Qualitative
Analysis

IP Chemistry (6092), Sec 3

By the end of this topic, you can answer

1

How can two reagents identify seven different cations?

2

Aluminium and zinc ions give the same result with sodium hydroxide. How do you tell them apart?

3

Why is dilute nitric acid added before testing for chloride, iodide or sulfate ions?

4

How does aluminium foil help to detect nitrate ions?

5

Which test identifies each of six common gases?

6

How do you combine the results of several tests to name an unknown compound?

Starter

PredictTwo bottles have lost their labels. One holds copper(II) sulfate solution and the other holds iron(II) sulfate solution.

How could you tell them apart without tasting or smelling them?

NoteAdd a few drops of aqueous sodium hydroxide to each. Copper(II) ions give a light blue precipitate and iron(II) ions give a green one. This topic shows how such tests identify ions and gases.
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.
AnalogyEach test is like a two-question quiz for the ion: "What colour is your precipitate?" and "Do you dissolve in excess?" No two ions give the same pair of answers to both reagents.
1

Testing for Cations

1.1Where the precipitate comes from

  • The precipitate is the metal hydroxide, which is insoluble in water.
  • The hydroxide ions come from the reagent and join the metal ions in the solution:
Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s)Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s)
  • Sodium, potassium and ammonium ions give no precipitate, as their hydroxides are soluble.
1

Testing for Cations

1.3Aqueous sodium hydroxide: a few drops, then an excess

  • The green precipitate of iron(II) hydroxide turns brown on standing.
  • In excess, the white precipitates of aluminium and zinc hydroxide dissolve to give colourless solutions. Calcium hydroxide does not.
1

Testing for Cations

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
1

Testing for Cations

1.3Adding aqueous ammonia drop by drop

  • A few drops: a light blue precipitate of copper(II) hydroxide forms and settles.
  • In excess: the precipitate dissolves, giving a dark blue solution.
1

Testing for Cations

Quick checkWhich cation gives a white precipitate with aqueous sodium hydroxide that is insoluble in excess?

Aaluminium, Al3+
Bcalcium, Ca2+
Czinc, Zn2+
Dammonium, NH4+
NoteB: the white precipitates of aluminium and zinc hydroxide dissolve in excess aqueous sodium hydroxide. Ammonium ions give no precipitate.
1

Testing for Cations

1.4Identifying a cation with aqueous sodium hydroxide

1

Testing for Cations

1.4Identifying a cation with aqueous ammonia

Exam tipAluminium and zinc ions give the same result with aqueous sodium hydroxide, so use aqueous ammonia to tell them apart.
1

Testing for Cations

Worked exampleA solution gives a white precipitate with aqueous sodium hydroxide, which dissolves in excess. With aqueous ammonia, it gives a white precipitate which does not dissolve in excess. Identify the cation.

Step 1A white precipitate that dissolves in excess sodium hydroxide: the cation is aluminium or zinc.

Step 2With excess aqueous ammonia, zinc hydroxide dissolves but aluminium hydroxide does not.

Step 3The cation is aluminium, Al3+.

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
2

Testing for Anions

2.1The anion tests, with their ionic equations

NoteSilver chloride, silver iodide and barium sulfate are insoluble salts, so they form as precipitates, just as in the preparation of insoluble salts.
2

Testing for Anions

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.
AnalogyIt is like wiping a whiteboard before writing a message: any old marks are removed first, so the only marks left are the ones you are looking for.
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.
2

Testing for Anions

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

Step 1Acidify a sample with dilute nitric acid, then add aqueous silver nitrate.

Step 2A white precipitate shows chloride ions. A yellow precipitate shows iodide ions.

2

Testing for Anions

2.3The nitrate test

  • 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.
2

Testing for Anions

Quick checkA solution is acidified with dilute nitric acid, then aqueous silver nitrate is added. A yellow precipitate forms. Which ion is present?

Achloride, Cl−
Biodide, I−
Csulfate, SO42−
Dcarbonate, CO32−
NoteB: silver iodide is yellow. Silver chloride is white, sulfate is tested with barium nitrate, and any carbonate is removed by the nitric acid.
3

Testing for Gases

3.1Tests for gases

3

Testing for Gases

3.1Tests for gases

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
NoteOxygen, hydrogen and carbon dioxide are colourless and odourless. Ammonia, chlorine and sulfur dioxide have a pungent smell, and chlorine is greenish-yellow.
3

Testing for Gases

Quick checkA pungent gas turns damp blue litmus paper red, then bleaches it. What is the gas?

Aammonia
Bcarbon dioxide
Cchlorine
Dsulfur dioxide
NoteC: chlorine is acidic and also bleaches. Ammonia turns red litmus blue, carbon dioxide is odourless, and sulfur dioxide is tested with acidified potassium manganate(VII).
3

Testing for Gases

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

Step 1The gas is ammonia, so K contains ammonium ions, NH4+.

Step 2A light blue precipitate that dissolves in excess ammonia to a dark blue solution: copper(II) ions, Cu2+.

Step 3A white precipitate with barium nitrate: sulfate ions, SO42−.

Exit ticket

1State the result when aqueous ammonia is added to a solution of iron(III) ions until in excess.

A red-brown precipitate, insoluble in excess.[1]

2Describe the test for sulfate ions.

Acidify with dilute nitric acid, then add aqueous barium nitrate: a white precipitate forms.[1]

3Describe the test for hydrogen.

Hold a lighted splint at the mouth of the tube: it is extinguished with a pop.[1]

4Name the reagent used to tell aluminium ions from zinc ions.

Aqueous ammonia (in excess).[1]