| CHEMICAL TEST FOR | TEST METHOD | OBSERVATIONS | TEST CHEMISTRY |
Chemical test for Carbonate ion CO32– or hydrogencarbonate HCO3– ion
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.
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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 gaswhich turns limewater milky – cloudy fine white precipitate (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) To identify 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 the hydrogencarbonate … HCO3–(s) + H+(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 that some give clear colour changes in the solid which might be useful to identify the metal and for sodium hydrogencarbonate … 2NaHCO3(s) ==> Na2CO3(s) + H2O(l) + CO2(g) |
| Sulphate ion or sulphate(VI) ion SO42–[sulfate, sulfate(VI)] chemical testIf 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 precipitateof barium sulfate.(ii) A white precipitate of lead(II) sulphate.
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(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. |
| Sulphite ion or sulphate(IV) ion SO32–[sulfite, sulfate(IV)] chemical testTest (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 fromorange to green.
(iii) A white ppt. of barium sulphite which dissolves in excess hydrochloric acid to give a clear colourless solution. |
(i) To identify 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) |
| Sulphide ion S2– (sulfide) chemical testIn test (ii) dangerous hydrogen sulphide (hydrogen sulfide) 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 precipitateof 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). |
| Chloride ion chemical testCl–
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 precipitateof silver chloride soluble in dilute ammonia.(ii) You get nasty fumes of hydrogen chloride which turn blue litmus red and give a white precipitate with silver nitrate solution.
(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) |
| Bromide ion chemical testBr– | (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.(ii) Orange vapour of bromine and pungent fumes of SO2, test for sulphur dioxide.
(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) |
| Fluoride Ion chemical testF–
Fluoride and hydrogen fluoride gas are harmful, irritating and corrosive substances. |
(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 chemical test 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 vapourand 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 ==> yellow 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) |
| Nitrate ion or nitrate(V) ion NO3–chemical test | (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 redlitmus blue, (ii) Where the liquids meet a brown ringforms
(iii) Nasty brown gas (beware!) ofnitrogen (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. |
| Nitrite ion or nitrate(III) ion NO2–chemical test | (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) and gives ‘brown ring’ test | ||
| Alkali:Hydroxide ion chemical test 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 litmusblue, variety of colours univ. ind. dark green – violet for weak – strong.(ii) If strongly alkaline ammonia should be released, | (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) |
| Chromate(VI) ion chemical testCrO42– (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 yellowsolution turns orangeas 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) |
S2O32-(aq) + dil.HCl → SO2(g) + S(s)
S2O32-(aq) +AgNO3(aq) → Ag2S2O3 (aq) →Δ Ag2S (black precipitate)
S2O32-(aq) +Pb(NO3)2(aq) → PbS2O3 (aq)(white) →Δ Ag2S(black)
• To a solution of add conc. HNO3, excess of ammonium molybdate and warm. A yellow precipitate will be formed.
