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Chemistry revision notes on qualitative chemical tests for anions (negative ions)

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Doc Brown's Chemistry: CHEMICAL TESTS FOR IDENTIFYING ANIONS (negative ions - mainly non-metal ions)

 CHEMICAL identification TESTS Part 5 Qualitative tests to test for, and identify, anions (negative ions)

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Revising Inorganic Chemistry: The following tests for the following anions are fully described including explanations, methods, observations and equations, so described are how to test for the following anions – negative ions: carbonate ion CO32–, hydrogencarbonate ion HCO3–, sulphate(VI)/sulfate(VI) ion SO42–, sulphite (IV)/sulfite(IV) ion SO32–, sulphide/sulfide ion S2–, fluoride ion F–, chloride ion Cl–, bromide ion Br–, iodide ion I–, nitrate(V)/nitrate ion NO3–, nitrate(III)/nitrite ion NO2–, hydroxide ion/alkalis OH– and the chromate (VI) ion CrO42–

Chemical Tests Index

 Part 1 Introduction to chemical tests

 Part 2 Qualitative tests to identify organic molecule functional groups of homologous series

 Part 3 Metal cations (positive ions), metal carbonates, ammonium ion and hydrogen ions (acids)

 Part 4 Gases, water and non–metallic elements

 Part 5 Anions (negative ions) including hydroxide (alkalis) (this page)

APPENDIX 1. IDENTIFYING ELEMENTS from LINE SPECTRA (non-chemical test method)

some associated revising links

GCSE/IGCSE Revision QUIZ on chemical tests for identifying ions, gases and compounds

A Level Quantitative analysis: acid–base, silver nitrate–chloride, EDTA titrations

A Level Quantitative analysis: Redox titrations

Full list of KEYWORDS for inorganic/organic identification methods in alphabetical order e.g. test/reagent for: * acid ==> H+ * acid/acyl chloride RCOCl * alcohols – general ROH/prim RCH2OH/sec R2CHOH/tert R3COH) * aldehyde RCHO * prim aliphatic amine R–NH2 * aliphatic/aromatic carboxylic acids * alkali ==> OH– * alkane/alkene >C=C</alkyne –CC– (saturated versus unsaturated) * aluminium/aluminum ion Al3+ * amide RCONH2 * prim aliphatic amines R–NH2 * ammonia gas NH3 * ammonium ion NH4+ * prim aromatic amine C6H5–NH2 etc. * barium ion Ba2+ * Benedict's solution * Brady's reagent * bromide ion Br– * bromine Br2 * caesium ion Cs+ * calcium ion Ca2+ by flame or hydroxide ppt. * carbonate CO32–/hydrogencarbonate HCO3– with acid or effect of heating metal carbonate e.g. MCO3 * carbon dioxide gas CO2 * carboxylic acid RCOOH * carboxylic acid (aliphatic) salts e.g. RCOO–Na+ * chloride ion Cl– * chlorine gas Cl2 * Chromate(VI) ion CrO42– * copper(II) ion Cu2+ by flame or hydroxide ppt. * 24DNPH (for aldehydes/ketones test) * esters RCOOR * Fehling's test/solution * flame test for metal ions * fluoride ion F– * haloalkanes/halogenoalkanes R–X * hydrogen gas H2 * hydrogen sulphide H2S * hydrogen ion, acids H+ * hydrogen bromide gas/hydrobromic acid HBr * hydrogen chloride gas/hydrochloric acid HCl * hydrogen iodide gas/hydriodic acid HI * hydroxide ion, alkali OH– * hydroxy/alcohol/phenol (organic) * iodide ion I– * iodine I2 * iodoform test – formation of CHI3 * iron(II) ion Fe2+ * iron(III) ion Fe3+ * ketone R2C=O * lead(II) ion Pb2+ * lithium ion Li+ * lime water Ca(OH)2(aq) * magnesium ion Mg2+ * metal carbonates–heating e.g. MCO3 * metal ions via hydroxide precipitate * nitrate or nitrate(V) NO3– * nitrite or nitrate(III) NO2– * nitrogen dioxide or nitrogen(IV) oxide NO2 * oxygen gas O2 * phenols C6H5OH etc. * potassium ion K+ * rubidium ion Rb+ * reducing sugars * saturated/unsaturated * silver nitrate AgNO3 (see chloride, bromide, iodide tests) * sugars (reducing) * sodium ion Na+ * strontium Sr+ * 'sulphate/sulfate' or sulphate(VI) SO42– * sulphide S2– * 'sulphite/sulfite' or sulphate(IV) SO32– * sulphur dioxide gas SO2 * Tollen's Reagent * unsaturated/saturated * water H2O * zinc ion Zn2+ *

