bromide, only soluble in concentrated ammonia.
(iii) A white ppt. of lead(II) bromide is formed. |
(i)
Ag+(aq) + Br-(aq)
==> AgBr(s)
Any soluble silver salt + any soluble bromide gives a cream silver bromide precipitate. (ii) The bromide ion is oxidised to bromine and the sulphuric acid is reduced to sulphur dioxide. (iii)
Pb2+(aq) + 2Br-(aq)
==> PbBr2(s) |
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Fluoride Ion F-
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(i) If the suspected fluoride is
soluble add dilute nitric acid and silver nitrate solution. (ii) You can warm a solid fluoride with conc. sulphuric acid and hold in the fumes (ONLY!) a glass rod with a drop of water on the end. |
(i) There is NO precipitate! (ii) Look for etching effects on the surface of the glass rod. |
(i) Silver fluoride, AgF, is
moderately soluble so this test proves little except that it isn't
chloride, bromide and iodide! (ii) Hydrogen fluoride gas is produced by displacement F- + H2SO4 ==> HSO4- + HF which reacts with the glass silica to form silicic acid, silicon oxyfluoride, silicon fluoride. The chemistry is messy and complex BUT the glass rod is clearly etched. |
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Iodide ion I- |
(i)
If iodide soluble, add dilute nitric acid and silver nitrate solution.
The silver nitrate is acidified with dilute nitric acid to prevent the
precipitation of other non-halide 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. |
(i)
Yellow precipitate of silver
iodide insoluble in concentrated ammonia. (ii) purple vapour and rotten egg smell! (iii) Yellow precipitate of lead(II) iodide. Not too definitive -Test (i) best. |
(i)
Ag+(aq) + I-(aq)
==> AgI(s)
, any soluble silver salt
+ any soluble iodide ==> silver iodide precipitate,
(ii) iodide ion is oxidised to iodine and the sulphuric acid is reduced to 'rotten eggs' smelly hydrogen sulphide, (iii) insoluble lead(II) iodide formed Pb2+(aq) + 2I-(aq) ==> PbI2(s) |
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Nitrate
ion or nitrate(V) ion
NO3- |
(i) Boil
the suspected nitrate with 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) (iii) Strongly heating nitrates of M2+ salts. |
(i)
the
fumes contain ammonia, which turns red litmus blue,
see
ammonia test
details
(ii) Where the liquids meet a brown ring forms (iii) Nasty brown gas (beware!) of nitrogen (IV) oxide (nitrogen dioxide) |
(i) The
aluminium powder is a powerful reducing agent and converts the nitrate
ion, NO3-, into ammonia gas, NH3
(ii) NO complex of iron(II) formed (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. |
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Nitrite
ion or nitrate(III) ion
NO2- |
No simple test to clearly i.d. it, (i) in acid solution it decomposes to give colourless NO gas which rapidly oxidises to nasty 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. | |||
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Ammonium
ion NH4+ |
If no smell at first, add COLD sodium hydroxide solution to the suspected ammonium salt and test any gas with red litmus. |
Smelly
ammonia released! and red litmus turns blue. |
Ammonia
gas is evolved: NH4+(aq) + OH-(aq) ==> NH3(g) + H2O(l) |
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Acids
Hydrogen
ion i.e. H+ or H3O+ ion (note:
to completely identify acids you need to test for the anion e.g. chloride
for HCl etc.) |
(i)
Litmus or universal indicator or pH meter.
(ii) Add a little sodium hydrogencarbonate powder. |
(i) Litmus turns red, variety of colours with univ. ind. strong - red, weak
- yellow /orange, depending on strength of acid. (ii) Fizzing with any carbonate - test for CO2 as above. |
(i) A
pH meter reading gives a value of less than 7, the lower the pH number the
stronger the acid, the higher the H+ concentration,
(ii) HCO3-(aq) + H+(aq) ==> H2O(l) + CO2(g) |
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Alkali:
Hydroxide
ion i.e. a soluble base (alkali) which forms the OH- ion in water (note:
to completely identify alkalis you need to test for the cation e.g. sodium
for NaOH etc.) |
(i)
Litmus or universal indicator or pH meter. (ii) Add a little of an ammonium salt. |
(i)
It turns litmus blue, variety of colours univ. ind. dark green - violet for
weak - strong. (ii) If strongly alkaline ammonia should be released, see ammonia test for rest of details |
(i) A pH
meter gives a value of more than 7, the higher the pH number the
stronger the alkali, the higher the OH- concentration, (ii) ammonia
gas is evolved:
(ii) Ammonia released from the salt. NH4+(aq)
+ OH-(aq) ==> NH3(g) + H2O(l) |
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Chromate(VI) ion CrO42- (yellow) These tests are not very definitive, but collectively they are a good 'pointer'! |
(i) Add dilute sulphuric acid. (ii) Add barium chloride/nitrate solution. (iii) Add lead(II) nitrate solution. |
(i) The yellow solution turns orange
as the dichromate(VI) ion is formed. (ii) A yellow precipitate of barium chromate(VI) is formed. (iii) A yellow precipitate of lead(II) chromate(VI) is formed. 'lead chromate' |
(i) CrO42-(aq)
+ 2H+(aq) ==> Cr2O72-(aq) (ii) Ba2+(aq) + CrO42-(aq) ==> BaCrO4(s) (iii) Pb2+(aq) + CrO42-(aq) ==> PbCrO4(s) |
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Fluoride
and hydrogen fluoride gas are harmful, irritating and corrosive substances.
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