Topic 11 · Qualitative Analysis

Qualitative Analysis

1

Testing for Cations

1.1How the tests work

Qualitative analysis identifies the ions in a substance, and the gases it gives off, from what is observed when reagents are added.
  • Cations are identified with aqueous sodium hydroxide and aqueous ammonia. Add a few drops, then add the reagent in excess.
  • Note the colour of any precipitate, whether it dissolves in excess, and whether ammonia is given off on warming.
  • The precipitate is the metal hydroxide, which is insoluble in water. Sodium, potassium and ammonium ions give no precipitate.
Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s)Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s)

1.2Results for each cation

CationAqueous sodium hydroxideAqueous ammonia
aluminium, Al3+white precipitate, soluble in excess giving a colourless solutionwhite precipitate, insoluble in excess
ammonium, NH4+ammonia produced on warmingno change
calcium, Ca2+white precipitate, insoluble in excessno precipitate
copper(II), Cu2+light blue precipitate, insoluble in excesslight blue precipitate, soluble in excess giving a dark blue solution
iron(II), Fe2+green precipitate, insoluble in excessgreen precipitate, insoluble in excess
iron(III), Fe3+red-brown precipitate, insoluble in excessred-brown precipitate, insoluble in excess
zinc, Zn2+white precipitate, soluble in excess giving a colourless solutionwhite precipitate, soluble in excess giving a colourless solution
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1

Testing for Cations (continued)

1.3The colours of the precipitates

whiteAl³⁺whiteCa²⁺whiteZn²⁺light blueCu²⁺greenFe²⁺red-brownFe³⁺noneNH₃ on warmingNH₄⁺
  • The green precipitate of iron(II) hydroxide turns brown on standing.
  • In excess aqueous sodium hydroxide, the white precipitates of aluminium and zinc hydroxide dissolve to give colourless solutions. Calcium hydroxide does not.
  • Only copper(II) hydroxide dissolves in excess aqueous ammonia to give a dark blue solution.
light blue precipitatedark blue solutiona few dropsof NH₃(aq)excessNH₃(aq)Cu²⁺(aq)
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1

Testing for Cations (continued)

1.4Identifying a cation: the complete flowcharts

coloured: the colour tells you the ion directlyCu²⁺insolublein excesslight blueFe²⁺insolublein excessgreenFe³⁺insolublein excessred-brownwhite: test furtherDissolves in excess?whiteAl³⁺ or Zn²⁺test withNH₃(aq)Ca²⁺insolublein excessyesnoAdd aqueous sodium hydroxide: a few drops, then in excessIs there a precipitate?NH₄⁺warm it: ammonia turnsdamp red litmus blueWhat colour is the precipitate?noyes coloured: the colour tells you the ion directlyCu²⁺dissolves in excess:dark blue solutionlight blueFe²⁺insolublein excessgreenFe³⁺insolublein excessred-brownwhite: test furtherDissolves in excess?whiteZn²⁺colourlesssolutionAl³⁺insolublein excessyesnoAdd aqueous ammonia: a few drops, then in excessIs there a precipitate?Ca²⁺no precipitateWhat colour is the precipitate?noyes
Exam tipDescribe a result in full: the colour of the precipitate and what happens in excess, e.g. "white precipitate, soluble in excess giving a colourless solution". Aluminium and zinc give the same result with aqueous sodium hydroxide, so aqueous ammonia is needed to tell them apart.
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2