Use the alphabetical test list above for identifying anions, cations, gases, molecules etc. to find what you require! for your KS3–KS4 Science–GCSE–IGCSE– Chemistry and GCE–AS–A2–IB–US grades 9–12 K12 advanced subsidiary chemistry course etc. and help you to identify unknown inorganic and organic compounds–molecules for qualitative analysis.

EMAIL query? comment? test missing? * Gas Preparations *  Hazard warning signs/symbols–examples of labelling

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 5. INORGANIC Qualitative TESTS Anions and Alkalis

CHEMICAL TEST FOR TEST METHOD OBSERVATIONS TEST CHEMISTRY
Chemical test for Carbonate ion CO32– or hydrogencarbonate HCO3– ion

test for CO2

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.

 

 (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 fine white precipitate (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) 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 (see heating carbonates in metal cation section)

and for sodium hydrogencarbonate ...

2NaHCO3(s) ==> Na2CO3(s) + H2O(l) + CO2(g)
Sulphate ion or sulphate(VI) ion SO42– [sulfate, sulfate(VI)] chemical test

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.
Sulphite ion or sulphate(IV) ion SO32– [sulfite, sulfate(IV)] chemical test

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) 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 test

In 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 precipitate 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).
Chloride ion chemical test

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.

(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 test

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.

(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 test

F–

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.

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 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  ==> 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 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.
Nitrite ion or nitrate(III) ion NO2– chemical test 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.
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 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)
Chromate(VI) ion chemical test

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) 2CrO42–(aq)  +  2H+(aq) ==> Cr2O72–(aq)  +  H2O(l)

(ii) Ba2+(aq) + CrO42–(aq) ==> BaCrO4(s)

(iii) Pb2+(aq) + CrO42–(aq) ==> PbCrO4(s)

 

 

 

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Keywords & phrases Tests for CO3/CO32–, HCO3/HCO3–, SO4/SO42–, SO3/SO32–, S2–, F–, Cl–, Br–, I–, NO2/NO2–, NO3/NO3–, OH/OH–, CrO4/CrO42– What is the test for a carbonate ion? How do you test for carbonate ion? What is the test for the hydrogencarbonate ion? How do you test for hydrogencarbonate ion? What is the test for the sulfate ion? (sulphate, sulphate(VI)) How do you test for sulfate ion? What is the test for the sulfite ion? (sulphite, sulphate (IV)) How do you test for sulfite ion? What is the test for sulfide ion? (sulphide ion) How do you test for sulfide ion? What is the test for fluoride ion? How do you test for fluoride ion? What is the test for chloride ion? how do you test for chloride ion? What is the test for bromide ion? How do you test for bromide ion? What is the test for iodide ions? How do you test for iodide ions? What is the test for nitrite ion (nitrate(III) How do you test for nitrite ion? What is the test for nitrate ions? (nitrate(V)) How do you test for nitrate ions? What is the test for hydroxide ions? How do you test for hydroxide ions? What is the test for chromate(VI) (chromate ion) How do you test for chromate(VI) ions? 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