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Analytical Chemistry — Notes

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1. Colours of salts and their solutions

Qualitative analysis identifies the ions in a substance from observations such as colour and precipitates.

  • White solids, colourless solutions: salts of Na⁺, K⁺, Ca²⁺, NH₄⁺, Zn²⁺ and Pb²⁺.
  • Copper(II): hydrated CuSO₄ blue crystals, blue solution; CuCl₂ green to blue-green; CuO black; CuCO₃ green.
  • Iron(II): FeSO₄·7H₂O pale green crystals, pale green solution.
  • Iron(III): FeCl₃ yellowish-brown solid and solution; Fe₂O₃ reddish brown.
  • Zinc and lead oxides: ZnO white (yellow when hot); PbO yellow; Pb₃O₄ bright red; PbO₂ dark brown.
Table of four groups of ions and the colours of their solutions
IonTypical colour of its solution
Cu²⁺Blue
Fe²⁺Pale green
Fe³⁺Yellowish brown
Zn²⁺, Pb²⁺, Ca²⁺, NH₄⁺Colourless

Key terms

Qualitative analysis

Finding out which ions or substances are present, without measuring how much.

Precipitate

An insoluble solid that forms when two solutions are mixed.

2. Sodium hydroxide: coloured precipitates

Sodium hydroxide supplies OH⁻ ions; insoluble metal hydroxides precipitate. Add it drop by drop, then in excess.

  • Fe²⁺: dirty green Fe(OH)₂, insoluble in excess (turns brown on standing in air: oxidised to Fe(OH)₃).
  • Fe³⁺: reddish-brown Fe(OH)₃, insoluble in excess.
  • Cu²⁺: pale blue Cu(OH)₂, insoluble in excess.
  • Ca²⁺: white Ca(OH)₂ (sparingly soluble), insoluble in excess.
Table of precipitates formed by sodium hydroxide with four cations and their behaviour in excess
CationPrecipitateColourExcess NaOH
Fe²⁺Fe(OH)₂Dirty greenInsoluble
Fe³⁺Fe(OH)₃Reddish brownInsoluble
Cu²⁺Cu(OH)₂Pale blueInsoluble
Ca²⁺Ca(OH)₂WhiteInsoluble

Formulas

  • Dirty green precipitate
    FeSOX4+2 NaOH→Fe(OH)X2+NaX2SOX4\ce{FeSO4 + 2NaOH -> Fe(OH)2 + Na2SO4}
  • Reddish-brown precipitate
    FeClX3+3 NaOH→Fe(OH)X3+3 NaCl\ce{FeCl3 + 3NaOH -> Fe(OH)3 + 3NaCl}
  • Pale blue precipitate
    CuSOX4+2 NaOH→Cu(OH)X2+NaX2SOX4\ce{CuSO4 + 2NaOH -> Cu(OH)2 + Na2SO4}

Key terms

Drop by drop

Adding a reagent a little at a time so that the first change can be seen before an excess is present.

3. Sodium hydroxide: zinc and lead

Zinc and lead salts give white precipitates that dissolve in excess NaOH, because their hydroxides are amphoteric.

  • Zn²⁺: white gelatinous Zn(OH)₂ → dissolves to sodium zincate, Na₂ZnO₂ (colourless solution).
  • Pb²⁺: white chalky Pb(OH)₂ → dissolves to sodium plumbite, Na₂PbO₂ (colourless solution).
Table of zinc and lead ions with a little and excess sodium hydroxide
CationA few drops of NaOHExcess NaOH
Zn²⁺White, gelatinousDissolves (sodium zincate)
Pb²⁺White, chalkyDissolves (sodium plumbite)

Formulas

  • A few drops
    ZnSOX4+2 NaOH→Zn(OH)X2+NaX2SOX4\ce{ZnSO4 + 2NaOH -> Zn(OH)2 + Na2SO4}
  • Excess: sodium zincate
    Zn(OH)X2+2 NaOH→NaX2ZnOX2+2 HX2O\ce{Zn(OH)2 + 2NaOH -> Na2ZnO2 + 2H2O}
  • A few drops
    Pb(NOX3)X2+2 NaOH→Pb(OH)X2+2 NaNOX3\ce{Pb(NO3)2 + 2NaOH -> Pb(OH)2 + 2NaNO3}
  • Excess: sodium plumbite
    Pb(OH)X2+2 NaOH→NaX2PbOX2+2 HX2O\ce{Pb(OH)2 + 2NaOH -> Na2PbO2 + 2H2O}

Key terms

Sodium zincate

Na₂ZnO₂, the soluble salt formed when zinc, its oxide or its hydroxide reacts with sodium hydroxide.