Testing for Anions

2.1Tests for anions

AnionTestResult
carbonate, CO32−add dilute acid, and bubble the gas into limewatereffervescence: the gas is carbon dioxide, which gives a white precipitate with limewater
chloride, Cl−acidify with dilute nitric acid, then add aqueous silver nitratewhite precipitate of silver chloride
iodide, I−acidify with dilute nitric acid, then add aqueous silver nitrateyellow precipitate of silver iodide
sulfate, SO42−acidify with dilute nitric acid, then add aqueous barium nitratewhite precipitate of barium sulfate
nitrate, NO3−add aqueous sodium hydroxide, then aluminium foil, and warm carefullyammonia is produced: it turns damp red litmus paper blue
CO₃²⁻fizzes; limewater turns milkyCO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)add dilute acid, then bubbleany gas through limewater1Cl⁻white precipitateAg⁺(aq) + Cl⁻(aq) → AgCl(s)I⁻yellow precipitateAg⁺(aq) + I⁻(aq) → AgI(s)acidify with dilute nitric acid,then add aqueous silver nitrate2SO₄²⁻white precipitateBa²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)acidify with dilute nitric acid,then add aqueous barium nitrate3NO₃⁻ammonia: damp red litmus bluethe nitrate is reduced to ammoniaadd aqueous sodium hydroxideand aluminium foil, then warm4

2.2Why dilute nitric acid is added first

  • The acid removes any carbonate (and hydroxide) ions, which would also form a precipitate with silver or barium ions and give a false result.
  • Nitric acid is used because all nitrates are soluble, so it adds no ions that form a precipitate.
Exam tipDo not acidify with hydrochloric acid in the chloride test, or with sulfuric acid in the sulfate test. They add the very ions being tested for.

Worked exampleAn unknown solution contains either sodium chloride or sodium iodide. Describe how you would find out which anion is present.

  1. Acidify a sample with dilute nitric acid, then add aqueous silver nitrate.
  2. A white precipitate shows chloride ions. A yellow precipitate shows iodide ions.
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2

Testing for Anions (continued)

2.3The nitrate test

damp red litmusturns bluenitrate solution+ NaOH(aq)aluminium foilwarm
  • Aluminium reduces the nitrate ions to ammonia, which turns damp red litmus paper blue.
  • Ammonium ions also give ammonia when warmed with aqueous sodium hydroxide, even without aluminium. Test for them first.
  • Hold the litmus paper at the mouth of the tube. It must not touch the alkali in the tube.
3

Testing for Gases

3.1Tests for gases

oxygenglowing splint relightshydrogenlighted splint: poppop!carbon dioxidelimewater turns milkyammoniared litmus turns bluechlorineblue litmus bleachedsulfur dioxidepurple to colourless
GasTestResult
oxygen, O2insert a glowing splintthe splint relights
hydrogen, H2hold a lighted splint at the mouththe splint is extinguished with a pop
carbon dioxide, CO2bubble the gas into limewaterwhite precipitate, which dissolves with excess carbon dioxide
ammonia, NH3hold damp red litmus paper at the mouthit turns blue
chlorine, Cl2hold damp blue litmus paper at the mouthit turns red, then is bleached white
sulfur dioxide, SO2bubble into, or hold filter paper soaked in, acidified potassium manganate(VII)it turns from purple to colourless
  • Oxygen, hydrogen and carbon dioxide are colourless and odourless. Ammonia, chlorine and sulfur dioxide have a pungent smell, and chlorine is greenish-yellow.
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3

Testing for Gases (continued)

3.2Putting the tests together

Worked exampleA solution of compound K gives these results.
(1) Warmed with aqueous sodium hydroxide: a light blue precipitate, and a gas that turns damp red litmus paper blue.
(2) With aqueous ammonia: a light blue precipitate, soluble in excess to give a dark blue solution.
(3) Acidified with dilute nitric acid, then aqueous barium nitrate added: a white precipitate.
Identify the ions in K.

The gas is ammonia, so K contains ammonium ions, NH4+.The light blue precipitate that dissolves in excess ammonia to a dark blue solution shows copper(II) ions, Cu2+.The white precipitate with barium nitrate shows sulfate ions, SO42−.
Exam tipWhen a gas is given off, name it and give its test and result. The tables in this topic are printed in the practical paper (Paper 3) only, so learn them for Papers 1 and 2.
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