Sodium plumbite

Na₂PbO₂, the soluble salt formed when lead, lead(II) oxide or lead(II) hydroxide reacts with sodium hydroxide.

4. Ammonium hydroxide, a few drops and in excess

A few drops of NH₄OH give the same precipitates as NaOH (an ammonium salt stays in solution). In excess:

  • Cu(OH)₂ dissolves → deep (inky) blue solution of tetraamminecopper(II) sulphate, [Cu(NH₃)₄]SO₄.
  • Zn(OH)₂ dissolves → colourless solution of tetraamminezinc(II) sulphate, [Zn(NH₃)₄]SO₄.
  • Pb(OH)₂, Fe(OH)₂, Fe(OH)₃ do not dissolve.

Plain-text equation for copper in excess: Cu(OH)₂ + (NH₄)₂SO₄ + 2NH₄OH → [Cu(NH₃)₄]SO₄ + 4H₂O (zinc follows the same pattern).

Table comparing the behaviour of five cations in excess sodium hydroxide and excess ammonium hydroxide
CationExcess NaOHExcess NH₄OH
Fe²⁺, Fe³⁺Precipitate staysPrecipitate stays
Cu²⁺Precipitate staysDeep blue solution
Zn²⁺DissolvesDissolves
Pb²⁺DissolvesPrecipitate stays

Formulas

  • Dirty green precipitate
    FeSOX4+2 NHX4OH→Fe(OH)X2+(NHX4)X2SOX4\ce{FeSO4 + 2NH4OH -> Fe(OH)2 + (NH4)2SO4}
  • White precipitate, insoluble in excess
    Pb(NOX3)X2+2 NHX4OH→Pb(OH)X2+2 NHX4NOX3\ce{Pb(NO3)2 + 2NH4OH -> Pb(OH)2 + 2NH4NO3}
  • White precipitate, soluble in excess
    Zn(NOX3)X2+2 NHX4OH→Zn(OH)X2+2 NHX4NOX3\ce{Zn(NO3)2 + 2NH4OH -> Zn(OH)2 + 2NH4NO3}

Key terms

Ammonium hydroxide

A solution of ammonia in water; a weak alkali.

Tetraamminecopper(II) sulphate

The deep blue soluble complex salt formed when copper(II) hydroxide dissolves in excess ammonium hydroxide.

5. Sodium hydroxide and ammonium salts

Any ammonium salt warmed with an alkali gives ammonia: the stronger base drives out the weaker one.

Identifying ammonia: pungent smell; turns moist red litmus blue; gives dense white fumes of NH₄Cl with a rod dipped in concentrated HCl (NH₃ + HCl → NH₄Cl).

Table of three tests that identify ammonia
Test on the gasObservation for ammonia
Smell (waft gently)Sharp, pungent
Moist red litmusTurns blue
Rod dipped in conc. HClDense white fumes

Formulas

  • Ammonium chloride, warmed
    NHX4Cl+NaOH→NaCl+HX2O+NHX3\ce{NH4Cl + NaOH -> NaCl + H2O + NH3}
  • Ammonium sulphate, warmed
    (NHX4)X2SOX4+2 NaOH→NaX2SOX4+2 HX2O+2 NHX3\ce{(NH4)2SO4 + 2NaOH -> Na2SO4 + 2H2O + 2NH3}
  • Ammonium nitrate, warmed
    NHX4NOX3+NaOH→NaNOX3+HX2O+NHX3\ce{NH4NO3 + NaOH -> NaNO3 + H2O + NH3}

Key terms

Test for the ammonium ion

Warm the substance with sodium hydroxide solution: ammonia is given off, which turns moist red litmus paper blue.

6. Amphoteric nature of aluminium, zinc and lead

Amphoteric: reacts with both acids and bases to form a salt and water. Examples: Al₂O₃, ZnO, PbO and Al(OH)₃, Zn(OH)₂, Pb(OH)₂.

  • Metals with hot concentrated NaOH give hydrogen: aluminium (with water) → sodium aluminate; zinc → sodium zincate; lead → sodium plumbite.
  • Oxides and hydroxides with NaOH give the same salts and water (no hydrogen).
  • With KOH the products are potassium aluminate, zincate and plumbite.
Table of the products when aluminium, zinc, lead and their oxides or hydroxides react with sodium hydroxide
Reactant with NaOHSaltOther product
Al (with water)Sodium aluminate, NaAlO₂Hydrogen
ZnSodium zincate, Na₂ZnO₂Hydrogen
PbSodium plumbite, Na₂PbO₂Hydrogen
Oxide or hydroxideSame saltWater

Formulas

  • Aluminium with hot NaOH
    2 Al+2 NaOH+2 HX2O→2 NaAlOX2+3 HX2\ce{2Al + 2NaOH + 2H2O -> 2NaAlO2 + 3H2}
  • Zinc with hot NaOH
    Zn+2 NaOH→NaX2ZnOX2+HX2\ce{Zn + 2NaOH -> Na2ZnO2 + H2}
  • Lead with hot NaOH
    Pb+2 NaOH→NaX2PbOX2+HX2\ce{Pb + 2NaOH -> Na2PbO2 + H2}
  • Aluminium oxide
    AlX2OX3+2 NaOH→2 NaAlOX2+HX2O\ce{Al2O3 + 2NaOH -> 2NaAlO2 + H2O}
  • Zinc oxide
    ZnO+2 NaOH→NaX2ZnOX2+HX2O\ce{ZnO + 2NaOH -> Na2ZnO2 + H2O}
  • Lead(II) oxide
    PbO+2 NaOH→NaX2PbOX2+HX2O\ce{PbO + 2NaOH -> Na2PbO2 + H2O}
  • Aluminium hydroxide
    Al(OH)X3+NaOH→NaAlOX2+2 HX2O\ce{Al(OH)3 + NaOH -> NaAlO2 + 2H2O}

Key terms

Amphoteric substance

A substance that reacts with both acids and bases to form a salt and water.

Sodium aluminate

NaAlO₂, the soluble salt formed when aluminium, its oxide or its hydroxide reacts with sodium hydroxide.

7. Telling solutions apart

Choose a reagent that gives a different observation for each member of the pair:

PairReagentObservations
Fe²⁺ / Fe³⁺NaOHdirty green / reddish brown
Zn²⁺ / Pb²⁺excess NH₄OHprecipitate dissolves / stays
Zn²⁺ / Ca²⁺excess NaOHprecipitate dissolves / stays
Cu²⁺ / Fe²⁺excess NH₄OHdeep blue solution / dirty green precipitate stays
NH₄⁺ / Na⁺NaOH, warmammonia evolved / no gas
Table of a sequence of tests for identifying a cation
StepObservationConclusion
A few drops of NaOHColoured precipitateFe²⁺, Fe³⁺ or Cu²⁺ by colour
Excess NaOHWhite precipitate dissolvesZn²⁺ or Pb²⁺
Excess NH₄OHWhite precipitate dissolvesZn²⁺
Excess NH₄OHWhite precipitate staysPb²⁺

Key terms

Distinguishing test

A single test whose observation differs for the two substances being compared.

Exam tips

Common mistakes

  • Swapping the colours of Fe(OH)₂ (dirty green) and Fe(OH)₃ (reddish brown).
  • Saying Cu(OH)₂ dissolves in excess NaOH (it dissolves in excess NH₄OH).
  • Using excess NaOH to tell zinc from lead (it dissolves both).
  • Leaving water out of the aluminium–NaOH equation.

How marks are usually awarded

  • Observation questions (1 mark each): colour and behaviour in excess where asked.
  • Equations (1 mark): correct formulae and balancing; name the salt (zincate, plumbite, aluminate) if asked.
  • Distinguish questions (2 marks): one reagent, one observation for each substance.

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